BMS cutting out at nominal V?

I just finished building two 8S3P batteries using Molicel 21700 p42a with Daly BMS. Both 8s batteries will charge but BMSs are cutting out at nominal 28V instead of full charge (or are reaching some limit? Temp?) Tried 2 chargers and verified output V is 33.6v, same result. Any ideas? Here is the BMS:

I’ve seen lots of daly bms issues, it’s not what i would get… but if it’s the same with both batteries then probably a setting or protection.

There are multiple issues:
Max charge 7.5A, how much do you charge with?

Max discharge 45A will mean they will cut the power during starts, which can kill your esc.
It’s not the right bms for you if you want to use it both for charge and discharge

Charger at 36V is not right for 8S - which shouldn’t be charged at over 33.6V.
This probably is your basic issue, bms is likely cutting out due to overvoltage.

I think they can be programmed, if so then you can read out the error code and confirm why it is cutting out.

Check each grouped voltage to make sure they are all the same with in .02V when the BMS is cutting out. It’s could just be doing what it is supposed to do and turning off because a group is fully charged.

If cells are all the same, it sounds like it has been programmed for lifepo4 not li-ion.

Typo. I meant 33.6v at charger. Charging at 5 amps. Wired for charge-only. Separate discharge connector.

Don’t think this bms will cut power at 0.02V unbalance. It seems to check and balance at 4.18V

Can you connect it to a PC?

I would measure the voltage of each 3p row. I don’t trust Daly BMS any more, I had one that unbalanced a pack of new cells, it would never bring a couple of the rows up to the correct voltage.

What I mean, make sure each group is balanced. If one group is a 4.2V and the rest at 3.5V then the BMS will cut off at 7 X 3.5 + 4.2 V = 28.7V.

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I don’t think the bms should cut off though. It should only cutoff the p group that is fully charged. At least that is how I understand a BMS should work. The remaining p groups would then still be charging until full

I should have put a balance y adapter for easy troubleshooting. Next time :slight_smile:

The charging is through all cells in series, so as soon as one group is over voltage the BMS cuts out. The BMS balances with a resistive load, not by putting power into individual groups.

During “balanced” or “supervised” charging, I’m surprised that a BMS doesn’t charge P-packs individually where needed. Is it the same principle with RC chargers like ISDT iCharger x12 for example that is charging through all cells in series and not packs individually ?

So much complication in the circuit design to charge individually. Active balancers move energy from over charged cells back to lower energy cells but are a lot bigger physically than you would want on a small efoil battery, and not really necessary when using new, matched cells.

At 28v your pack is sitting at nominal shipping voltage and not charging at all. You most likely have a connection error somewhere or the BMS doesn’t work at all.

  • check balance cable is in correct order from cell groups 1 through to 8

Love the ”shit” autocorrect😄 happens to me also.

This is a common port bms so there are no charge/disharge lead options.

Haha actually edited the whole thing now because I realised the pack is just sitting at shipping cell voltage so no charge is happening. It’s just shutting down immediately.

No, it took awhile before charger light turned green. Maybe 20 min and in that time it did charge up the pack maybe 0.5v. It must be some limit is reached. Charger will bulk charge through the discharge connector no problem, just not through BMS. I checked bms connector wiring after pack completion and it measured correct. It is wired like this: (discharge wiring added)

This is not how my bms works in my Vesc board. As each group finishes charge, the resistive load connects- LED light turns on for that group, one group at a time until all lights turn on and pack total voltage is reached and charger turns off. The pack would never charge up completely if the BMS cut out at the first group reaching full voltage.

Solved. Somehow I ordered LiFePO4 version! That must be it. Thanks for the help!

Good call :+1:

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The resistive load will be turning on at around 4V for the highest voltage groups, slowly bleeding the voltage off while the others catch up stopping when one group gets to 4.2V or what ever it is set at. This is how almost every BMS works to balance cells, apart from bigger ones used for home energy storage.

Almost no BMSs charge via the balance wires, these are just for measuring voltage and applying resistive load.

Do you have the model number for your BMS?

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