Noob Question re BMS

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bobbill   100 W

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Noob Question re BMS

Post by bobbill » Jul 19 2021 12:31pm

I sorta know what it should do....does it function when charging, all the time or only discharging???

I figure if hooked up is working to balance cells all the time.

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Re: Noob Question re BMS

Post by john61ct » Jul 19 2021 1:33pm

A BMS is just a collection of functionalities to protect a battery

and differ widely between various instances and price ranges.

LVC is to stop discharging before voltage gets so low as to cause damage

HVC is to stop voltage going too high while charging

Either or both can be based off the lowest/highest **cell level** voltage respectively

usually both.

Good BMS let you both see the voltage values and adjust the setpoints.

Some just open/close external contactors (relays, solenoids), most have internal MOSFETs, and thus charge more to handle high currents.

Very few actually limit current, usually just shut down like a breaker, some burn like a fuse.

Some also have temperature sensor input.

Above are "protective BMS" functions.

Balancing can be done in many ways, few BMS do it well.

Start voltage, delta for stopping, and balance current are the key ratings, and again better ones make those adjustable.

Balance chargers, dedicated active balancers and manual balancing are other alternatives.

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Re: Noob Question re BMS

Post by E-HP » Jul 19 2021 9:11pm

bobbill wrote:
Jul 19 2021 12:31pm
I sorta know what it should do....does it function when charging, all the time or only discharging???

I figure if hooked up is working to balance cells all the time.
Assuming a run of the mill BMS, it will function when charging, by stopping the charger when any group reaches 4.2V, assuming lithium ion cells. For discharging, it will function by cutting off the battery if it load exceeds the continuous current rating, or exceeds the peak current that it's set to.

Only a pretty expensive BMS will actively balance the pack at anything less than full charge, since they siphon off the voltage from the highest charged group, and then allow the charger to resume charging, until the next highest group reaches 4.2V (then repeats). A better BMS may charge the lowest group, while siphoning off from the highest group. Faster, but the balancing current is pretty low, so still takes forever.

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Re: Noob Question re BMS

Post by bobbill » Jul 20 2021 4:47am

John and HP thank-you!

I have some 30 cells. Cells vary in voltage from 6.1v to 6.9v, in 5 groups, which I have segregated to limit BMS' smoothing. Am about to assemble to form a 36V pack, doing parallel first, then series, but not connecting BMS, until later.

Was just curious if BMS will do its work and when,,,appears it functions all the time. Which makes sense now that ai think of it - thanks again.

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Re: Noob Question re BMS

Post by john61ct » Jul 20 2021 9:11am

You need to at least roughly balance the cells before putting them in parallel, say under 0.1V delta, at which point they will start to naturally equalize.

Then those groups need to be equalized with each other, in effect one big group

so they are each exactly the same voltage before assembling the pack.

The BMS protective functions are active all the time.

IF you choose to use a cheap BMS for balancing,

it only happens after voltage is above the start-balance setpoint

and stops once the delta is reduced to that setpoint.

As mentioned you want to be able to observe the per-group voltages, anytime really,

but in this case, so you do not hold the charging current at such high-SoC any longer than necessary.

With too low a balancing current, when the delta is high, it can take many hours, even days to complete balancing.

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Re: Noob Question re BMS

Post by TNC » Jul 20 2021 10:00am

Being somewhat new to the ebike battery arena, I'm still surprised that most batteries don't come with a detailed note about the specific BMS in each pack. You know...kind of a data sheet showing the exact functions of the onboard BMS. There is a thread on the forum here where it did show a BMS that you can purchase for your own build, and it did show great detail on the capability of that BMS. I realize there are some generalities in many/most BMS units, like overcharge and short protection. However, it would be good to know if there is a functional balance component in the BMS and how it functions to achieve balance. Maybe I'm overthinking it, but I believe this would be helpful for those of us who want to know information like this as it would provide better knowledge for our charging habits and maybe better selection of aftermarket chargers. One may not need a manual the size of "War and Peace", but a little detail might be helpful.

A poster on another site mentioned that you can probably know if you have a balance capable BMS in your pack if you have a charger that has a display for charging amps. I have the Luna Advanced Charger that has volts and amps on the display. It also has that capability of charging at 80-90-100 percent selection along with an amp selection. I was told that charging at 100%, you could keep tabs on the amp display, and if the amperage dropped and fluctuated to some degree as max voltage was reached, you could pretty well count on the BMS having a balance funtion. I notice when I charge at my more common 80% level, the amperage never changes all the way to full 80% charge when the charger turns off. If I understand correctly, the balance function only operates at 100% charge, and the amp drop and fluctuation one sees on an amp display indicates a balance function in operation. So I guess I can assume my pack is balancing due to that observation. I'm learning here, not teaching...LOL!

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Re: Noob Question re BMS

Post by john61ct » Jul 20 2021 3:15pm

Usually there is no one who both speaks English and also has a clue technically involved in the supply chain.

The outfit selling the goods often understand nothing about what they are selling.

Everything driven by cutting costs, swapping parts middle of a production run, no documentation at all available for anything.

Get a BMS that displays your per-cell voltages.

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Re: Noob Question re BMS

Post by E-HP » Jul 20 2021 6:34pm

john61ct wrote:
Jul 20 2021 3:15pm
Usually there is no one who both speaks English and also has a clue technically involved in the supply chain.
If they hired a person like that, they'd probably double their labor costs, since the person would be making 5 time what the non-English speaking minimum wage laborers assembling the packs make.

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Re: Noob Question re BMS

Post by bobbill » Jul 22 2021 5:59am

Ex observations. Thing is if one asks ?, one gets an answer, so someone is privy. I have one ready made pack, that is so small, BMS might not be...

But, it is why some of us ask ?s.

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bobbill   100 W

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Re: Noob Question re BMS

Post by bobbill » Jul 22 2021 6:01am

John, cell cannot be off a tenth or three??? Tenth of a volt is not much to balance, seems.

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Re: Noob Question re BMS

Post by TNC » Jul 22 2021 4:31pm

john61ct wrote:
Jul 20 2021 3:15pm
Usually there is no one who both speaks English and also has a clue technically involved in the supply chain.

The outfit selling the goods often understand nothing about what they are selling.

Everything driven by cutting costs, swapping parts middle of a production run, no documentation at all available for anything.

Get a BMS that displays your per-cell voltages.
I got my battery pack from Bicycle Motor Works. The guy who owns that company...they supply gas conversions as well as batteries for ebikes...builds his own packs inhouse. They seem to be of very good quality, and he offers several options. I do the backpack setup for off roading and got a 52V/17.5AH version that has been giving very good service. He describes the BMS as being a smart BMS with various functions...one being a balance capability. But no data sheet is supplied.

BMW also sells some individual components as evidenced by this BMS that is listed with quite a bit of detail. Not sure if it's the one in my pack or not.

https://www.ebay.com/itm/52V-14S-45A-Li ... 3799457857

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Re: Noob Question re BMS

Post by john61ct » Jul 22 2021 10:32pm

bobbill wrote:John, cell cannot be off a tenth or three??? Tenth of a volt is not much to balance, seems.
Cells should all be exactly the same before putting in series.

a delta of 0.1V can take a very long time for a cheap BMS or balancer to eliminate

given the usual pitifully low balance current rates.

Thus their requiring holding a stressful, higher than normal CV setpoint the whole time,

shortening cell lifespan

is a pretty stupid design, again only the norm to keep costs low.

If a 100mV delta shows up in normal use, something is wrong

I usually reckon a balance is due when delta grows to 50mV, assuming balancing gear is readily available each charging session

but maybe use a greater tolerance when maintenance sessions are less frequent.


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Re: Noob Question re BMS

Post by john61ct » Jul 22 2021 10:41pm


TNC wrote:BMW also sells some individual components as evidenced by this BMS that is listed with quite a bit of detail. Not sure if it's the one in my pack or not.

https://www.ebay.com/itm/52V-14S-45A-Li ... 3799457857
Fine if you trust him and he supports the product.

Fact that the description is left in barely comprehensive Chinglish. . .

And you'd need something else to monitor the cell-group voltages, determine when balancing is complete

setpoints adjustable?



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E-HP   100 MW

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Re: Noob Question re BMS

Post by E-HP » Jul 22 2021 11:26pm

bobbill wrote:
Jul 20 2021 4:47am
I have some 30 cells. Cells vary in voltage from 6.1v to 6.9v
What kind of cells?

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Re: Noob Question re BMS

Post by TNC » Jul 23 2021 12:36am

john61ct wrote:
Jul 22 2021 10:41pm
TNC wrote:BMW also sells some individual components as evidenced by this BMS that is listed with quite a bit of detail. Not sure if it's the one in my pack or not.

https://www.ebay.com/itm/52V-14S-45A-Li ... 3799457857
Fine if you trust him and he supports the product.

Fact that the description is left in barely comprehensive Chinglish. . .

And you'd need something else to monitor the cell-group voltages, determine when balancing is complete

setpoints adjustable?
No...no adjustability by me via an app or anything else I'm aware of. As I mentioned in my first post in this thread, monitoring the gauge on the charger, it does appear to be going through a balance process toward the end of a 100% charge. I realize to actually observe the detail in each cell during and after the charging/balancing process would be ideal, is not a BMS with a balancing function adequate in general use?

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Re: Noob Question re BMS

Post by bobbill » Jul 23 2021 8:41am

Couple things: 30 30Qs and 30 Pane NPT. Separate packs.
None of my gizmo reads beyond .0 of a volt, so I sorta figure hat is normal and satisfied when cells are near each-other. Am I goofy?

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Re: Noob Question re BMS

Post by john61ct » Jul 23 2021 11:25am

You can maybe guess that balancing has started, but it is bad for cells to sit at that high voltage for long

so really, IMO it is critical to know when balancing is complete

and for that you need an accurate per-cell level volt readout.

If I used a BMS to do my balancing - and no, as I've stated most do not do it well enough - I would prefer that a BMS offered the readout, rather than having to add another device for that.

And I also would not use a BMS that did not offer adjustments of its setpoints.

But, most people do not know nor care about these issues, just accept what the pack builder installed.

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Re: Noob Question re BMS

Post by john61ct » Jul 23 2021 11:27am

bobbill wrote:
Jul 20 2021 4:47am
I have some 30 cells. Cells vary in voltage from 6.1v to 6.9v
There are no lithium cells that go that high.

Maybe those are 2S packs of LFP?


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Re: Noob Question re BMS

Post by tomjasz » Jul 23 2021 12:44pm

bobbill wrote:
Jul 23 2021 8:41am
Am I goofy?
Building out a battery instead of just buying a decent pack qualifies.

:lol:
Thanks Justin_le we're here thanks to you. All the best to the mods for their tireless work keeping it on an even keel.

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Re: Noob Question re BMS

Post by Chalo » Jul 23 2021 12:55pm

bobbill wrote:
Jul 22 2021 6:01am
John, cell cannot be off a tenth or three??? Tenth of a volt is not much to balance, seems.
As he pointed out, balancing only occurs at the top charging voltage. With most BMSes I've used or replaced, the balancing current is very small-- no more than 30 milliamps and usually less. So it takes a long time and sometimes a lot of restarting the charger to even out relatively small imbalances.

One device that can be used with or without a BMS is an "active balancer", which evens out voltage imbalances whenever they occur, and which can use much larger currents to do so.

https://www.ebay.com/itm/184869799584

This kind of board has a single function, and can't protect your battery against overcharging, overdischarging, short circuit, or anything besides voltage imbalances.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.

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bobbill   100 W

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Re: Noob Question re BMS

Post by bobbill » Jul 23 2021 1:08pm

Thanks,

The cells are 60 in number in two groups with BMS for each 36v pack of 30. The indiv, cells are 30Gs znd P-sonic INRs only and all near same voltage. give or take one-tenth of a volt.

Does it make a diff if they vary in voltage by 10ths, say from 6.6 tp 7.1, arranged in simialr groups voltage-wyz, say all 6.6 in set...for example.

I ask as it seems it seems to me, the variance should not make much diff. for BMS which balance all the time...

BTW, packs are only (10S 3 and 4P) and are in Parellel sets first, S later...

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Re: Noob Question re BMS

Post by Chalo » Jul 23 2021 1:13pm

I don't understand where 6.6 and 7.1 are coming from.

If it were my project, I'd charge all the cells to the same voltage, give them some rest to identify (and cull) any that show faster self-discharge, then assemble the pack.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.

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Re: Noob Question re BMS

Post by bobbill » Jul 23 2021 2:40pm

Chalo, Thanks...had reply but it disappeared...??? The 6. and 7. voltages were mine. Was advised to make equal and 6.5 was decent storage voltage...also advised that tenth of a volt is decent...and to set these suckers grouped together so BMS is not taxed in balance work...is all.

BWIW have Grin hub/controller klt and Justine gets it...to me who gets nada, apparently.

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Re: Noob Question re BMS

Post by Chalo » Jul 23 2021 2:59pm

bobbill wrote:
Jul 23 2021 2:40pm
Chalo, Thanks...had reply but it disappeared...??? The 6. and 7. voltages were mine. Was advised to make equal and 6.5 was decent storage voltage...
So these are Panasonic cells in 2S units? Because the voltages don't make sense for single cells.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.

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Re: Noob Question re BMS

Post by HK12K » Jul 24 2021 2:58am

30Q is a standard lithium ion battery made by Samsung with an absolute maximum safe charge voltage of 4.2 volts, 4.1v being better for longevity. I have absolutely no idea which Panasonic cell you're using, but if it is a cylindrical 18650 lithium ion cell it will likely be the same as far as safe voltage range is concerned.

In what format are you planning to connect the 30Q and Panasonic packs together? If in parallel, and with a bms that is able to prevent you from over discharging, chances are the worst you'd experience is range far less than what one would expect for the stated capacity of the pack. If a single cell in the pack fails to perform at a similar level to the others that group will quickly fall behind the rest and your ride will be over, regardless of overall pack voltage or the voltage of the rest of the cells. That is a best case scenario and assumes the bms steps in to cut power to prevent you from over discharging and damaging a cell group. If it doesn't and you drain a cell below it's rated minimum voltage it could lead to a potential fire or explosion during use or at a subsequent recharge

If however you were planning to run these packs in series you are literally playing with fire. Cells or packs connected in series should always be of closely matched characteristics. Meaning same brand and model of cell, similar internal resistance, preferably from the same production lot.

As far as balancing is concerned you need to look more closely at what you're working with and the appropriate voltages. While not a consummate professional when it comes to BMS' I can say that most of this stuff is geared towards typical lithium ion voltages. The voltages you're referring to do not have any basis in typical 1s lithium ion cell voltages whatsoever. If you attempt to charge a lithium ion cell to 6 volts it is going to go into thermal runaway and violently vent long before it reaches that point.

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