Need advice on BMS24

[jusp]

Hi,

I am constructing a 16s 48V lifepo battery bank for powering a 10kW electromotor on my sailboat. The bank is currently in a test setup at home, please see attached pictures. Each of the 16 's' consists of 4 cells in parallel, so 64 individual cells total. I am (currently) charging with a 48V charger for lead acid golf chart batteries, charging voltage up to 58V, 15A max, which should be a trickle charge for the 400Ah bank.

I am using a Chargery BMS24T as controller/balancer. I am fairly new to lithium batteries and everything around them, and do not want to risk burning down my house (or worse, my boat). So I could use some advice.

0) The BMS24 seems to support LiFe and LiPo. I have the impression that it autodetects, because I am unable to select the type, and it starts up with LiPo when connected to my batteries. Neither of these two is LiFePo, but the selection should only affect initial configuration values which I will be able to change anyway, and for selecting a charge profile that is used for displaying SOC. Correct?

1) The meager data sheet for my batteries lists a maximum charging voltage of 3.65V, but I am unable to set the overcharge P voltage of the controller lower than 3.9V. Reading information on LiFePo charging curves on the internet I have the impression that this is not a problem, and that I can still safely charge up to 4.2V per cell. That would mean that the datasheet is a bit overprotective. Am I correct in assuming this?

2) Although I have the battery more than 50% charged already, the BMS insists that the SOC is still 0%. As a result, the discharge relay is kept open, although the charging relay works fine. The BMS also refuses to be powered by the batteries themselves: power selector to Bat keeps the BMS dead, so I currently have the selector to Ext and am powering it with an external power supply. However, this will be impractical when the bank is rigged on my boat. This inability to power from the battery could be related to the fact that the SOC thinks that the battery is empty. What am I doing wrong here?

Thanks for any input,

Juul

 

[john61ct]

selecting a charge profile that is used for displaying SOC

Those two are not really related.

I would get a dedicated Battery Monitor for SoC guesstimation if you want reasonable accuracy, but set that aside for now, get everything working for now just using at-rest voltage, as read by a confirmed accurate DMM, ideally regularly calibrated.

No auto-configuration afaik, you need to set the adjustable parameters using the display unit, as per the manual.

LFP is LiFePO4

"LiFe" is older term from tiny RC hobby cells

LiPo completely different, higher voltages.

Please link to the maker's or purchase URL for your cells.

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> The meager data sheet for my batteries lists a maximum charging voltage of 3.65V, but I am unable to set the overcharge P voltage of the controller lower than 3.9V. Reading information on LiFePo charging curves on the internet I have the impression that this is not a problem, and that I can still safely charge up to 4.2V per cell. That would mean that the datasheet is a bit overprotective. Am I correct in assuming this?

No, you are really likely going to start a fire messing around with such stupid ideas!

The slower the charger (lower current), the easier it is to overcharge, if not in danger of starting a fire, certainly reducing cycle lifespan.

3.45Vpc or 55V is the highest I would go, do not hold CV at all, just stop when reaching that voltage.

Do not rely on the BMS for charge control HVC, nor LVC when discharging, use it as a last-ditch failsafe for when your primary hardware fails.

Stop discharging long before the first cell hits 3.0Vpc, but actually depends on the C-rate.

______
Sorry I can't help specifically with that BMS, @jonyjoe303 hopefully will show up within a few days.

Meantime, have you contacted the support email listed in the manual?

Do some serious googling too, not just YT videos.

 

[jonyjoe303]

On the display unit, unplug the data cable, that will restart the unit. When it does pick "life" for your system. I have the bms8 which is similar to the bms16 and bms24 as far as programming. I just unplug my data cable and as soon as I plugged it back up, it restarted and gave me the choice to pick my battery type.
As far as the SOC, I don't even use it. In 2 years I have had my chargery it has been unreliable. I use a seperate coulombmeter to read SOC. But it should record amps going into the battery and show something. When I first got the chargery. I mis-wired the shunt and it wasn't recording the amps. That might be something to check. When charging is the display showing amps going into the battery?
In the past 2 years I have only use one relay for the charging and didn't bother getting a relay for the discharge protection. I never let my battery get too low and just rely on the alarms to warn me of a low battery.

 

[jusp]

Thanks for both your answers. I bought the batteries from EvLithium in China. They shipped, but play dummy for requests on technical information or for a proper charger and BMS. I posted a request on a forum instead of contacting Chargery for their BMS because I had bad hopes on a response. Companies seem eager to sell, but less so on providing info. And the BMS24 manual is pretty cursory. I could not even find specs on the relays, I had measure the relay wires and guess myself what I needed.

Lacking reliable information I have charged the batteries until now by staying right next to them watching the maximum cell voltage and unplugging the charger when this reached 3.65V, trusting the data sheet. I have done a few bursts this way,l but according to john61ct I should have played safer. Thanks The balancer seems to work (after enabling it). Data cable unplugging did not fix the SOC indicator, although having the BMS powered by battery suddenly worked. The SOC is not really worrying me, but I do want a safe charging system. I have not yet wired the shunt because my current problems were more related to configuration. That is, I have hooked up the probes to the shunt because it could be that the BMS was thrown off by not detecting it. I did not wire the charging cables to it. But my battery charger itself shows the amps, 15A, it cannot do more.

On the relays, my motor controller checks the battery voltage and shuts down when this drops too low, although this monitors the entire bank, not individual cells, so I think I need that fixed. The charging relay seems to work, but I cannot set the voltage parameters to anything safe.

> Do not rely on the BMS for charge control HVC, nor LVC when discharging, use it as a last-ditch
> failsafe for when your primary hardware fails.
So this would mean that I should rely on a proper LiFePo charger, instead of the setting BMS parameters (provided that I can configure them correctly) and use the relays?

> Do some serious googling too, not just YT videos.
I agree, that is what I tried, but Googling still gives pretty confusing info.

So based on your answers I should search for a reliable, matching BMS for my batteries, and for a reliable 48V LiFePo charger. 15 Amps is as far as I want to go here, do not want to risk overloading the electrical wiring of the boat, and a charge time of 24 hours or so is fine for me. Do you know a reliable supplier? Googling around shows mainly components for small systems.

 

[john61ct]

Many if not most vendors have no clue about what they sell, just distribution in & out.

From China the tech side is often outsourced, rarely available to the sales staff, hardly anyone speaks English, and asking Qs loses face, you and your boss.

Can't trust spec sheets need to test yourself.

Or pay lots more for Western sourced products.

I have yet to see a LFP specific charger that isn't hard-wired too high.

All you need is a charger that is adjustable, so you can calibrate it to stop at the point you want.

Or an adjustable HVC wired to cut off the charger's source, could then be a dumb DCDC converter or PSU.

With BMS then acting as backup failsafe for when primary fails.

LVC or HVC based on bank voltage is fine, just stay on top of balancing and use conservative settings, e.g. 3.45 for 100% and 3.05Vpc for 0%

A good quality set of matched cells in a low C-rate use case may only need rebalancing every few years.

 

[john61ct]

When you find you do need faster charging, anywhere up to 150A is perfectly fine as far as the cell longevity is concerned.

If your charge termination is just voltage based HVC thus CC stage only, and 15A being dangerously low 0.04C

Best to set the cutoff a bit lower, say 3.42Vpc

If you did use say 50+A then 3.50 would be fine.

For a precise definition of 100% SoC, say for capacity benchmark testing

Start out above 0.1C, CV setpoint of 3.45Vpc and hold Absorb stage until current drops to 0.05C, in your case 20A.

 

[jonyjoe303]

Without the shunt connected, the SOC will never work. The shunt is what lets the bms count the amps going in and out of the battery. What happened when you disconnected the data cable? on my chargery bms8, it resets the SOC to zero and it also reboots the bms and lets you choose the battery type. Lifepo4 or li-ion. When you wire up the shunt, make sure and follow the diagram in the chargery manual. If you connect it wrong it won't give you good readings, but it won't damage anything.
The chargery bms is a very good bms, I've had mine for 2 years and has been extremely reliable. Once you set it up the first time, you never have to mess with it again, it just works. I don't think you will find a better bms that can do what the chargery does. As far as relays, I been using the cheap 4 dollar 30 amp automotive relays, it works good on my system.
Also make sure the temp sensors tips (made of metal) don't touch the battery terminals, it will short out the circuit board. Maybe wrap them in electrical tape for extra safety.
If you need more assistance with chargery the diysolarforum has alot of users there that use the chargery bms. Maybe somewhere there encountered the same problem of not being able to choose the battery type. It might be a simple fix thats not on the manual.

 

[jusp]

Ok, that all is useful information that I can start working with. It seems I can keep using my existing power supply, but add an HVC, and maybe an LVC, and rely on balanced cells. I could use my BMS24 for balancing and keeping an eye on the individual cell voltages, because it seems to do that well, but I will not be able to use it as a backup HVC/LVC, because it I am unable to set the proper cutoff voltages on it.
One question: john61ct hinted a few times on that a low-C charging current (15A in my case) may be dangerous. Why is that exactly?

On the shunt connection: having to connect the shunt for proper SOC readout makes no sense to me, but it appeared to be the solution. Connecting it made the SOC meter come alive. That means that the meter does not work when the battery is connected to an open circuit? For example my electro motor switched off and charger disconnected when sailing. I assume that SOC percentage is based on the measured voltage? I will play a bit more with the BMS24 now the shunt is connected, to see what else has changed. And thanks for your confidence statement on the Chargery BMS, I started to feel a bit uncomfortable about it. I bought my relays at Digikey, based on what I measured I needed a coil voltage of 12V. Were a bit more pricy https://www.digikey.com/en/products/detail/te-connectivity-potter-brumfield-relays/K10P-11D15-12/254541.
This one is a bit on the edge for my 15A power supply, but I risked it for my experiments.

I will have to stall this project for a week or so, have other things to attend to, but in the mean time I will send some info request to Chargery, hopefully they will respond. Will report later what happened.

 

[john61ct]

Such a low charge rate is not dangerous on its own

but depends on the stop-charge algorithm you use.

Go back and read what I wrote, see if you can write it out in your own words

 

[jusp]

> The slower the charger (lower current), the easier it is to overcharge, if not in danger of starting a fire, certainly reducing cycle lifespan.

It was this that I could not place. I would expect that a lower current would raise the voltage more slowly, giving more time for the HVC to react (if reaction speed is even an issue).

 

[jusp]

> I assume that SOC percentage is based on the measured voltage?

The voltage of a certain SOC is of course also dependent on the current. I had assumed that not connecting the shunt would let the BMS think that the current was zero, which would be fine for my test setup, but because SOC display needs both voltage and current I can imagine that the BMS tries to detect the presence of the shunt, for which it has to be connected to the battery, not only to the probe wires. So that issue seems explained.

 

[jonyjoe303]

for lifepo4 SOC is based more on amps then on voltage. Lifepo4 spend the majority of its time resting at 13.1 volts (between 10 and 90 percent) thats why you can't rely on voltage for SOC, you need a meter that can count the amps going in and out of the battery. By adding and subtracting amps it gives you the SOC. Voltage only gives you the high 14.6 volts (full) and the low 12 volts(empty).
The chargery will run fine without the shunt, it will still measure the cell voltages and pull the relays if anything abnormal happens. All the shunt does is count amps in/out of battery.

Lower charge current is actually better to charge your battery. Like you mention the voltage goes up more slowly resulting in getting a full battery. Charging at high amps usually results in 1 cell reaching 3.65 volts before the others which stops the battery from getting fully charged. If you have a good charger and the bms is setup right, the risk of overcharging your battery at low amps is next to impossible. A good cc/cv charger will always reduce the amps the closer the battery gets to the max voltage.
For lifepo4 3.65 per cell its the highest you want to go, they can handle being overcharged better then li-ion, but it reduces there lifespan.

This is the meter I use to measure the SOC on my 220ah lifepo4. The tk15 (cost about 30 dollars) can handle up to 50a. Its alot better then the chargery SOC.

 

[BlueSeas]

Some of my thoughts reiterate what others have already said.

The shunt on the BMS provides the sensing for the number of Amps going in and out of your battery bank. This is the only way presently to estimate SOC on LFP batteries. Why...during discharge in the range of about 20% to 85%, it's not possible to determine SOC on voltage since the discharge curve is flat. The shunt allows measurement of current flow, amps in and amps out. So if you specify the capacity of your bank, it can provide a reasonably accurate SOC.

I'd try to get that working to your satisfaction. If it isn't accurate enough, the Victron BMV 700 series battery monitors are the best in the business for this. Not even that pricy, under $200 for a panel meter and a little over $200 with a Bluetooth interface for your smart phone.

In previous installations for customers, we try to have 3 levels and a override at the Nav station. First level the charger should be smart enough to terminate charge at the point you want, usually CCCV to 3.5V and C/20. For a 500 Ah bank, that's typically 3.5V and 25A. Second, if the charger overruns, usually just voltage, have an audible alarm, perhaps visual as well. This gets you involved to decide what to do before finally, Stage 3, the BMS shuts down the bank. Finally provide an override for the operator to bypass all protection. Not normally recommended on charge. It's the same scenario on discharge, but the override more important. Imagine navigating a challenging inlet in poor conditions. You probably don't want to lose propulsion at the whim of the BMS. Or in most of my installs, the navigation system. Priorities....

You don't have to have all this...but that puts the requirement on you to perform these tasks with whatever instrumentation you have available.

Good luck! And smooth sailing!

 

[jusp]

Solid information, I am glad I started this post I now understand how the SOC determination works, I was under the impression that it was based on some high precision voltage measurements. So current counting probably means that I should never depower the BMS.

In the mean time... I fried my BMS. Don't ask, from now on I should take better care of preventing people from walking in and starting to talk while I am busy with a piece of high energy. Anyway, the good thing that came out of all this is that I detected that I was working with a BMS manual that was waaay out of date. The one I had was a 14 page, undated EvLithium- branded version 1.05 of the Chargery manual. But when I reordered the BMS replacement from a different vendor I got version 4.2, extended to 66 pages, dated Nov 25, 2020, and edited and reviewed by Steve_S from dysolarforum. Haven't read it yet, but the English appears to have cleaned up and it seems promising. Problem is, I have to wait a while for the replacement to arrive. But gives me time to read some more, and let your responses sink in.

 

[john61ct]

Voltage while in use is hopeless at SoC% guesstimation.

At rest isolated for an hour is better but very very rough, needs a calibrated table, and that tale changes for each cell model and as the pack ages.

Counting coulombs is a bit better, but I doubt any chea BMS does it with accuracy better tgan say 10-15%.

_______

giving more time for the HVC to react (if reaction speed is even an issue).

No it is not.

Accurate charge termination requires both voltage and trailing current to be specified.

Your rate is lower than the lowest endAmps spec I would use, usually 0.05C.

For normal usage charge cycles, CC-only voltage-based is fine.

However the lower the current, the higher the SoC% is for a given voltage

in your case using 0.04C letting the voltage go past 3.40V would be overcharging, in the sense of harming longevity.

At 0.4C even a 3.55V HVC would result in a lower (better) capacity utilization.

 

[jusp]

Aha, ok. I will keep it low for now