Burnt BMS (Pics included) Please help?

[Watchamacallit]

Hi all.

I should probably warn you that i'm fairly anxious right now. Im very aware of how dangerous these batteries can be and i'm desperately trying to find solution(s) to this problem. So a bit of patience and understanding will be hugely appreciated:

So for the last 2 years, ive owned two identical 48v, 17.5ah ebike hailong style batteries with little to no problems.

However, a month or so ago, while charging one of them (on their 4a charger) i left a rather slim carrier bag on top of the battery and it steadily overheated and slightly melted the casing. Visually it seemed quite minor and i then decided to just be more careful in the future, and strictly monitor them as they charge.

fast forward a few more months (the "melted" battery still operating just fine with regular use) I decided to take apart the hailong battery that had melted to inspect it (I'm reasonably technical, I build PC's and often solder things) so approached with caution as i've never taken apart an ebike battery before.

Anyway, after disassembly, and peeling back the blue heatshrink wrap, to my surprise i saw a rather toasted charge mosfet on the BMS. Even more surprisingly was the lack of any heatsink which, i presume was removed so it would fit in the casing (it's extremely tight) what maybe is why it overheated??




I dont feel comfortable leaving this thing on the pack now, and have been desperately trying to source replacements for both batteries (I fear the same outcome for the other one as they are identical!)

As i stated though, i've not done this before and i want to approach this as carefully and safely as possible. So what i'm hoping for is:

1.) To immediately remove the BMS's, in the safest way possible (Do i remove the balance connector first? or B-? what is the process?) if anyone can offer me any guidance on this it would be HUGELY appreciated!!

2.) source replacement BMS's as, overall, i do think the packs are built well but just heavily let down by the BMS's in them. They are all branded samsung cells, with neatly organized and tidied cables. spot welds and good soldering. however i'd rather be corrected if i'm wrong by those more experienced.....



I will post below a sheet of infomation i had on purchase with the batteries, as it may prove useful:

1. Lithium Battery pack with Samsung 35E cells 

2. Built-in BMS board 

3. US/EU/AU Charger (54.6V 2.0A) 

Battery Model 48V 17.5AH 
Nominal Capacity (AH) 13 
Nominal Voltage (V) 48 
Source Resistance (m?) 40 
Cell Combination 5-parallels 
13-series 
Cell Quantity (parallel*series) 65 
Discharge Cutoff Voltage (V) 41 
Charge Cutoff Voltage (V) 54.6 
Rated Discharge Current (A) 15 
Instantaneous Maximum Discharge Current (A) 40 
Maximum Continuous Discharge Current (A) 20 
Maximum Continuous Charge Current (A) 5 
Charge Mode CC-CV 
Standard Charge Current (A) 2 
Charge Time under Standard Charge Current 6 hours 
Fast Charge Current (A) 5 
Charge Time under Fast Charge Current 2.5 hours 
Charge Temperature Range -20-55°C 
Battery Net Weight 3kgs 
Battery Gross Weight(with all parts and package) 4.5kgs 


Again many thanks for any help and assistance with this, it's very much appreciated.

 

[999zip999]

Where did you get the battery ? What brand is it ? Do you have a link ? So we could check it out. That looks toast all over as over cooked in the oven as opposed to one item shorting out. It looks more like the inside of a overheated hub motor.

 

[Watchamacallit]

Where did you get the battery ? What brand is it ? Do you have a link ? So we could check it out. That looks toast all over as over cooked in the oven as opposed to one item shorting out. It looks more like the inside of a overheated hub motor.

It was through an online UK shop that no longer exists so i'm afraid i couldn't tell you.

No branding on the batteries either, only the typical Hailong casing logo's, So my theory is it was the uk shop that built them and they used whatever junk BMS was available at the time to cut down costs.

 

[amberwolf]

Anyway, after disassembly, and peeling back the blue heatshrink wrap, to my surprise i saw a rather toasted charge mosfet on the BMS. Even more surprisingly was the lack of any heatsink which, i presume was removed so it would fit in the casing (it's extremely tight) what maybe is why it overheated??

It's not all that uncommon for a BMS to generate a lot of heat inside a heatshrinked pack that is also inside a sealed unventilated case, especially if it's in a hot environment or otherwise unable to dissipate the heat as it was designed to do.

With low-discharge (and charge) rate packs, that's not normally much of an issue. When temperatures outside the casing are high, and if the pack is used (ridden) in high temperatures, or used hard (high load heating cells and BMS up) and then charged right away, it can heat the BMS FETs up a fair bit.

It can be worse for packs with common charge and discharge ports, because the FETs are in series, so the charge FETs are being used as diodes during discahrge, and the discharge FETs are bieng used as diodes during charging. Your is probably separate ports and not used that way, because you have wires on C- (charge) and P- (discharge).



There isn't always a heatsink on the FETs; it's pretty common not to have one. They don't do nearly as much to remove heat when sealed up inside packs. They do spread the heat over the whole surface area of the heatsink and anything it's touching, so can be better than nothing if there is a fairly hot spot in a small component...but a better design is to use parts (or enough of them) that won't get hot in that usage scenario, or change the scenario (pack design) to allow at least internal airflow to occur within the casing (not put the BMS inside the heatshrink, but instead somewhere air can flow around it even if it's just internal convection).

Some BMSs have a metal plate over teh whole thing that acts as a cover and heat spreader, and sort of heatsinks the discharge FETs, and sometimes the charge FETs are also sinked by this, but not all of them are. (not all of them have one on the discharge FETs either, and just use the PCB itself as the heatsink). yours doesn't look like it ever had a cover like that, so it probably just uses the PCB as the heatsink.

The white rectangular part with two white wires (out of focus close to the camera) is the temperature sensor (really a thermal "breaker" switch) that allows the BMS to shut itself off if it overheats. Where was that sensor within the pack? (it's usually placed within the center of the cell mass, so that it detects cell overheating to prevent pack fires during charge or discharge; sometimes it's just stuck on the surface of the pack under the heatshrink, sometimes it's loosely held in the battery case, sometimes it's right up on the BMS).



As far as just sitting there, or even using it to ride with, there's not really anything to worry about with a BMS that has a damaged charge FET. All it does is pass the negative wire of teh charge port (C-) to the negative wire of the pack (B-). So the only problems it causes are: If it is failed open, the BMS won't let the cells charge. If it is failed shorted, the BMS won't be able to stop charging, even if a cell group is already full (unbalanced) or the temperature sensor detects overheating, etc.

If balancers failed, *then* it could damage cells just sitting, if a balancer is stuck "on", by draining that group continuously whether it needs it or not, eventually emptying it completely. If a balancer is stuck "off", it can't drain that group if it is overcharged, and eventually that cell group could be severely overcharged if the charge FET is failed shorted, because the pack can't stop an overcharge or imbalance situation as long as a charger is connected and running.

1.) To immediately remove the BMS's, in the safest way possible (Do i remove the balance connector first? or B-? what is the process?) if anyone can offer me any guidance on this it would be HUGELY appreciated!!

Since you presumably don't care what happens to the old BMS, disconnect wire order doesn't matter. (it's possible to damage some BMSs during connection or disconnection if done in the wrong order). For most BMSs, just disconnecting the balance connector is sufficient to disconnect the BMS from all the cell positives, so it is then not affecting anything to do with the pack. Yours appears to be made this way. Probably you can peel away the hot-glue or silicone that secures that connector to the BMS, and just gently pull the plug out of the BMS (by the connector, not the wires).

If you feel the need to disconnect the B-, P-, and C- wires, just make sure you tape each one off as you do this, to ensure they can't touch any positive wires anywhere, or cells, etc, and so that you can mark them for where they came from to put them on the new BMS the same way.

I recommend using "hammerhead" or "chisel" tips for a high-wattage (80w+) soldering station or iron, to be able to cleanly unsolder and solder large-gauge wires like those that are on large-area / heavy-conductor-trace PCB pads. Weller makes good ones. I use a Weller WE1010, and a pack of assorted styles of tips for different jobs that work on most things.
https://www.amazon.com/gp/product/B077JDGY1J
https://www.amazon.com/gp/product/B07P1B2535
For really heavy wires or pads for power traces on PCBs I use an 80w large-chisel-tip Weller iron dedicated to this. mine is pretty old and just uses a neon lamp in the handle to tell me it's on; this is the newer version:
https://www.amazon.com/Weller-SP80NUS-80-Watts-Soldering-Iron/dp/B00B3SG796
The reason for the large tips is so they hold more heat for the initial heating up of a pad or wire so it happens really fast, and has less chance of damaging surrounding parts (which can happen if you have to hold an iron on a spot a long time to make it hot enough to melt the solder).

I use two different Kester solders depending on size of part being soldered; thick stuff for things like the BMS power wires, thin stuff for most components and small wires. (60/40 tin/lead is the "standard" I find works most often; you can try lead-free if you want, but I've had limited success with it):
https://www.amazon.com/gp/product/B0032V15WQ
https://www.amazon.com/gp/product/B005T74XSY

For things that aren't soldering well, I use Kester flux to help clean the area during soldering
https://www.amazon.com/gp/product/B07RH967LY

To remove stuff that you can't just pull out while heating with the iron (unlike wires), fluxed braid works well in most cases; solder suckers *can* work but you have to practice a fair bit to get the knack of doing it just right, even with a good one (and a lot of them suck, no pun intended). I can't find the one I last bought but this one is similar
https://www.ebay.com/p/2254497717?iid=274647272035

If you haven't soldered or unsoldered things with heavy wiring / large power pads, etc., a fair amount, I recommend youtube tutorial videos on that, and practice on the old BMS before you put the new one on, or on other old electronics you don't need. It only takes a few hours' practice to get pretty good at it, and make very quick work of good solder joints that make great connections. (it's different than doing small connections, though similar).

2.) source replacement BMS's as, overall, i do think the packs are built well but just heavily let down by the BMS's in them. They are all branded samsung cells, with neatly organized and tidied cables. spot welds and good soldering. however i'd rather be corrected if i'm wrong by those more experienced.....

FWIW, a number of people call a BMS (Battery Monitoring System) a Battery Murdering System because there are a lot of bad designs out there (probably most of them) that don't fail safe from various kinds of problems. Some are even made of parts intended only for single-cell use, but are instead used in series for a stack of cells in the BMS (so when anything goes wrong or stuff is connected in a different order, even at the factory, than required by this, stuff fails and cells get damaged or are left unprotected). There are some threads around talking about this problem, including one with some detailed descriptions and schematics, etc., but I don't have any links ATM.

One way to at least be able to keep an eye on things is to use a Bluetooth BT BMS so you can monitor it from your phone, etc. Some of those arent' reliable either, but at least you can see voltages, etc., during charge and discharge, of the actual cells, and if something isn't "normal" you have the opportunity to open it up and check on it independently to be sure where the problem lies.

I don't know which specific BMSs are good these days, so I'll leave that for others to come answer for you.

 

 

[Watchamacallit]

.....
I don't know which specific BMSs are good these days, so I'll leave that for others to come answer for you.

 

Massive thank-you for taking the time to write this out, just what i needed and has put me at ease a little. So the overall gist is that it's mostly OK in it's current state but, preferably, maybe it's better to replace it? Or is it ok to just clean it up and reassemble it in the pack?

I assume based on this infomation it would be ok to get another BMS without a heatsink (or remove it myself) so it will fit easier?

I gotta admit when i caught glimpse of the BMS in the state it was in, my heart started racing having visions of a huge house fire!

I'm fairly handy at soldering and i do it quite regularly - have done for years now so I think i've a number of transferable skills to be able to do a decent job of this. I'm just not 100% ready yet and still don't know enough about these batteries other than the clear danger they can pose if they malfunction or arn't treated right. I've a healthy respect for them.

The white rectangular part with two white wires (out of focus close to the camera) is the temperature sensor (really a thermal "breaker" switch) that allows the BMS to shut itself off if it overheats. Where was that sensor within the pack?

Yeah you're right, it's a temperature sensor (75 degree's C written on it)

When i removed the heatshrink, it was located in the upper portion of the BMS (with part of it's white wire touching the FET hence the brown discolouration).

I did think that it had poor placement when i unwrapped it and maybe that was a contributing factor? Where would it be best located on the BMS PCB?


Again, MANY thanks for your assistance. Hugely appreciate it.

 

[amberwolf]

Massive thank-you for taking the time to write this out, just what i needed and has put me at ease a little. So the overall gist is that it's mostly OK in it's current state but, preferably, maybe it's better to replace it? Or is it ok to just clean it up and reassemble it in the pack?

I wouldn't even "clean it up", unless it has corrosion on it (and in that case it's easier to just disconnect the balance wires, because you won't know what damage the corrosion has done, or where it might be that you can't see).

I assume based on this infomation it would be ok to get another BMS without a heatsink (or remove it myself) so it will fit easier?

If it has a heatsink I'd leave it there, because it will at the least spread the heat that is otherwise concentrated in the parts it's touching/mounted to, over it's entire surface, even if the heatsink doesn't have a lot of airflow.

Regardless of type, etc, I would make sure it is not inside the heatshrink (which prevents any airflow around it at all, trapping more heat right there), and if possible is in the most "open" area of the case, meaning the spot that has the most air around it (even if it doesn't flow much, convection will do *something* to at least spread the heat it generates around, and not concentrate it in just the parts making the heat).

Some packs that don't generate cell heat (high current capability cells used in a low-demand application so they never heat up) may put the BMS heatsink/spreader against the pack itself, to let the large thermal mass of the pack absorb the heat. Whether this is a good idea for your pack, you'd have to judge based on your knowledge (or testing) of cell general temperatures during and after being used on a ride, and charging.

I gotta admit when i caught glimpse of the BMS in the state it was in, my heart started racing having visions of a huge house fire!

Unfortunately that does happen. Why specific fires happened isn't usually possible to diagnose after the fact because there's so little left to examine, without forensic skills and tools. But we know sufficient overheating, overcharging or overdischarging can damage a cell to the point it fails internally and begins to heat catastrophically. So things that can cause any of those, I would consider "bad".

A failed charge FET that's open doesn't matter. It just means you can't charge the pack, so you can't overcharge it either.

A failed charge FET that's shorted, meanign always on, is a problem--it means it will never stop charging in the cases the BMS detects a problem of any kind.

You can test to see if the FET is open or shorted, if curious. Open test just connect charger when pack is drained enough that charging would normally start. If charging doesn't start, and charger works on the other pack, and any fuse/etc between charger and C- on BMS is ok, then FET is probably open. (you'd have to also verify thermal switch is closed and that all balance wires at the BMS pads show normal voltage range, not below BMS LVC (probably 2.8v or so) and not above HVC (probably 4.2v or so), but those are probably ok if it's been working normally before).

Short test is a little more involved; you'd wait for charge to fill up cells to full, past HVC, and see if charger current is still flowing. Or trigger thermal switch to turn off (it's usually marked with it's temperature range). If charger still supplies current in either case, then charge fet is probably stuck on.

You can test for the stuck on condition with an ohms / continuity test (measuring across the Source and Drain pins, usually the tab and "righthand" pin with the tab on "top", FET label facing you) but you'd want to ensure there is no voltage across it first (this can damage your meter).

Yeah you're right, it's a temperature sensor (75 degree's C written on it)

When i removed the heatshrink, it was located in the upper portion of the BMS (with part of it's white wire touching the FET hence the brown discolouration).

I did think that it had poor placement when i unwrapped it and maybe that was a contributing factor? Where would it be best located on the BMS PCB?

It's not usually meant to measure the BMS temperature, it's meant to measure the cell temperature, so the best place for it is between the cells as close to the middle of the pack as it will reach.

If a BMS is well-designed, it would already monitor it's own temperatures in one or more of a number of ways (though I don't expect this of the typical cheap ones, especially those that come installed in packs).

If you prefer to have the BMS monitor it's own temperature, the best place to put that is as close to the discharge and cahrge FETs as possible. For instance, in the one you have, if that's what you want, I'd stick it between the charge FET and the discharge FETs to the left of it in that picture.

But if it were my pack I'd stick in between the cells as close to the middle of teh pack as possible.


If you want to "upgrade" functionality of any BMS using those thermal switches, you can actually put several of those thermal switches in series, with the two end ones each supplying one of the wires that go to the board itself. Then you install each one in a place that you think will get hot enough to shut off the BMS when there is a problem. You could put a few within the pack between different groups of cells, and a couple or more on the BMS for different heat-producing areas (FETs, balancers, shunts, etc).

Use thermal switches with appropriate ratings for the specific areas you're monitoring, so that they'll shut off only when there is actually a problem, but that they *will* shut off if there *is* a problem.

 

[Watchamacallit]

Massive thank-you for taking the time to write this out, just what i needed and has put me at ease a little. So the overall gist is that it's mostly OK in it's current state but, preferably, maybe it's better to replace it? Or is it ok to just clean it up and reassemble it in the pack?

I wouldn't even "clean it up", unless it has corrosion on it (and in that case it's easier to just disconnect the balance wires, because you won't know what damage the corrosion has done, or where it might be that you can't see).

Would it be worth me replacing the charging FET at least? Just to rule out any possible risk of failure or damage from the first overheating?

Would it be the same FET as the others? the writing on the charge FET has disappeared from the heat damage.

 

[amberwolf]

It is probably the same FET model. If you want to be certain, you can open the other pack and expose the BMS to see that, *and* to verify it hasn't failed, too.


You can try replacing it, but without knowing why it failed it could just do it again at any time (it's possible that the covering of the pack with the bag during charging did it, but that shouldn't be the case unless it's already so hot inside the pack that it's right on the edge anyway). If it's gate drive circuitry either failed with it, or is what killed it, replacing it won't do much.

Before changing it, I would test it as noted previously; at least you'll know which failure mode it had, so you know whether it could have overcharged the pack or not.

NOte that since it is surface mounted to the PCB, soldered across the back face of the FET thoroughly to the copper layer, it's giong to take some significant heat to remove it, and also to install a new one. Sometimes that damages the PCB itself if it takes too long. If there's no vias in the area or heat sensitive parts, it's not that big a deal, but it could separate the copper from the PCB material.


Regarding "possible risk of failure or damage from the first overheating?" I don't know what you mean.

If you don't intend to use this BMS to charge the battery (and are replacing it with a new BMS instead) then there is no reason to fix the FET. The only thing it does is allow current to pass from the charge port to the battery (C- to B-).

If there was damage to the rest of the BMS from the heat, replacing the FET itself is not going to help that, because you'll be heating it up to do that, too. If there was damage to the pack (cells) from the heat, it won't fix that either.

Changing the FET will only fix a problem or risk of damage if the charge fet is failed shorted (or damaged in a way that is likely to make this happen during charge), *and* you intend to keep charging it before replacing the BMS (or you intend to keep using this BMS but with the charge FET/etc repaired, instead of replacing it)

If it's failed open then the port simply won't let you charge at all, so you would have to replace the FET (and anything that is also be damaged, such as gate drive could be), to continue using this BMS to charge the battery, but otherwise it doesn't matter if it's failed open or not.

 

[Watchamacallit]

You can test for the stuck on condition with an ohms / continuity test (measuring across the Source and Drain pins, usually the tab and "righthand" pin with the tab on "top", FET label facing you) but you'd want to ensure there is no voltage across it first (this can damage your meter).

So i checked the voltage of the burnt out FET between the source and the drain (the tab at the top of the fet itself) and it measured 0 voltage, so i then did a continuity test and got a positive/beep.

Based on this, I guess the fet has failed on and would charge the pack to annihilation given the opportunity? Dont the chargers for these ebikes just charge it to a set voltage then cut off? wouldn't that mitigate this major issue? (and explain why i've not had a fiery inferno over the last few months?)

Out of curiousity i also measured all the other FETs. I removed the balance cable first (I successfully removed all the glue) but they all were reading 43v (the current charge of the battery)

I thought by disconnecting the balance leads this would now be disconnected from the battery? I wasn't expecting a reading of 43v from the other FET's

 

[amberwolf]

So i checked the voltage of the burnt out FET between the source and the drain (the tab at the top of the fet itself) and it measured 0 voltage, so i then did a continuity test and got a positive/beep.

Based on this, I guess the fet has failed on and would charge the pack to annihilation given the opportunity?

That's what it sounds like. (it's the most common failure mode for a FET).

Was the continuity test done in both directions? (some continuity tests use enough voltage to activate the body diode of a FET, so a test in one direction on even a good one will beep or read continuity, but the other way will read open, if the FET is off).

Dont the chargers for these ebikes just charge it to a set voltage then cut off? wouldn't that mitigate this major issue? (and explain why i've not had a fiery inferno over the last few months?)

They should. But...if the cells are out of balance; say one group is at 4.2v but the rest are still only 4v, the BMS detects this and turns off the charge input, then drains that 4.2v cell down to some level, then restarts charge, and cycles this wya until they all match and none exceed 4.2v. But...if the charge cant' be stopped, and the charge current is greater than the balancer can shunt aside for the full cell group, then that cell group will keep charging. This charge cycle that might not be a problem, it might only get to say 4.25v. But if it is then used again right away (or before the balancer finishes it's job), it will never be drained down to balance the others, so it will always be higehr than thye are, and each time it's used and recharged it'll get a little higher, and at some point (potentially a very long time from the beginning of the problem), damage to the cell occurs, which in some cases is catastrophic and dramatic.

Sometimes it never causes a problem, because the balancers get a chance to fix this after charging but before it's used again.

But the cell group already has a problem because it's full first (meaning it has less capacity and probably higher Ri), and that will nly get worse with time, and eventually it's possible the BMS wont' be able to deal with the problem in it's damaged state, it will need to be able to shut charging off to prevent overcharge and wont' be able to, and....

It can still stop discharging to prevent overdischarge of a low capacity group, because those FETs (P-) are still working.

Out of curiousity i also measured all the other FETs. I removed the balance cable first (I successfully removed all the glue) but they all were reading 43v (the current charge of the battery)

The other FEts will all read the same because they're paralleled.

When the balance connector is disconnected (for a BMS that checks for this; they don't all, but they should), the BMS will turn off both C- and P- FETs, because it can't know what state the cells are in so it won't allow the pack to be used.

When the FETs are off, they will read the difference in voltage between the B- and the P-. It's a "ghost" voltage, meaning any load at all will drain it down (even the meter will, if you leave it there long enough), but it means the FETs are off, like they should be.

That does mean the pack is not usable to ride with, either, until the BMS is reconnected or replaced, or the B- wire on the BMS is connected to the P- wire with a heavy enough connection to carry the full bike current (bypassing the BMS entirely).

With the balance leads disconnected, even if the FETs were all stuck on, then the BMS cannot drain the pack of anything, because it doesn't have any other positive connections to the cells. (no B+ on the BMS itself; that goes straight from the most positive cell to the discharge and charge ports, other than any fusing/etc in the battery case).

 

[tomjasz]

Red fish paper is exclusively used by UPP, Unit Pack Power
Rechie@unitpackpower.com should be able to help.
I have 8-10 of the BMS used 3-4 years ago, none match.

 

[Watchamacallit]

Was the continuity test done in both directions? (some continuity tests use enough voltage to activate the body diode of a FET, so a test in one direction on even a good one will beep or read continuity, but the other way will read open, if the FET is off).

By both directions i assume you mean reversing the multimeter cables on the drain/source?

In which case i get the same result.

Red fish paper is exclusively used by UPP, Unit Pack Power
Rechie@unitpackpower.com should be able to help.
I have 8-10 of the BMS used 3-4 years ago, none match.

Doesn't sound good then?

I was hoping to find replacements that at least exceed the quality of this BMS but i suppose thats always going to be a gamble.

Looking on Ebay, it seems as though most BMS's sold are just the same as ones on AliExpress, of which i'd guess their quality varies dramatically.

Adding the fact that i have specific requirements (switched, 30a, 48v, seperate c/d ports, strict space restrictions etc) it just limits my options more.

The closest i've been able to find is this but it still likely wont fit with it's heatsink:

https://www.aliexpress.com/item/1005003362901037.html?spm=a2g0o.9042311.0.0.27d34c4dEXH0nL

Should i pull the trigger? Even once i get a new BMS i'm gunna have the grow the balls to actually desolder/solder this thing, lol.


Anywhoo, again tremendous thank-you's for all the help.

 

[Chalo]

Adding the fact that i have specific requirements (switched, 30a, 48v, seperate c/d ports, strict space restrictions etc) it just limits my options more.

Those aren't requirements unless you require them for some reason.

Me, I'd use a cheap common BMS that I could replace later without drama, and use whatever features it has rather than holding out for the same features as my old failed BMS.

This one is small. I've installed a bunch of them:
https://www.ebay.com/itm/324632188628

 

[Watchamacallit]

Those aren't requirements unless you require them for some reason.

Nope, it really is a requirement lol, hence me saying so.

My setup is very specific and space is of a premium so they're all hard requirements without giving myself a greater headache. The bicycle it's all powering is a highly custom build. Everything else i've managed to modify and change but i never had to delve into modifying or altering the battery itself in any way (And im extremely hesitent to do so) but this now is going to force my hand.

Me, I'd use a cheap common BMS that I could replace later without drama, and use whatever features it has rather than holding out for the same features as my old failed BMS.

This one is small. I've installed a bunch of them:
https://www.ebay.com/itm/324632188628

How often do you have to change BMS's using the cheaper units?

Although i sort of agree with your logic particularly if the "cheaper" bms's are all the market has to offer (i dont know, as i'm just getting my feet wet in this) but my experience with building other things tells me buying cheap you just buy twice.

For such a crucial (And highly dangerous) Component like a powerful battery, i'd rather spend triple or greater for a solid well made BMS then take unecessary gambles with cheaper BMS's or just overall waste time and money replacing them because of repeated failures.

But thats just me, if all the market has to offer is cheap BMS's then i obviously dont have this luxury - it is after all why i'm here, to learn .

 

[amberwolf]

By both directions i assume you mean reversing the multimeter cables on the drain/source?

In which case i get the same result.

THen the FET appears shorted, and could not be used to control charge.

I was hoping to find replacements that at least exceed the quality of this BMS but i suppose thats always going to be a gamble.

My guess is the BMS is of typical quality; the specific failure you got is more likely to be caused partly by the placement of the BMS inside the heatshrink, inside the plastic case. It could also be because it was then used while under something that kept in what heat had been able to get out before; not possible to tell for sure without repeating the conditions with the other BMS (which will damage it if that is what caused it).

You may find better mechanical designs for them that let heat exit better relative to the placement within your battery case, but the heat still has to go somewhere.

You can get ones that are sized for much higher charging currents than you will use, so that the heat they generate is less (lower resistance on the charge FETs), but these will be generally larger, so if space is already tight they may not fit. Or change the FET(s) used for ones better suited to the heat, and that make less of it with lower RDSon, etc.

https://www.aliexpress.com/item/1005003362901037.html?spm=a2g0o.9042311.0.0.27d34c4dEXH0nL

Should i pull the trigger? Even once i get a new BMS i'm gunna have the grow the balls to actually desolder/solder this thing, lol.

It does state it has two FETs on the charge port, which halves the current either sees, and lowers the internal temperature generated by each...but it still is a 5A charge limit, so my guess is the total RDSon of the two FETs is similar to that of the single FET on the typical BMS (and probably like yours). Unless they state which FETs they use, you cant' know without testing.

I don't see any specs for cell HVC or LVC, balancing capabilities (including how much current they can shunt during balancing; typically in the 50mA range, and also what cell voltage they actually shut charging off at, and what voltage they resume charging at), or cutoff temperatures, so you may want to ask them about those, if it's important to your system.

They also only list Maxes for charge/discharge current, not continous ratings. They might mean continuous when they say max, but that's not typically what max means unless it is stated as "max continous".


Unfortunately getting complete and accurate specs clearly stated is difficult; most of the sale pages appear to be written by marketers or other non-technical people, rather than those that designed them or know much of anything about what is important to know, so I can't say which places would be any better. It's made harder by the often poor translations, since technical stuff doesn't translate directly very well.

There are some brandnames that pop up more often in threads for BMSs over the years, like BesTech, http://www.bestechpower.com/ , and a couple different ones (at least) in the Bluetooth BMS thread(s) that I can't recall ATM.

I don't know which ones are better than others, though.

 

[Chalo]

How often do you have to change BMS's using the cheaper units?

Rarely. Usually it's a clear case of either overheating or short circuiting the battery leads. Almost always, the ones I'm replacing are cheap OEM BMSes rather than cheap replacement BMSes.

In my experience, "nice" BMSes have added features (like Bluetooth, onboard displays, programmability, etc.) rather than better components or construction.

 

[Watchamacallit]

How often do you have to change BMS's using the cheaper units?

Rarely. Usually it's a clear case of either overheating or short circuiting the battery leads. Almost always, the ones I'm replacing are cheap OEM BMSes rather than cheap replacement BMSes.

In my experience, "nice" BMSes have added features (like Bluetooth, onboard displays, programmability, etc.) rather than better components or construction.

Ah gotcha, thanks for the reply


Being curious (and trying to grow more comfortable with the battery) i decided to measure the balance lead voltages, which gave me 3.5 to 3.4v variance increases going up each bank

Is this a good sign? Would I be right in thinking this shows that the battery cells are at least pretty well balanced?

 

[amberwolf]

Being curious (and trying to grow more comfortable with the battery) i decided to measure the balance lead voltages, which gave me 3.5 to 3.4v variance increases going up each bank

Is this a good sign? Would I be right in thinking this shows that the battery cells are at least pretty well balanced?

If it's off by as much as a tenth of a volt per cell, then not really, even as far from full as that is.

That is what you see in an aged pack, or one with too high a load for the cells it has, or poorly matched cells (so that each group averages the same capacity and internal resistance as all the others; preferably the cells themselves do as well), or for packs made of non-genuine and/or cheap cells, if the pack is new(ish) and already that imbalanced.

Typically they shoudl read the same within a few hundredths of a volt or less--the closer, the better.

I'm assuming by "variance increases" you mean the actual cell group voltages, as measured between adjoining pairs of balance wires? If not, let us know.

Most likley there's nothing wrong, but I'd recheck those making sure hte meter is set to 20VDC (5vDC if it has that range but unlikely).

If you see a pattern to the difference between cell voltages (what I would expect variance to mean in this context ), like just the most negative few groups are all the same lower-voltage, but all the more positive groups are all the same higher voltage, then it might mean the BMS is powered only by the lower ones (some are, some are powered by the whole pack); this kind of thing happens when a pack sits for a long time unused / uncharged.

Either way, normally a BMS will keep a good well-built pack balanced well, as long as the cells aren't being discharged at a higher rate than is healthy for them, since they'll all be equal in response to the load. At higher rates than they're really meant for, the small differences, even in good "matched" cells, start to show, and over time the imbalance can grow, if the packs aren't left on the charger long enough for the BMS to fix this (assuming the typical resistive drain-down-high-cells type of BMS). How long that takes depends on the imbalance, and the amount of current the BMS balancers can handle (50mA ish probably typical for this size of pack). For a really bad imbalance, it could take days, or even weeks, but that's usually for poorly built packs, really old ones, or ones with failing cells/groups, or ones being severely abused.

Since it sounds like your BMS isn't keeping them balanced, there are a few possible things:
--BMS doesnt' have balancers.
--BMS defective (not likely, since most of these resistive balancers just work, or they fail shorted on which drains just the group attached to that balancer, continuously, whether pack is on, off, or even attached or not to the bike. This doesn't always take long; assuming full 50mA constant drain, that's 50mAh per hour, so one day is 1.2Ah (.05 * 24), which is a pretty large imbalance. It also means the pack would be shutting down a lot sooner than expected, getting worse all the time; you'd notice this kind of problem pretty quick.
--BMS defective differently: if it's capacitive charge transfer type instead (less common), the transfer mechanism could've failed, and all balancing would cease (most likely). But in this case, it would still require a cell-level problem that crreates the imbalance in the first place, and continues to make it worse over time.
--Cell level problem; mismatched cells discharging differently from each other in different groups. Damaged cells, same. Overloaded cells, same.
--cell interconnect problem; some cells in parallel groups not connected, or high resistance connection, prevents those cells from participating in charging/discharging and rest of group is less capable. You'd notice this one, too, becuase it takes out however much capacity is in each "lost" cell from total pack capacity.

 

[docw009]

Hate to butt into a good discussion, but I looked at the aliexpress BMS listing. There is no specification for balance current, and I will bet that BMS doesn't balance. I've been tripped more than once by that. It might say "balance BMS" in the description or on a label, but if there is no spec, it won't balance.

I'll also bet that the BMS in your Hailong didn't balance either. If they were pressed for room, and always pressed for price, they leave it out.

 

[docw009]

Here's a BMS with on/off switch, thermister, and 40A capability, plus balance. Note how your first one had no specs at all, while this one includes a list.

https://www.aliexpress.com/item/1005002742479483.html?spm=a2g0o.productlist.0.0.24dd5cf5qhFXPk&algo_pvid=48ff589f-652b-483b-8c93-52f64ceb3971&algo_exp_id=48ff589f-652b-483b-8c93-52f64ceb3971-0&pdp_ext_f=%7B%22sku_id%22%3A%2212000021936167107%22%7D&pdp_pi=-1%3B19.5%3B-1%3B-1%40salePrice%3BUSD%3Bsearch-mainSearch

 

[Watchamacallit]

I'm assuming by "variance increases" you mean the actual cell group voltages, as measured between adjoining pairs of balance wires? If not, let us know.

Yes thats correct. I've put the multimeter to 20 vdc and i'm seeing anywhere between 0.00-0.02 difference between each pair.

Thanks again for all the infomation, i'll be coming back and forth from this thread for quite some time to absorb all of it.

Here's a BMS with on/off switch, thermister, and 40A capability, plus balance. Note how your first one had no specs at all, while this one includes a list.

https://www.aliexpress.com/item/1005002742479483.html?spm=a2g0o.productlist.0.0.24dd5cf5qhFXPk&algo_pvid=48ff589f-652b-483b-8c93-52f64ceb3971&algo_exp_id=48ff589f-652b-483b-8c93-52f64ceb3971-0&pdp_ext_f=%7B%22sku_id%22%3A%2212000021936167107%22%7D&pdp_pi=-1%3B19.5%3B-1%3B-1%40salePrice%3BUSD%3Bsearch-mainSearch

awesome. Thanks for this. I've spent hours so far looking for a BMS and this looks ideal.

Curious but why does it state in the title "constant current" ? Is this just reffering to CC/CV charging?

 

[tomjasz]

All do respect to the brain trust, but better BMS have me in better battery life. Each to your own, but watching hundreds if not more battery sales I saw LOTS of issues. Anyone remember having to “flash” Luna packs with the same budget BMS UPP used!

I’ve messed with a few...

EM3ev and others stopped selling replacement when clueless DIY repairers toasted their BMS purchases and then claimed warranties. I toasted a few in a dry static charged basement shop. Until I finally learned...

 

[tomjasz]

Hate to butt into a good discussion, but I looked at the aliexpress BMS listing. There is no specification for balance current, and I will bet that BMS doesn't balance. I've been tripped more than once by that. It might say "balance BMS" in the description or on a label, but if there is no spec, it won't balance.

I'll also bet that the BMS in your Hailong didn't balance either. If they were pressed for room, and always pressed for price, they leave it out.

Eggs Akley!

 

[amberwolf]

Yes thats correct. I've put the multimeter to 20 vdc and i'm seeing anywhere between 0.00-0.02 difference between each pair.

Just to be sure, this is a different set of measurements than previously done?

I'm checking, because the results are definitely different than 3.4-3.5v per cell, and it makes a difference to whether the cells are healthy / balanced or not.

Only a couple of hundredths of a volt difference, that is pretty well balanced for a typical ebike pack (especially if there aren't any balancers on the BMS).

A tenth of a volt, however....

 

[amberwolf]

Curious but why does it state in the title "constant current" ? Is this just reffering to CC/CV charging?

If you mean the " 40A constant discharge current " in the item name at the top right, then that matches this spec further down (and also in the attached chart)

Brand Name: LiRooter
Certification: CE
Accessory Type: Battery Accessories
Origin: CN(Origin)
Model Number: Lirooter14S001
Application: for 13S or 14S 48V Lithium Ion Battery Protection
Function: with full set of protection
Dimension: 90*57*9mm
Constant discharge current: 40A
Constant charge current: 10A

Which would mean it can handle a continuous load of 40A pulled from the battery by the controller/motor system, if necessary. That's good in that it means it probably won't heat up as much under the smaller load you'll use it under (the actual limiting is done by your controller).

Similarly, it can handle as much as 10A continously for charging, so it should also run cooler there, too.

But the best part of their page is that they give you complete specs of how the BMS works; all it's limits and shutdown conditions. Makes it easier to troubleshoot problems, and more predictable behavior (because if the system shuts off when none of these limits are in play, something else is wrong).