I have my new packs all set. Been running them for a few days. This evening, I was checking the batteries before bed and noticed one of my battery cells was crazy low. They were perfectly balanced for two days. I flipped off the pack, because I have more than enough night power in my other pack to run or small draw overnight. When I flipped the discharge off, my cells showed almost balanced. Is there something wrong with the BMS? It keeps giving a "cell under voltage" with a 5a draw, and keeps kicking on and off. When I manually turn it off, shows the second picture. I'm lost....
Overkill 24v Under Voltage issue
- Thread starter TankFish
- Start date
I was about to reply and say, "tried that". But, you were right!
The first thing I did was go out there and check the wires, everything was still nice and snug. So, after you said the same thing I thought, I went out again, with the BMS app open on my phone. Checked every single lug on every single cell. All tight. When I closed the lid, I noticed the voltage change on the app out of the corner of my eye. The ten pin had one wire slightly loose. After 10 minutes of digging the silicone off they use at overkill, I was able to reset the pin where it needed to be and violas! Works perfect! Thanks for giving me a reason to go double check sir! Probably would have never noticed that tiny little loose pin if I hadn't gone out to check my work.
MisterSandals
Participation Medalist
Just out of curiosity, why are you using your BMS as a battery switch?
I'm not. I just used the available feature while I was inside laying in bed.
Out of curiosity.. Not a dig... Why can't you use your BMS as a battery switch? Isn't that one of the core functions of a BMS? Literally without it, it'll just be called an "active balancer"... So I'm not seeing the adverse effect here...
curiouscarbon
Science Penguin
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personally i think if there’s less than like 40A going through it, the risk is really low. 40/100A seems safe in my arbitrary feeling
when 200A surge and turn off mid way, that should hopefully be ok, but my understanding is that the turning off is not necessarily perfectly synced across all the individual MOSFETs, so if you turn it off with a really big load, some late switching MOSFET might get hit with a surge when most of rest are fully off. take this with a huge grain of salt, someone else probably has a much more accurate explanation
when 200A surge and turn off mid way, that should hopefully be ok, but my understanding is that the turning off is not necessarily perfectly synced across all the individual MOSFETs, so if you turn it off with a really big load, some late switching MOSFET might get hit with a surge when most of rest are fully off. take this with a huge grain of salt, someone else probably has a much more accurate explanation
Ahh ok... My (JK) BMS makes a very unique sound when it engages.. It sounds like clanking two water-filled glass bottles together. The way I've configured my BMS, it cuts and engages every time the battery's low (long story), one morning i got over 800+ email notifications from my inverter that my battery's disconnected 800 times, that's why i was wondering what would be the adverse effect. So far it's still working and still doing it's connect/disconnect cycling every night...
That sounds like a contactor based BMS, the FET based BMS make no sound when the transistors turn on or off. Just like a FET based BMS, you can damage the contactor by switching on and off under a heavy load.
This is why most people try and set the inverter low-voltage disconnect to avoid having the BMS activating the disconnect.
The BMS is the last line of defense for your battery, ideally you should configure components (like the inverter and charge sources) to be your first line of defense.
there's a reason why that won't work for me because of cell imbalance and the inherent discharge curve of LFP cells which I responded to in another thread (https://diysolarforum.com/threads/d...nly-connected-to-my-battery.21890/post-258816)
tl;dr tho, basically the BMS will cut discharge when any individual cell drops below threshold, and that almost always happens before the whole bank is below inverter's threshold...
but in any case i take your point, im not going to go out of my way to use the BMS as a shutoff before the inverter, but if it's part of the BMS's normal operations i'll let it do what it needs to.. i just hope I dont fry my BMS prematurely.
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