Float charging & why you shouldn't bypass the BMS

Many more builders are using huge packs today, two years ago stuffing 60+ cells in a board would likely get you blasted on the forums for not needing the range and having a heavy board but people are beginning to see the benefits and going big.

Maintaining a large pack is a little different than 100 to 200 watt-hour pack. Most BMS circuits only output around .09 amps when balance charging which may work fine for a XXs1p battery but it takes many times longer to balance a larger battery with big P groups. Overtime insufficient balance charging will lead to a large enough drop in voltage to significantly reduce your range due to the low cell voltage cut-off found in a good BMS.

In this scenario, if the board has a bypassed BMS, you are running the risk of dropping the cell voltage below recommended levels and damaging the cells or worse. I have noticed many builders now using this method and relying on the VESC software to do all low voltage monitoring without any fail safe or accounting for individual cells. This may seem like a sparkling idea on a fresh board and battery but life is never perfect, cells will drift and if you are not monitoring your cells you need to be using a BMS and leaving it on the charger longer once it reaches its target voltage.

It can take weeks for a BMS to balance a large pack if it has drifted significantly. .09 amps is a snails pace when charging a 500+ watt-hour pack. Keep this in mind next time you hear the fan in your power supply turn off, that is usually the moment the charger has reached the target voltage and float charging commences.


This is great info. Thank you. So this is the case even though the light turns green? The bms is still balancing? I’m using a torqueboards 12s3p it occurred to me that my range was lower than I thought it should be. It’s pretty cold here now though and i’m using six shooters.

depends on your bms, but yes, most of the time it will still be balancing even when the charger goes green.


It’s worth noting that (at least) a Bestech BMS will start balancing as soon as a group hits 4.2v and once a charger goes green, even if you unplug it right after that, the BMS will still balance the pack. I had a chat with the Bestech guys and they confirmed balancing happens even when the charger is unplugged.


How much drift has anyone actually observed here? As long as we bottom balance the cells, then they should stay relatively in sync right? I can see this being a problem with LIPO’s but not with 18650s.

it seems very rare for people to have balancing done by any other method than just discharging balancing cells only when they hit 4.2 and when this happens its a very slow discharge balancing. With this method if youre charging fast the whole pack voltage will rise to the whole pack cut-off voltage and some cells are too high and then come down slowly so you might have cells at as an example: 4.0,4.0,4.3,4.3… and the discharger function keeps going even off the charger as just said. so you now have 4.0,4.0,4.2,4.2. A very slow charge must be done to truly balance. But i havent used mine yet to see how it plays out or to see what kind of drift happens with li-ion

I would think that you would want a charger that can do 3 to 4A when charging you never want something super fast at charging as you will burn out the cells but you also dont was 1A or maybe even 2A because it will take someone a very long time to charge totally. Now I might be wrong but I believe if you have a 4p battery and a 2A charger it will do .5a per P.

When did people start straying away from balance chargers with balance leads on each pack

i dont get that either. its usualy jsut bypassed while using the pack, not while charging it. everything else would be “skipping” the BMS alltogether.

edit: dont paralel packs balance each other anyway??

Parallel pack self balance itself for lion tho. not sure on lipo but then they dont voltage sag much and cheep

That is what I do on my build. I have two 6s4p lg hg2 packs that I charge up individually, and discharge together as 12s4p.

You are not hearing me correctly, I am talking about the charge rate while on a float charge. It is related to how much your BMS can burn off with the balancing resistors.

@Achmed20 you are correct parallel cells will discharge as one. The point I am trying to get across is the fact that a large parallel group will take forever to bring back into balance with the .09 amps a BMS will provide and if you do not pay attention you could end up with a dangerously low cell if you do not have any other fail safe than the VESC low voltage settings.


I charge and discharge through a 12S BMS. But I also build my packs so they can be disconnected from the BMS and broken down into 6S sub-packs. Basically it’s 2x 6S4P connected in series. So then you can also service the pack with a balance charger if needed. Best of both worlds


Thanks @chaka this is great info. I think we need to clear up some nomenclature:

Standard use of BMS: BMS is connected to pack for charging and discharging. Discharge rate is limited to BMS discharge capacity.

"Bypassing" BMS: Describes using a BMS for charging but bypassing for discharge. Allows for a cheaper and smaller BMS

Skipping BMS: No BMS is used at all. Cells are charged and discharged directly

I’ve never quite understood how discharge through the BMS works to protect the cells. Does a typical BMS have the ability to monitor and control P group output? (I still discharge through BMS, because I’ve understood it to provide additional safety, but I admit that “I don’t get it.”)


Unless cells degrade, it’s impossible for individual cells in a parallel pack to not be balanced…

As soon as you place cells in parallel they will automatically start to balance themselves, so inherently blasting some current in to one of those cells (in parallel) (which you can’t actually do because they’re all connected, but let’s pretend you can), will be shared evenly amongst the parallel cells…

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Yes, the BMS monitors all the parallel cells (s) and if one of those cells drops below the cutoff voltage the power will be regulated or cut. If your pack isn’t well made, one or more cells in series could discharge at a faster rate than the others, therefore their voltage could drop to the cutoff level quicker than the others…


For the sake of not derailing the thread please think of a parallel group as 1 cell. When we talk about cell voltage monitoring we are talking about one parallel group not the individual cells.

@treenutter The BMS will cut power if 1 cell/“p group” drops below a specific voltage. If you bypass the BMS you lose this fail-safe.


i just leave the board charging over night, at least 10 hours, to ensure the float brings all P group into balance. at least that’s what i picture is happening.


oh yea, learned this the hard way

That’s why a set my soft at 3.3v and hard at 3v and leave the BMS as a secondary governor. Same with the over current.