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.
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.
Lower current certainly helps reduce phantom voltage rise but I think you should be able to help things by lowering your charging voltage or getting all settings up and away from the charger’s voltage (which might not be a good idea, depending on the charger voltage).
You have a very complex set of thresholds set up now, all trying to handle the symptoms you are seeing, but IMO never addressing the cause of all the symptoms…which hasn’t been discovered yet.
Try a lower charging voltage, even if you will never use it. This is just for testing to see if the charge finishes properly, going through a full CV stage down to the termination (charge stop) current setting level…if there is such a setting.
Keep your protection settings high and start balancing about 0.2V/cell below your charge voltage, e.g., 3.8V/cell balance start for a 4.0V/cell charge voltage.
The p26a delivers as much energy total as a 30q at 10 amps, more at 15 amps, and can be pushed to higher currents. It is also much cheaper from what I’ve seen. The 30q may be better below 10 A, but it’s still much more expensive and most of the time we’re looking to use more power than that.
I use Wh because it takes voltage into account. it’s better than saying “I have a 10Ah battery” and that meaning different things when considering the Voltage. 1Ah on 12s isn’t the same as 1Ah on 10s. Wh however is more representative of capacity, so I use that.
what affects capacity values are things like internal resistance and the load applied. if we want to pull tonnes of amps from our cells, the capacity will reduce faster as a result. I don’t know of a way to accurately communicate capacity without caveats like that.