Help, something is draining my pack

So some time ago I took my spud out for a spin… but for some reason it wouldn’t fire up.

Went back and found out that my 10S2P had been completely drained… 0.3V per cell left… At that time I thought perhaps my BMS was busted.

So I replaced it with a new 10S2P and new BMS and monitored it. For some reason, the pack was still slowly draining at about 0.5V per day. So I thought maybe it was my anti-spark switch (electricskateboard repair), I swapped that to a brand new flipsky smart anti-spark switch and the same thing is happening, 0.5V draining off each day.

I tried leaving the pack unplugged and voltage was stable, I also tried connecting it to a different VESC and it started to drain slowly again.

Any ideas what the hell is going on?

Thanks in advance! Erv.

maybe you could post a pic of your internals?

very strange indeed

Bad bms? Maybe, or short? Weird

what kind of bms?

which vesc?

my suspicion in on the anti-spark switch. not sure if possible, but if you replaced that with a loop key, i’d bet money the slow drain will be gone.

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@banjaxxed I think the instance when the pack got drained to zero in a matter of a few days might have been BMS issue. But I have since changed to BMS.


It’s the cheap waterproof red colour one from China.

VESC is hobbyking 4.12 and flipsky dual 4.2… both still drain slowly. As does new BMS.

I have it now transplanted to another enclosure with an antispark I know works… voltage is stable…

Now what I don’t get is why two brand new antispark from different manufactures would have the same problem… Is it something I did?

This is in no way a high powered board… Single 6374 on 10S…

My current option is to change to a loopkey but that means having to open a larger hole in the enclosure… And also the hassle of a loopkey…

I measured both antispark outputs when the switch was off and both exhibited a small less than 1V leak… WTH…

Transferred battery and BMS to another deck with an antispark I know doesn’t leak… Voltage stable again…

So basically I have 2 brand new antisparks which are leaking… Why did this happen?

Interesting find! So who knows the answer?

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  1. Meassure what resistance the the FETs show when off. (Should be Mega Ohm or “overload”)
  2. Meassure on “OUT” terminals with battery connected, if you have any active output.
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this would be better answered by an electrical engineer, but here goes.

antispark switches uses mosFET(s) in the open position to complete the circuit. the gate that determines on / off is the thinnest filament and could allow the tiniest amount of current despite it being in the off position:


that’s my guess. please tell me how i’m wrong. i’d actually like to know as well.

anyway, this is why i never use antispark switches that have this tradition design.

Lets say the off state is 1Mohm. You’ll have a “tiny current” of 0.0042mA(10s). So…from full to empty would be…2000+ years something for a 10Ah battery?

Lol hang on, let me get a calculator.

calculator said 27 years, few zeros wrong xD


How I measure the FET resistance?

As for the OUT terminals I connected a 4S battery, when the switch is off one anti-spark shows 0.2V, the other 0.5V… I’m pretty sure this is not supposed to happen…

Yes it should be negligible in real world terms… but I’m seeing my pack being drained noticeably…

I tried every combination to weed out the problem, pretty sure it’s the anti-sparks leaking… and somehow both of them (from different manufacturers) are exhibiting the same problem…

just meassure between source and drain, nothing connected.

Across the positive terminal both problematic switches show zero ohms… it’s completely pass thru…

Across ground, one of them was OL.

The other was weird. When I first measured it read 5Mohms… Then after I probed it more it went over limit…

Gave up… 100% sure both my almost brand new antisparks are the cause of the leaks.

Have since gone back to the primitive but proven XT90 loopkey… no more antisparks for me…


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