What could kill batteries in a 24V series?

As described in this post, my dad has a 24V system for his RV using 12V Litime batteries in a 2s2p configuration. A couple months ago, he had a problem where both the series got out of balance. We removed the batteries and tried balancing, but the "low" batteries from both series wouldn't take a charge, either in a passive balancing configuration or being attached directly to a Litime 12V charger. He's working on getting the two bad batteries replaced by LiTime.

In the meantime, we put the two good batteries in a series and added an active balancer. We tested the system on Thursday, running the air conditioner from the inverter, and charging the batteries using a generator and the 1600 watts of solar on his RV roof. After finishing charging, the two batteries were reading 13.62V and 13.38V (measured with the battery disconnect switch off so that there was no load or charging), but then we ran the air conditioner long enough for the shunt to record 4 aH consumed and re-measured the batteries, and they were 13.36 V and 13.38 V, so we felt like the balancer was doing its job.

However, after camping for a night (with electric hookup), they tested the batteries by disconnecting from the campground power and the inverter wouldn't run the air conditioner. When they got home, we checked and with no load/charge (using the battery disconnect switch) the batteries read 13.4V and 13.2V. BUT when the battery disconnect switch was turned on and the shunt reported a 0.4 amp draw, the battery that had previously been 13.2V dropped to 12.2 volts. Even worse, when the inverter tried to turn on (no active load on the inverter, just the inverter trying to turn on) that battery's voltage dropped below 10 volts.

So with now 3 of 4 batteries seemingly dead (all of them were purchased less than a year ago), we're trying to figure out if something in the system could be killing them? We had already checked that all the connections were tight when the first two batteries started acting up. Is there anything else we should be checking?

How often were these batteries getting fully charged? 14.2-14.6v measured on the battery terminals.

I don't have measurements, but presumably they were fully charged most of the time, because he has 1600 watts of solar on the roof (so they would be charged in storage) and had not yet boondocked very often. I double-checked, and the Victron solar controller was set for absorption charging at 28.8V and float at 27.2V (per Litime's documentation).

Could this be a BMS trip? My Two thoughts:

1) Are these batteries dead, or is the BMS preventing them from charging? I suspect the dead battery is overcharged and has tripped a BMS. Probably can discharge, but not charge.

2) BMS over amperage trip. In the day, the inverter received power from both the charge controllers and the batteries, so perhaps just barely avoided a BMS trip. At night exceeded those limits. My 15k BTU AC pulls 1400 watts from the inverter, but with DC conversion losses, 1750 watts. Start up amperage can be much more, enough to trip a 100 amp BMS. More than 100 amps surge per battery.

TomPanning said:

added an active balancer

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Sounds like most of the damage was done before adding the interbattery balancer. #TooLateNow.

I forgot to mention, these are 12V 200 aH batteries, rated for charging and discharging at 200 amps. So when he had 2s2p, he could do 400 amps at 24 volts, feeding a 3000 watt inverter. Now with only 2s1p, it's still 200 amps at 24 volts, so 4800 watts. So that all seems within bounds.

wpns said:

Sounds like most of the damage was done before adding the interbattery balancer. #TooLateNow.

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But damage from what? From not having a balancer before? They were only installed for less than 6 months, during which they weren't doing very much (the RV was in storage).

TomPanning said:

But damage from what? From not having a balancer before?

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That’s my guess. Clearly, at least half of the batteries were damaged. Maybe those batteries just aren’t any good?

From reports on this forum, Li time batteries seem to be delivered with out of balanced cells. Before connecting in series its advised to charge each at 12v. The charge voltage set below BMS protection level and applied for several hours or more to give the cells time to balance. A final charge with all batteries in parallel may be needed.
It's probable from the start the cell balance was poor and without a battery balancer to help, the batteries were not charged
equally and cell out of balance increased.
The 'bad' batteries may have entered protection, connecting to a dumb charger, power supply or 12v 'good' battery, would 'wake up' the BMS and alow charging.
From reports, it seems the recomended Li time charge volts almost always cause cell overvolts on new batteries.

mikefitz said:

From reports on this forum, Li time batteries seem to be delivered with out of balanced cells. Before connecting in series its advised to charge each at 12v. The charge voltage set below BMS protection level and applied for several hours or more to give the cells time to balance. A final charge with all batteries in parallel may be needed.
It's probable from the start the cell balance was poor and without a battery balancer to help, the batteries were not charged
equally and cell out of balance increased.
The 'bad' batteries may have entered protection, connecting to a dumb charger, power supply or 12v 'good' battery, would 'wake up' the BMS and alow charging.
From reports, it seems the recomended Li time charge volts almost always cause cell overvolts on new batteries.

Click to expand...

We’re trying this now. We have one “bad” battery hooked up to a dumb charger that is holding at 13.8 volts and a clamp meter is reporting 0.31 amps flowing. We have another bad battery (12.6 volts) connected in parallel with the good battery (13.9 volts) and the clamp is consistently reporting 0.16 amps for the past couple hours. So there is a small amount of electricity flowing. Does that seem like enough to wake up the BMS?

Oke here in the boat world we have a lot of broken battery .
Reason is that the systeem is out of balance.
If you use 24 volt all stuf have to run on 24 volt.
Some people try to use a 24 setup and connect some 12 volt on one battery.
That will use that battery and pull both battery out of balance.
Even if your both battery use a bms .
By this reason it never gone work.
I know this is little off topic.
But its good to know that if you need 12 volt on a 24 volt systeem to use a dc to dc (24v to 12 v systeem.)

Do your bms have a way to connect to a pc or by bluetooth.
Some you can set the charger and load on or of.

My bms i can set the charger function on or of by the phone.

From the LiTime manual:



Did you do this? You mentioned that you might have skipped this step in your other post.

Somebody else mentioned that the batteries might have gone into a "protect mode". Have you tried connecting a 12V load to each of them individually? Something that would draw 10A or so?

Suijkerbuijk said:

I know this is little off topic.
But its good to know that if you need 12 volt on a 24 volt systeem to use a dc to dc (24v to 12 v systeem.)

Do your bms have a way to connect to a pc or by bluetooth.

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All the 12 volt stuff is connected to a 24-12 dc-dc converter. All loads go through a distribution bus bar; no loads are directly connected to the batteries.
Unfortunately, none of these batteries have any way to connect to the BMS.

v_green57 said:

From the LiTime manual:

View attachment 224259

Did you do this? You mentioned that you might have skipped this step in your other post.

Somebody else mentioned that the batteries might have gone into a "protect mode". Have you tried connecting a 12V load to each of them individually? Something that would draw 10A or so?

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He charged them all separately overnight before installing them, but can’t remember if they got fully charged. He doesn’t think that he put them in parallel before installing them, because the multimeter was reading the same voltage on each of them (we now know that that doesn’t necessarily mean that they’re balanced).

TomPanning said:

holding at 13.8 volts and a clamp meter is reporting 0.31 amps

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This suggests that the BMS may be in protection, lower the charge volts and see if the battery takes current.
Once the BMS is in protection mode, the voltage measured may not be the actual internal volts as the BMS is 'in the way'.

If the batteries are well out of balance the BMS could terminate charge at a low voltage, well below the recomended 14.4 volts.

Idealy use a charger or power supply that can be adjusted to a lower voltage, example 13.7 to 13.9 volts, and apply that voltage for several hours or more. ( BMS protection will be indicated by a sudden drop in charge current to a very low value). The idea is to set a charge voltage just below protection level to give the cells time to balance.

TomPanning said:

none of these batteries have any way to connect to the BMS.

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It's well worth paying a premium to buy a battery with BMS, easy to see what's happening and diagnose issues it they occur.

Even is charged in parallel before the series connection, there will be issues with cell balance in each of the batteries. Using 24 volt batteries from the start is the ideal option. It's a pity Li time and other manufactures gave advice on possible issues that may/will occur with series set ups. Battleborn , at one stage, requested buyers to state if series connection was to be used, in that case batteries with near identical performance would be supplied.

mikefitz said:

This suggests that the BMS may be in protection, lower the charge volts and see if the battery takes current.
Once the BMS is in protection mode, the voltage measured may not be the actual internal volts as the BMS is 'in the way'.

If the batteries are well out of balance the BMS could terminate charge at a low voltage, well below the recomended 14.4 volts.

Idealy use a charger or power supply that can be adjusted to a lower voltage, example 13.7 to 13.9 volts, and apply that voltage for several hours or more. ( BMS protection will be indicated by a sudden drop in charge current to a very low value). The idea is to set a charge voltage just below protection level to give the cells time to balance.

It's well worth paying a premium to buy a battery with BMS, easy to see what's happening and diagnose issues it they occur.

Even is charged in parallel before the series connection, there will be issues with cell balance in each of the batteries. Using 24 volt batteries from the start is the ideal option. It's a pity Li time and other manufactures gave advice on possible issues that may/will occur with series set ups. Battleborn , at one stage, requested buyers to state if series connection was to be used, in that case batteries with near identical performance would be supplied.

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I think this confirms that the BMS is in protection, but I'm definitely interested to hear everyone's thoughts. We hooked up a simple 12V cooler (the kind that you plug into your car and can either do heat or cool mode) that claims to draw 40W. You could hear the cooler attempt to start for a millisecond, and the amp draw would go to 1.9 amps before dropping to 0.7 amps and the battery voltage dropped to 0.0V and then went to 2.2V-2.4V.

Unfortunately, at the moment we don't have an adjustable power supply. We have an old DieHard battery charger for car batteries that we were using to try to "wake up" the BMS. It seemed to send 13.8V according to the multimeter when it was connected to the bad battery, but there's no way to adjust the voltage. We have a bench power supply on order that should get here this week. We could attempt to draw the good battery down to a desired voltage and then hook it up in parallel with a bad battery. Would that be effective/worthwhile?

Thanks for everyone’s help so far. I have a bench power supply now (a Riden), so I can attempt to revive the batteries that aren’t charging/discharging correctly. I’m hoping someone can give some more details about how I should do it and how I can tell if it’s working.

I watched some how-to videos on the power supply, and tried it out on two working batteries to make sure things worked as expected. I was able to set it with a max voltage of 13.7V and max current of 10.0A, and I could see the voltage climb while the amperage held at 10.0 amps, and then the amperage slowly falling once the voltage reached 13.7V. I also tried it with two separate batteries: one that I had recently been able to charge to 14.54V (so presumably the cells are fairly well balanced) and one that previously stopped charging around 14.2V (presumably containing mildly unbalanced cells). As expected, the power supply showed the amperage dropping much more slowly for the unbalanced battery (particularly in the 0.1A-2.0A range), presumably because the BMS was working on balancing the cells.

So now I’ve moved on to the batteries that won’t take a charge from the normal charger and also won’t give power, even to a small 3.5 amp load. I set the power supply to 13.8V and 1.0A, and it’s showing that the battery is taking 0.25A. Does that sound right? Do I just leave it like this until the current drops? What’s the next step, or how do I know that it has or hasn’t worked?

Sorry for all the questions, but I searched the forums and the general internet and couldn’t find a good explanation of this process (I’m probably missing the right keyword).

TomPanning said:

So now I’ve moved on to the batteries that won’t take a charge from the normal charger and also won’t give power, even to a small 3.5 amp load. I set the power supply to 13.8V and 1.0A, and it’s showing that the battery is taking 0.25A. Does that sound right? Do I just leave it like this until the current drops? What’s the next step, or how do I know that it has or hasn’t worked?

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IME, this is not normal. I can't tell you what to do next because you have a sealed manufactured batteries with procedures I don't know

1) I have used the power supply on BMS Under voltage home made lithium batteriesto "jump start" the charging. Usually removing the discharge load and attaching the power supply to leads will start the charging. If it doesn't, its time for me to bypass the BMS and touch the positive of the power supply to the first positive cell and the negative of the power supply to the opposite negative cell. Basically, the positive and negative of the 24 volt cell.

In order for you to bypass the BMS, I think you need to cut open the battery, and then you will be able to "jump start" the battery by bypassing the BMS on the positive and negative cells. I AM NOT RECOMMENDING YOU DO THIS NEXT. I recommend you read the insturction for the battery pack to see what else can be done.

2) If it were one cell with an overvoltage, you may need to put a small load across that one cell like a resistor or Light Bulb to drain this cell. I have not done the math to see what would be the best resistor ohms or wattage rating for a 3.2 VDC cell. Once the cell is below the reset value, you'd be able to charge. I AM NOT RECOMMENDING YOU DO THIS NEXT. Still requires the battery to be cut open the way I mention I did this. I recommend you read the instruction for the battery pack to see what else can be done.

3) If it were one cell with an undervoltage, you may need tothe power supply across this to return it to a good state.. I have not done the math to see what would be the best resistor ohms or wattage rating for a 3.2 VDC cell. Once the cell is below the reset value, you'd be able to charge. I AM NOT RECOMMENDING YOU DO THIS NEXT. Still requires the battery to be cut open the way I mention I did this. I recommend you read the instruction for the battery pack to see what else can be done.

4) If the power supply wires are not 10 AWG, you may need to build your own. Much voltage loss happens with the leads that come with it. If this is your case, you should notice wires being warmer than normal.
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That may bring your battery back to life, but the cells could still be unbalanced and I don't know if the BMS has balancing or a top balance would be the right stup.

Wrong topic