Over Voltage fix (and hopefully explanation)

AlanL

10 mW
Joined
Aug 12, 2022
Messages
28
Hello, I cannot find any discussion about over volted batteries other than that they are destroyed. Mine are not and I am hoping to get the voltage down before they are permanently damaged, Please Help!!!

I have a LifePo4 24s1p battery with the Daly 100a 72v BMS. It was working properly and charging and discharging with a LifePo4 84v max charger. For reasons not worth explaining, I discharged the battery and charged it with an 84v max Litium Ion charger. To my surprise the battery charged to 120v!!! I did some research and realized that was a huge mistake but can anyone explain why the Lithium Ion charger was able to charge the batteries over its own limit and over the the 88V limit on the BMS?

More importantly can any one tell me how to get the voltage down? When I discharge the battery it drains until it cuts off but still reads at least 110v. The battery is working, and I have now discharged and recharged it with the correct LifePo4 84v max charger but even that now charges it to 110 volts!

What is going on and why cant I get this voltage down??? What could be done to get it back in range?
 
Can you give us more specific information about your specific battery? Brand, maybe pictures?

The short answer is that it is not possible for your 24s LiFePO4 battery to have reached 120V. That would be 5V per cell... Then you say you discharged the battery. How did you discharge? Dummy load, ran your ebike to drain it? You said it drains until it cuts off.. Do you know what is cutting off? The BMS, the device you are using?

Last clarification question: what are you using to read voltage? And where are you reading voltage? The positive and negative terminals of the battery directly?

If you are concerned, move the battery outside to a safe place where it can't cause damage. LiFePO4 is not known for being pyrotechnic, but there is still a chance of overheating and smoke if it is drastically overcharged.

Without knowing more info, it seems like it's possible that you are reading the voltage incorrectly.
 
It looks like whatever meter your using is reading the battery voltage wrong. You said you used the pack and the BMS disconnected, so low voltage disconnect seems to be working.

double check with a multimeter. I had a previous 4s lifepo4 battery that was giving me all kinds of erroneous readings. After checking everything I found one of the balance connecters wasn't connected to the battery and that was causing the wrong readings.

Also check the battery voltage with a multimeter when your charging. It should never read more than the chargers max voltage, if its reading 5 or 10 volts higher the charger is bad.
 
Change the battery in your voltmeter. When the battery is low, voltages you test with it show higher than they should. The closer to dead the meter battery is, the higher the voltage tested will show.



AlanL said:
I have a LifePo4 24s1p battery with the Daly 100a 72v BMS. It was working properly and charging and discharging with a LifePo4 84v max charger. For reasons not worth explaining, I discharged the battery and charged it with an 84v max Litium Ion charger. To my surprise the battery charged to 120v!!! I did some research and realized that was a huge mistake but can anyone explain why the Lithium Ion charger was able to charge the batteries over its own limit and over the the 88V limit on the BMS?

More importantly can any one tell me how to get the voltage down? When I discharge the battery it drains until it cuts off but still reads at least 110v. The battery is working, and I have now discharged and recharged it with the correct LifePo4 84v max charger but even that now charges it to 110 volts!
 
amberwolf said:
Change the battery in your voltmeter. When the battery is low, voltages you test with it show higher than they should. The closer to dead the meter battery is, the higher the voltage tested will show.
Ooo, that's new knowledge for me, good to know!
 
OMG, I just read this at work and I want to run strait home and test it!

I actually did some more measurements before leaving for work directly on the battery because Harrisonpatm made me start wondering if my measurements at the BMS were wrong due to a bad BMS (who knows how that could result in a high reading) but strait on the battery I was getting 115v this morning. I also spent a few minutes looking for my old volt meter just to rule out the new volt meter but it didn't turn up easily enough.

If you are right I will feel slightly stupid but I will have learned some thing new. It is surprising how consistent the high voltage reading is from day to day and with many attempts if its just a low battery. I will find out at lunch and report back!
 
It would help if you snapped a couple pics of the entire battery, also showing your Voltmeter leads probing the battery POS and NEG terminals and also showing the Voltmeter display.
 


AmberWolf nailed it. It was pretty amazing timing for my voltmeter to start miss reporting right after I used the wrong charger...

Great lesson learned.
 

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Guy goes to the doctor. "Doc, everything hurts! I touched my ankle, ow! I touch my side, ow! I touch my head, ow!"

Doctor says, "Your finger is broken."
 
I am going to attempt a follow up question since I have started to explain my setup. My 24s1p battery consists of LifePo4 batteries with a Daly BMS. It charges to 88V as expected when each cell is fully charged to 3.7 volts. However, the system shuts off when it reaches only 70 volts. I believe it is the Daly BMS switching it off because the controller just looses power and display shuts off. The controller would not do this at under voltage it would just cut power to the motor and stay on. The controller is a Votol EM150 set to cut off for under voltage at 60 volts.

This is a link to the Daly BMS https://a.co/d/7NMYz0U it's specs say it cuts off at over voltage of 3.7 and under voltage of 2.5v per cell which would put me right at 60 volts.

Can anyone guess or give me a trouble shooting tip why my BMS would cut off at 70v instead of 60v? It is not programmable, that is just hard set. I will also say that I have tried limping up to the limit, drawing it down with very low current so as to avoid voltage sag. I am not seeing voltage sag either, it is just cruising along at 70v and low amps and shuts off. The battery is completely cool as well and when it sits for a few min and the battery regains some charge it comes back on at about 74v, if that helps any...

Ideas?

Thanks in advance!
 
Just a suggestion. There is almost no capacity gained between a charge termination Voltage of 3.5 Volts per cell vs 3.6 VPC. You'll get more life cycles out of the pack. Really should not charge to 3.7 VPC.
 
BVH said:
Just a suggestion. There is almost no capacity gained between a charge termination Voltage of 3.5 Volts per cell vs 3.6 VPC. You'll get more life cycles out of the pack. Really should not charge to 3.7 VPC.
Thanks for the advice, im not sure the controller can stop it charging and the BMS isnt programable, ill dial down the over voltage in the controller and see but realistically how could it? the charge lines go through the BMS not the controller.

In the detailed description of the BMS theres stuff i dont know about like discharge release voltage and discharge detect voltage and the +/- 1v notation. which might some how add up to 70v???

MAIN CIRCUIT INNER RESISTANCE: ≤ 20MΩ ; CHARGE VOLTAGE: S*3.75V ; OVER CHARGE DETECT VOLTAGE: 3.75 ± 0.05V ; OVER CHARGE PROTECTION DELAY: 1 SEC ; OVER CHARGE RELEASE VOLTAGE: 3.75 ± 0.05V ; BALANCE DETECT VOLTAGE: 3.65V ; BALANCE RELEASE VOLTAGE: 3.65V ; OVER DISCHARGE DETECT VOLTAGE: 2.5 ± 0.1V ; OVER DISCHARGE DETECT DELAY: 1 SEC ; OVER DISCHARGE RELEASE VOLTAGE: 2.65 ± 1.0V ; SHORT CIRCUIT DETECT DELAY: 250ΜS ; SHORT CIRCUIT PROTECTION RELAY CONDITION: OFF LOAD ; WORKING CURRENT: 100ΜA ; SLEEPING CURRENT (WHEN IN DISCHARGE): 20ΜA ; WORKING TEMP RANGE: -20 ̴ 70 ͦC ; SLEEPING TEMP RANGE: -40 ̴ 80 ͦC Short Circuit Protection BMS Self-Consumption Working Current :≤500 uA.


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You'd most likely have to buy a small hobby charger where you can set termination Voltage at any level you want and use it in-place of your standard, non-adjustable charger. You can get good hobby chargers that handle all the Lithium chemistries for well under $100.00
 
AlanL said:
The controller would not do this at under voltage it would just cut power to the motor and stay on. The controller is a Votol EM150 set to cut off for under voltage at 60 volts.

This is a link to the Daly BMS https://a.co/d/7NMYz0U it's specs say it cuts off at over voltage of 3.7 and under voltage of 2.5v per cell which would put me right at 60 volts.

Can anyone guess or give me a trouble shooting tip why my BMS would cut off at 70v instead of 60v?

I believe that you are correct in that it is the bms shutting off at low voltage rather than the controller. Just like the previous poster mentioned about 3.7v being a bit too high for LiFePO4, 2.5v is really really at the bottom end and maybe a bit to low.

Have you seen a discharge curve chart for LiFePO4? Flat, with steep cliffs at the top and bottom voltages. What this means in practice is, even with a new well-balanced 24s pack, when you start to get to the bottom of your capacity, the individual cell voltages will start to drop hard, unevenly and at different rates, depending on their specific microscopic ddifferences. You're aware of voltage sag, this is good, but 5% voltage sag in the middle of the discharge curve is different than 5% sag at the bottom.

So what's happening is that as you reach the bottom of your range, one of your cells in series will start to bottom out to 2.5v before the other 23. This is not because of bad or old cells,, this I just what LiFePO4 does at the bottom of its curve. The total pack voltage will still be 70v or so, but the Daly bms is reading B13 or whatever at 2.5v, so it cuts off the whole pack to prevent B13 from being discharged even further, which would damage the cell. Then you say it comes back up when you wait a bit; thats cuz you stopped pulling current, so the cells recover a bit (this is good, means your battery isn't degraded yet).

It's unfortunate that you can't alter the bms settings. I would actually recommend you set your low voltage cutoff to something like 2.9v or even 3v. Pushing your battery's capacity all the way to the top and bottom of its range every time is going to shorten its lifespan in the long run, to the order of hundreds of cycles. For example, on my 24s, i have have charging voltage set to 84v, 3.5v per cell. As mentioned earlier, the capacity stored between 3.45 and 3.65v is miniscule, and most of us choose to not stress our cells all the way up to 3.65 every time.

I would also recommend you set set controller low voltage cutoff to 68v, or something in that range. It's best not to rely on the BMS to turn off to avoid cell damage. It could fail closed without you knowing. Always good to have more than one safety in place. Another way of looking at it; my bms should kill power if it detects one cell at 2.9v. I know that if that happens mid ride, I can go into the bms and lower these settings to get me the last mile or two home for a charge, slowly and safely without damaging the battery. It's like setting up a reserve tank for emergencies.
 
Thats great stuff, maybe I will set my soft undervoltage to 74v so that it limps at that point and doesn't trip the BMS but hopefully I still get home! lol

I saw one post that suggested that weak connections could cause some resistance and thus individual voltage drop on individual cells causing the BMS to trip while overall voltage looks good. He found this by replacing the Daly with a smart BMS with nice realtime graphing for each cell and then he could see the drop.

Also, I am about to upgrade to 24s2p, is it reasonable to assume that having each cell in a parallel connection will mitigate the cell variance by averaging the differences in discharge rate between the two in parallel?
 
AlanL said:
I saw one post that suggested that weak connections could cause some resistance and thus individual voltage drop on individual cells causing the BMS to trip while overall voltage looks good.
Yes, totally true, and if you're worried about your connections you could give them a once over. But if you have no issues with amp draw in the middle of your range, and you're just seeing cutoffs in the bottom end of your capacity, the batteries themselves are going to be the main culprit.

AlanL said:
Also, I am about to upgrade to 24s2p, is it reasonable to assume that having each cell in a parallel connection will mitigate the cell variance by averaging the differences in discharge rate between the two in parallel?
You got it. Double the battery, same discharge rate, means half as much stress on the individual cells. Plus double the range. The math won't be exactly 2x; double battery also means double weight that now needs to be accounted for during acceleration, and the voltage sag won't be half as much as it was before, maybe like 20-40% less? Still great of course. Wish I could just double up on my battery, I'm out of space.

I don't think you mentioned yet, what is your build? Bike, trike, motorcycle, snowmobile?
 
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Thanks for your help and the interest. It’s an old YZ 80 frame I got for $100. So far its been a super fun project bike and goes faster and has better suspension than my Suron!


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