Help with Lithium system - charging issues

[Amp34]

I have a Renogy 20a DC-DC charger connected to a Battleborn 100Ah battery and a Victron 712 to monitor the system.

While we're driving around a lot, the battery does not seem to charge back up to full and stops charging soon after the car Is started. Voltage builds up to 14.4v pretty quickly, with corresponding 20A charge, then stays at 14.4v but does not draw any current. Once the vehicle is turned off (or the battery isolated from the charger) the voltage drops back down (currently to 13.00v).

As an example, today we drove 2 hours but it was only drawing a current for the first 10-20 minutes. I then stopped for an hour, turned some lights on for a few minutes and started the engine again. The battery was drawing 20A, but stopped about 30 seconds later, once voltage hit 14.4v (voltages verified across battery terminals with voltmeter).

The "final" voltage has been dropping each trip I do (done 3 so far). While I don't necessarily expect the battery to charge back to full ever time, I'd expect it to charge fully based on current usage (3-4 hour drive a day ~10-20A usage at night).

The Renogy charger is set to LiPo mode, with bulk/absorption set to 14.4v (14.40-14.45v measured) and no float. The battery settings for the 712 are based on Battleborns own suggested numbers (see image).

I also had a rather odd situation just now. Battery was at 12.98v (which is around 20% SOC?), turned the lights on and a few minutes later the 712 was showing 13.28v (verified with voltmeter). Once the engine was turned on shot up to 14.4v, then once turned off it dropped down to 12.98v again.

Does anyone have any suggestions as to what the problem is?

 

[jonyjoe101]

Without being able to check the individual cell voltage, the battery might have gone out of balance. If 1 cell is reading higher then the other 3 cells, the overall voltage would be 14.4 volts while the battery is still undercharge.

i know with my 4s lifepo4 (not a battleborn) I had trouble with it staying in balance, this resulted in it never getting a full charge. i had to get active balancers to fix the out of balance condition. And the battery always went out of balance when I was fast charging. In your case charging at 20 amps, you would reach an out of balance condition even quicker.



In the picture below it shows a 4s lifepo4 in good balance, if 1 of the cells was higher then the rest, it would reach 14.4 volts too early. Before the active balancers where installed 1 cell would always reach 3.65 volts while the 3 other cells where at 3.40 volts.

 

[01tundra]

How is the shunt wired and have you manually done a zero current calibration on it?

 

[Wheelman55]

OP. First thing state usage in amp hours and try to report voltages using the same measuring device. If it was me i would choose multi meter.

The BB 100ah. I’ve been using this same battery for some years now in my truck to run a fridge. Great battery!

A full batt will not accept any more amps once it reaches full charge, the amps do go to zero. This is normal. Pls excuse if you already know this.

With loads connected the batt will show a lower voltage. If you disconnect the loads the voltage will go back up. Make sure to check voltage with loads off.

Did you mean overnight usage is 10 to 20 amp hours? With 20 amp hours removed the BB voltage should read 13.2ish, providing loads are disconnected. It’ll show lower if the loads are running...say like a fridge running.

From the Voltage vs Capacity chart that BB gave me, 12.98v is 25/30% SOC.

Suggest you test again and measure the batt with loads off. Also...check all connections.

Best of luck!

 

[Amp34]

@jonyjoe101 Thanks, useful info. I assumed charging at 0.2C would be relatively slow, but perhaps that may be part of the problem. Checking the voltage today (after 24 hours of no charge/usage) and it's back at 13.3v (left at the 13.0v mentioned in the OP), which is 90% charged. Perhaps it took a while to equalize/balance the cells?

It's still a bit odd however. Going from 12.9v (yesterday before the drive) to 13.3v is 70Ah of charge, which is too much for the 2 hour drive, even if it charged the whole time. Something still doesn't make sense. I'm going to monitor it again this weekend and see what happens. Perhaps the initial voltage was wrong for some reason.

@01tundra Shunt is wired to the negative terminal on one side and to the charger and load on the other side. I haven't bypassed the shunt yet, may try that and monitor it by the multimeter instead for a while. That should take the 712 out of the equation. I hadn't zero calibrated it, but have now.

@Wheelman55 Thanks for the reply, voltages were measured with no load. All voltages measured with a digital volt/multimeter to cross check the reading from the 712 (both within 0.05v of each other). Voltage usually drops around 0.1v when a load is applied. And yes, 10-20Ah (I dropped the "h" accidentally). The lights were turned on to force a load and discharge to see if the battery would then accept more charge - which it did, for a very short time. How do you find the charging? Does it take a while to equalize/balance?

I guess the easiest solution would be to buy a mains charger and fully charge it, then discharge from there, but I'm reticent to buy spend a few hundred on a device I'll rarely if ever use again.

 

[ScottPC]

You might double check how the DC to DC charger is wired with respect to the shunt. I have a similar set up but with a Sterling BB1230 b2b charger. Initially is was wired with the negative from the Sterling to the negative battery terminal and not to the shunt itself so I wasn't getting an accurate current measurement in the BMV712. It also turned out there a was an updated version of the b2b charger so I replaced that at the same time. It wouldn't hurt to get a full charge on the battery (as another suggested) and sync it with the BMV 712 so you have calibrated starting point. After initial charge and once the BBs are under loads even small ones, the voltage tends to read in the 13.2 to 13.3 range.

 

[Amp34]

Just to update this. The voltage is all over the place. Perhaps I’m reading too much into it but it does seem odd.

Since the last post the voltage was sat at 13.3v as the vehicle hasn’t been moved and no load put on the battery. Vehicle was started and 20A went to the battery for about 30 seconds before the voltage went up to 14.4v and current draw stopped (according to 712). Vehicle was driven around for 20 minutes and turned off. Voltage was now 12.9v. An hour later the lights were turned on and off, and voltage went up to 13.4v (which would be 99% charged)...

So in all I have no idea what the actual charge of the battery is as it seems to vary significantly at apparent random. Is this normal?

 

[01tundra]

The voltage on our two BattleBorn's will drop quite a bit under a 15A load, but once the load is removed they will slowly creep back up to what the BB SOC/Voltage chart states.

So they definitely float back up quite a bit once load is removed, ours will do that for an hour or so usually.

Not sure if it really matters, but BB told me recently to set the discharge floor to zero.

 

[Rando]

Just a guess here (as the documentation from BB sucks and we don't actually know how they work), but you may be seeing a BMS quirk. We do know there are two paths between the battery terminals and the cells through the BMS - a charge path and a discharge path. Each of these paths has a power MOSFET in it (likely an array of smaller MOSFETS) to turn that path on/off. When it is charging up to 14.4V, it may be that the discharge path is off, so after it is charged, what you are seeing is the battery voltage through the body diode of the charge MOSFETS (so Vbattery - ~0.7V or about 12.8 - 13V for a fresh off the charger battery). Once you load it, the discharge path switches on and you are seeing the loaded battery voltage without the diode drop.

I have seen this briefly on my system, but I can also see the cell voltages so it is easy to tell what is going on.

I do remember someone mentioning a BB tear down article or video where you can see the BMS, and it appeared to be a pretty standard Chinese BMS. It would be great if someone could figure out which one it is and publish the actual specs.

 

[john61ct]

I am a very strong advocate of buying 4 bare cells from a known proven top-notch maker.

I do not care how long the guarantee is, I do not want cells made by an unknown vendor,

some unknown BMS that is not adjustable

and the status of the cells not even accessible

no idea if they are getting balanced properly

cap testing not possible to see how they are wearing

current level in and out severely crippled

and on and on.

Yes it seems "too hard" to take responsibility for caring for LFP cells yourself

but it really isn't if you just ignore all the disagreements between experts with different opinions on details that do not need to concern you unless you want to dive that deep.

The whole idea that you can just "drop in" a LFP bank and let it get taken care of by infrastructure designed for lead

is fundamentally flawed.

 

[jonyjoe101]

13.1 volts to 13.3 volts is normal for lifepo4, I have my lifepo4 for 3 years and thats the voltage I see when nothing is being used. If you see 13.3 volts with no load its good. Once you start seeing less then 13 volts with no load the battery is almost dead. With lifepo4 it goes from 13.0 volts to dead real quick thats why you need to use a coulombmeter at all times, you cannot tell SOC by voltage like li-ion. i would rely on the victron 712 to tell you the SOC since its keeping track of amps in/out of battery. Since the charger keeps shutting off after a few minutes of driving, you might actually have a full battery or as full as 14.4 volts will get you.

Though battleborn uses 14.4 volts as fully charge, in reality all lifepo4 are fully charge at 14.6 volts. They only use 14.4 volts to try and keep the bms from triggering and stopping the charge. I always go with 14.6 volts on mine for max performance.

 

[john61ct]

When you are in control,

3.33-3.35Vpc at rest is as Full as is healthy.

A 3.45Vpc charge will get you there, holding Absorb for a while to get a couple more % cap utilization is fine

but much higher voltage or holding CV stage too long is stressful, reduces longevity.

But drop-ins, with an inaccessible BMS, you are not in control, and really need to know the Start Balance voltage setpoint (usually way too high) and the balance current (usually way too low)

and ideally, a way to know whether balancing is complete.

Which you just don't have with drop-ins.