Sounds like a charger issue unless you’ve lowered the maximum voltage to me. I’m not actually sure how people deal with it, but if you’ve got say a 12s 50.4 V charger and only want to charge to 4.1 V per cell, so ~49.2 V total then your BMS will have to stop the charging at 49.2 V at which point the charger will still be trying to charge at full current. Someone who knows more about these things should probably get into that though because I don’t actually know what you’re supposed to do if you want to charge to a different voltage unless there is a pot inside the charger you can adjust to change where it goes into CV mode.
I just want to charge to 4.2V, but my “7A” charger (that LLT shows as 5.9A) seems to bring the voltage up until 2 cells hit 4.25V, them the BMS cuts off charging and balances down to 4.14V. the issue is with the full current the battery “reverse sags” up to the cell overvoltage cutoff but as soon as the charging is cut off the voltage drops to the actual level of ~4.18V
I feel like over ~4.15V the charger should be reducing the current towards the top end, so the cells can actually get to 4.2V and not over.
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Is the pack not well balanced? Are you actually getting the total pack voltage over 4.2 V times your S count? A charger should output in CC mode up to 4.2 V times the S count and then CV mode after that. If you’re getting a high imbalance at the top of the charge then that isn’t good, especially for a high current charger.
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I had a good balance but recently it’s been stuck at a ~0.12V delta, with 2 groups higher and the rest equal but lower. Al,psy drained the pack completely yesterday, charged up and the Delta is about the same. It’s been charging on and off on it’s own all morning and it doesn’t seem to be balancing, just jumping up then calming down but the lower cells don’t seem to have a chance to increase voltage with the full current.
That delta is probably the problem. You need to let it balance that out and it should eventually go into CV mode. If you have a lower current charger it would probably be better to let it balance on that first. Then try using the higher current in the future once the pack is better balanced. 0.12 V at the top is really large.
TBH maybe you should even wait for someone more knowledgeable to respond because that is getting to the point where something could be substantially wrong.
I don’t have a lower current charger handy, is there a way I can get my charger to output lower current? I’ve read that some have a pot inside I can adjust.
Just set it to balance off the charger (static balance) and leave it alone for a couple-few days.
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Wouldn’t cycling the battery a bit help it? Or no?
It will only start slowing down the charging current if the pack reaches the voltage set by the charger. This includes all voltage rises caused by cell and pack internal resistance, a “phantom voltage”, the opposite of voltage sag. This is the crossover point between the constant-current “bulk” charge stage and the constant-voltage “topping off” stage.
If the pack has not reached the charger voltage due to an overvoltage alert of some type (and the BMS stopping the charge) then the pack has never gotten to the point where it’s mostly charged and only needs to be topped off.
Raising your overvoltage settings (usually not good to do), and/or lowering your charger voltage, and/or lowering your charger current (less “phantom” voltage rise) can all help a charging setup get to the constant-voltage (CV) topping off stage before the BMS cuts things off.
As @bboybowzer mentioned you might also be able to balance when not charging, check the instruction manual for your BMS.
Something to consider…
Charging to a lower voltage, by skipping the CV topping-off stage, helps to extend cell life because it results in the cells dropping back down a bit more in voltage compared to letting them fully top off. Depending on the voltages you might lost a touch of range but if you don’t notice that then the lower voltage can help your pack last a bit longer. Adjust the balancing thresholds to take these lower voltages into account though.
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I think its been said but a 0.12 delta is very very high
That would technically be a full metric chonk. 12s3p a123 doesn’t exactly fit… @hummieee did you just use thick gaskets?
For once in my life(po4), I actually need a 12s lifepo bms instead of lithium ion. Any good sources where I don’t have to play lifepo/li-ion shenzen roulette?
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Ok, so I’m ready to close this beauty in a shrink wrap. I just need to solder the BMS (charge only) and I’m also waiting on some xt60ies for the charge port.
I’m also going to add an inline fuse on the charge positive line. What fuse should I use? I will be charging with a 4A charger.
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10A is what I use usually. Pack looks nice!
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How do you guys shrink wrap flexible packs? do you just shrink each individual section and have the series wires jumping between them?
I shrink them. The method depends on the spacing. For a Prototipo style battery, each segment gets its own wrap, and then I flatten the edges inward on the faces when the shrink is still warm.
For some packs where the whole thing has to be flec tolerant in a single compartment enclosure, I’ll wrap it and then carefully slice along the segment seams to allow the flex of things to happen.
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If you’re charging to a lower voltage won’t the BMS keep cutting on and off due to the “phantom” voltage? For example, if you want to stop charging at 4.1 V per cell, but the charger doesn’t do CV until 4.2 V per cell then won’t the BMS cut the charger off when you get to 4.1 V, the cells will drop back down some, the BMS will let it charge again until it hits 4.1 V per cell, and that will repeat many times until the cells stay at or near 4.1 V? Is that just how you’re supposed to charge in that case? Are there any issues with that cycling that will happen?
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this is exactly what seems like is happening with my battery, except with imbalanced cells.
my charger keeps going in CC until one cell reaches the cell overvoltage cutoff, then the phantom voltage leaves and the highest cell drops down to about 4.11V. it repeats this many times, adding 0.01V to each cell at best.
the only way I’ve found to deal with this in the LLT BMS app is to set the overvoltage release to about 4.05V, but keep the pack overvoltage release and overvoltage cutoff the same and set cell overvoltage to 4.25V-4.3V.
this way it charges slightly past 4.2V but then cuts off letting the phantom voltage release and the balancer kicks in bringing the cells closer. so far this has helped me cut my 0.12V delta of imbalance down to 0.06V in ~1.5 days.
My charger won’t go into CV until the pack is >50V at least. I have opened it up though and there are 3 potentiometers. if I knew which controlled the current I could tone it down for better, quicker, easier, safer, better balancing.
I may also just be completely misunderstanding phantom voltage and/or balancing, someone please tell me if I am.
This is a good reason to do your voltage adjustments on the charger end, rather than the BMS. If you lower the charger’s voltage, it will go into CV mode earlier.
Easily checked with a multimeter. A clamp-type meter that will measure DC is best (Uni-T UT210E for example), but any meter that can measure current and voltage will do.
Just hook up the charger to a suitable load (A partially charged battery will work) with the multimeter set up to measure current, then twiddle the pots till you find the one that controls the current. Label it, and put the other two back the way they were.
Then, without the charger attached to the battery and the meter measuring the output voltage, twiddle the other two pots until you find the one that controls the voltage. Label it, and put the other one back the way it was.
Et voila, you now know which pots control voltage and current.
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If you want to charge to a lower voltage, including a CV stage to top off the pack, then you need to set the charger to a lower voltage. Not the BMS.
The BMS will never allow the charger to go into CV mode if the BMS is set to cut off at below the charger’s voltage + the phantom voltage rise.
Yes, if the charger voltage + phantom rise is higher than the BMS cutoff then you can get lots of cycling on/off. This might not hurt anything, if it doesn’t happen very quickly, but it sure isn’t good.
Every time you stop current flow you get a voltage spike. At these relatively low current levels these spikes are small but they’re never going to increase the life of your BMS and could potentially shorten its life.
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