I have a really long thread here, with some of my build details and problems I ran into, but I felt like it was getting pretty long, so I decided to start a separate thread for this problem.
TL;DR of the BMS problems in the thread above - I was out of the country for a month, and despite the car being plugged in, I came back to a VERY dead battery. So dead that I had to individually charge cells to bring it back to life.
This problem continues to happen periodically (1 in 10 charge cycles?). The BMS will go into “DISCHARGE” mode while parked in my garage, plugged into the wall. The discharge rate is always 1.2A, and it will draw the batteries down to nothing if I’m not paying attention. It’s been wet and raining here lately, so I haven’t been using the gem car much in the last week, and came out yesterday to find it draining the batteries down.
I can always stop the drain by turning the Chargery BMS controller off, then on again.
Yesterday, when I took these pictures, I had to unplug the charger from the wall before cycling, in order to get the charger to kick on. Normally I can leave it plugged in when cycling the BMS.
I’ve tried adjusting the types of balance, and it happens in all the configurations I’ve tried (Including charge only balancing)
Here’s what the problem looks like when it is happening:
If the bms is powered from balance leads, it does not use power through the shunt. It does use power from the first 14-16 cells. So it should be off to minimize drain in storage. However it will not keep cells balanced when off.
1.2amps is a lot of current, you need to find the draw. Start with turning off battery switch. If it drops current draw it is most likely the dc to dc 13.5v circuit.
If you are concerned about long term storage disconnect battery and turn off bms switch. I have batteries that have not been charged in years. Self discharge of healthy batteries is very low.
If the problem is hard to find use an external ammeter to double check the shunt reading. The 300 amp shunt is not terribly accurate at small loads.
Hey Inwo, My assumption is that since the draw goes away only by cycling the the BMS, and nothing else, that the draw is coming from the BMS itself, and not from a another component. FWIW, when I left town for a week, I did remember to shut off the BMS, and when I turned it back on after a week, the voltage had dropped by less than 1V, so I don’t think there is a draw in the system.
Under normal “non problematic” situations, the BMS displays 0A draw. It’s only during these problematic times that it shows 1.2A draw.
These things lead me to believe it’s something malfunctioning (or incorrectly configured?) in the BMS itself.
Do you think I’m wrong in my analysis?
PS, when I say cycling the BMS, I mean switching it from it’s normal position of getting power from battery leads, to switching to External power (which doesn’t exist), then back again, which effectively reboots the BMS.
The bms reads current from shunt. Nothing in the bms can draw power through the shunt. Balance current does not register, as it is through balance leads only.
Two possibilities I can think of:
Something is drawing 1.2a through the shunt.
Measure mv across shunt 300a = 75mv.
Nothing is drawing current, and bms is reading phantom current.
Re calibrate bms amps, worth a try.
As a test you can disconnect shunt output connection to car. Amps should drop to zero.
Reconnect, amps go back up? =something is “on”.
Does not go back up? = something was “on”, but power cycle turned it off. That will be a tough one. You will need to disconnect things one at a time or get a clamp meter to trace the current.
When shunt is not plugged in to bms, I notice it will often read 1.2a. (open circuit)
This tells me that the shunt sense leads internal or external could cause a false reading.
I just finished building a Chevy Bolt Battery pack and hooked up a BMS 24 and as soon as it was hooked up the BMS showed a 1.2Amp draw. And the battery had no draw on it at all. So it makes me think that a 1.2- 2.0 amp draw is a factory setting that the BMS has, simply calibration USING A AMP CLAMP. Will make sure your BMS is accurate
Yes, but it doesn’t mean it’s drawing 1.2 amps. Only the it needs to be calibrated back to zero.
It will keep the the bms in discharge mode however.
Dave is correct. I was just saying the factory may accidentky calabrate them to have a factory default showing 1.2Amp draw and it’s totally in accurate. The draw would be closer to 0.0amps
I have never measured what the BMS takes as far as voltage to run it and keep it going
Back to the original topic
Try turning all
The balancing settings to ON and turn the screen to “Alwasy On” and keep a eye on it. Set the
Over charge turn off to 4.1 volts so as soon as a cell reaches 4.1 the charger turns off. That should give you a average pack cell voltage of 3.9-4.0
Is it possible the reason the charger never kicks on and recharges the battery is because it thinks that it is in discharge mode? (ie: will the charger refuse to kick on in discharge mode?).
That’s part of what confuses me… even if there is a draw coming from the BMS, or any other component, why wouldn’t the charger kick on and keep it from draining all the way down to zero?
Yes, most anything irregular will stop bms from enabling charger. It may not be necessary to control the charger from the bms.
I dont know anything about lithium profiles other than voltage. May or may not restart.
I believe if it is under bms control it does restart, but even then a fault will prevent charging.
Best solution might be ac control of charger from timer and/or bms.
The only thing that would stop your charger is the charger not correctly reading the battery’s voltage: example the charger thinks your battery is at its finish voltage. the BMS reporting or telling the charger.
I know this thread is old as dirt, but I’ve continued to have this problem off and on over the last couple years. Mostly off, which allowed me to ignore it. The amperage would show -1.2A, and I’d need to power cycle the bms and sometimes replug the charger before it would start charging.
Well today I got my DC ammeter out and confirmed I wasn’t actually drawing anything. So I finally did what @Inwo and @grantwest said to do 2 or 3 years ago. And what a shocker they were right. Thanks guys. I followed grants video step for step and my system started charging as expected.
