Provided I charge through a BMS that cuts off the power supply once the battery is charged, would it be OK to use a 12S charger for a 10S battery? Li-ion chargers provide constant current initially so until the battery pack voltage is close to 42V, both 12S and 10S charger should behave the same = charge with constant current. Right?
It won’t work, the charger needs to progressively decrease current while it’s reaching the EOC voltage, that what cc/cv chargers are for, if your charger is 50.4v it will hit 42V like a freight train and stop suddenly, causing a posible pgroup drift and just needs a small error/imperfection in the emergency system on the BMS to burn your house down. Do you really want to tell people you house was burned because you used an incorrect charger instead of buying a USD 30 one?
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It’s not about saving $30. It’s about understanding how things work. Say I only charge to a voltage where the 10S charger wouldn’t switch to the CV mode (that’s BTW how I charge my battery anyway), would there be any difference if I used 12S charger instead?
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Those two things are mutually exclusive. It is by definition CV at 10S voltage. If it isn’t then you’re already not using a 10S charger.
What are you currently using to charge?
Let me put it differently. Say I charge my 10S battery pack up to 41V (rather than up to full = 42V). While the battery pack voltage is under 41V, the 10S charger still charges in CC mode. It only switches to CV mode when the battery pack voltage is closer to 42V. Given I never ever want to charge to more than 41V, would there be any difference in using a 10S charger and a 12S charger?
I use a 10S charger but I have my smart BMS set up to stop charging when the highest cell voltage is 4.15V. I don’t balance the cells. This is to extend the battery pack life at the cost of slightly lower range.
I’m working on a build with 12S battery pack and will need to get a new charger for it. My 10S charger is a cheap one that only does a little over 1A. That’s mostly fine and I will continue using it. I’ll be getting a faster charger for the 12S pack though. What I’m after here is whether it would be OK to occasionally use it for the 10S pack if I need to charge it fast (with the BMS cut-off settings in mind).
Charger voltage =/= cell voltage. I’m not sure of the intricacies of how your charger and BMS interact, but most BMS’s expect a near CV input when approaching the max voltage they’re allowed to reach.
If your pack is pulling 2A from the charger then there is considerable voltage drop between the charger voltage (which the BMS reads, even at a cell level reading) and the actual state of charge.
If your BMS disconnects the charger once max voltage is reached and requires a reconnect to resume charging, then your pack will actually dip below 41V once it stops charging.
If the BMS is not so smart, then it will operate the fets like a voltage regulator, constantly turning it on and off so that the voltage the pack sees doesn’t exceed 41V despite the charger outputting more. If that’s the case then it will actually act as a full CCCV charger for the cells.
That will actually bring the pack to 41, while the previous possibility would not.
While a 12S charger would operate the same, it still should not be used. If anything in the BMS goes bad then it will not be able to stop the excess voltage from reaching the cells.
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I’m aware of what happens with the voltage once the power supply gets cut-off by the BMS. The BMS doesn’t re-enable charging until the pack voltage gets below certain threshold (can be configured), so it won’t do the repeated on/off switching.
OK, cool. We’re finally getting somewhere. 
That’s what I figured as well. I guess it would be OK if keeping an eye on the voltage readings while charging. I do appreciate there’s increased risk and this should only be done with extreme caution. I’m not even sure I’ll ever want to do it. I was merely curious if it’s an option at all.
Could it happen that the 12S charger would refuse to charge if the battery pack voltage is way lower than it would expect for a 12S pack? Say below 30V, which would imply below 2.5V per cell for 12S?
That’s very much possible.
An easy way to check is to hook up the charger to a power resistor that’s only a few ohms. The CCCV should drop the voltage really low, and if it does then you know it works.
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I think this is a really good question, and someone should experiment if you can use a 12s charger to charge a 10s pack just for a little bit and of course watching the voltage. This may help people in the time of need
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Problem is you only know if your BMS works if you use the on board mosfets as an E-switch. Otherwise you have no way to check except to open up your enclosure and jump the contacts to trip the fets.
No, it switches to CV mode when the battery pack charger voltage is closer to exactly 42V. If you disconnected the charger at that moment, the battery voltage would drop below 42V.
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The BMS will not let you charge as it has an over voltage setting which will be triggered by the 12s charger.
We have discussed this above. Depends on the BMS type and settings. On mine, when it cuts off the charger, the voltage has to drop below configured threshold before charging is re-enabled. Then it doesn’t end up in the on/off loop.
What do you mean by this? I thought the charger would gradually increase the voltage until it either reaches the desired constant current or the max voltage.
Probably depends on where the cut-off is. Sticking to 10S battery pack as an example, if the cut-off threshold is below 42V, it should charge with CC at all times regardless of whether it’s a 10S or 12S charger.
If the cut-off threshold is above 42V (which would be the case for most/all BMS-es that you can’t configure) then yes, 12S charger would charge faster than a 10S charger near the end since it would still be in the CC whereas the 10S would have already switched to CV. That’s definitely not what you want.
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Just to let anyone know who is curious if a 12s charger will charge a 10s battery it can. I did it last night we where out of town and forgot the 10s charger and only had my 12s charger. I have a d140 bms with charge only and used the vesc app to monitor the voltage and never left it un attended. I stop charging when my voltage hit 41v just to be safe. Battery didn’t get warm at all. I definitely wouldn’t advise anyone doing it but I wanted to report my experience and it worked.
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Those two sentences contradict each other ^
You can’t claim it will charge it when you only tested on a partial SOC.
This is a STRONG case of YMMV in regards to quality of charger and type of BMS, and anyone claiming otherwise is either a fool or lacks proper experience in this realm of mishmashed power electronics and China manufacturing.
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TLDR if you get a charger that you can open up, some of them have a pot inside that you can adjust the voltage down. Make it your max voltage (for LiFePO4 ONLY 0.5V more than full charge), combine with battery side BMS and you’re set.
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Absolutely not.
To anyone inexperienced coming across this thread, do NOT do this if you don’t have extensive knowledge of electronics. I wouldn’t even do this without consulting an EE.
For starters, cheap ESC’s may not have highly rated mosfets. Inputting a higher voltage, like 12S voltages , into a 10S BMS could overvolt the mosfets and just blow them open, resulting in ZERO overcharge protection in your pack. That would guaranteed start a fire if your charger actually output 12S voltage.
Lowering your charger voltage via a pot will help, but you certainly shouldn’t set it to 0.5V above what your maximum is! If you have control over it you should set it to what your pack actually needs. Don’t risk straining the system.
My final take on this entire debacle:
Do NOT plug in ANY charger that is a higher voltage into ANY battery unless you’re willing to take the risk that you can burn your house to the ground. Take necessary precautions.
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Did you even read my reply?
I’ve been thinking about how useful the MPPT charge controller will be for group rides. One person carries it and spare packs.
Also if the spare batteries have BMS the MPPT could be used as the charger for them if you wanted, using the 12S charger you might be carrying as a power supply.
Oh to be clear it would primarily be used for the Onewheel, but it could also charge the 12S boards.