Separate bettery packs in parallel

Sattva Ram

100 W
Joined
Jan 4, 2020
Messages
271
Given an engwe bike added another dp6 on top of the frame to the stock battery. I put them in parallel with commonport BMSes. It was working fine until it got depleted and i presume the low voltage got tripped with the aftermarket pack and ever since then the bms is dead. It's not my bike so it didnt happen under me but the battery is kinda empty now so i guess the LVC got triggered in it. Could it be that the parallel mode fked it up? I thought it through well and i see zero reason why it would kill the bms if it's commonport. After all one pack cuts off the other pushes some charge in it (at least if there's some voltage diff) and the cut bms senses it as if it was a charger and revives the bms worst case scenario. (At least if the low volt release voltage is reached). And if they deplete hand in hand then there isnt a voltage diff so the cut off bms should remain cut because the other doesnt push anything init. (Tho there might be some wire length problem the aftermarket pack has longer wires so there actually might be voltage diff on load also one is 52 cells the other is 65 so there must be IR diff too. But even then it shouldnt kill the bms by my thinkin...for even then the higher voltage pack acts as a charger which by no means should fk up the bms.) Or what i'm thinking could have happened is that there was some horrible feedback loop and a very rapid on off switching the load constantly triggering the bms lvc and the higher voltage pack constantly reviving it and this back and forth switching might have made the bms go berserk.

I thought about adding mosfet diodes but i decided against it for i thought two common port bmses must work in parallel no matter what. Also who knows if the reaction time of these pseudo diodes are good enough...if indeed it's the parallel usage is what fks it up then maybe even such a pseudo diode cant help it for it takes time for it to react. And there's no way i will put proper diode in it so the most i'd put init is these pseudo diodes but who knows if those can save the bms...

(Now to add some piss to the 💩 the pack had some chargeport problem and the owner took it to some "relative" to change it to a more sturdy one and he wired it back together badly. He put the red wire from the chargeport straight into the last (red) string of the bms but he didnt solder it on the battery positive it was just hanging in the air like this so positive charge wire only connected to B13 string of bms. After soldering it back correctly the BMS seemed to have been working but i dont know if it was the cause the death of the bms. It was operating well for a few weex after this repair but now it's caput as it got depleted. So i'm not entirely sure what caused the death of the bms. My first guess was the "work" of the relative but theoretically i couldnt really have caused it...so right now i think it's much more likely that it's the parallel usage and a triggered lvc of the bms)


I'm thinkin about bypassing the discharge on both pax and only use the bms as charge protection. The owner has two chargers anyway. I'll give him some handy small voltmeter that he can plug into each battery to see if there's no voltage diff before paralleling them. I dont wanna experiment anymore and kill another bms tbh. Or the other option is these pseudo diodes...
 
Sattva Ram said:
Given an engwe bike added another dp6 on top of the frame to the stock battery. I put them in parallel with commonport BMSes. It was working fine until it got depleted and i presume the low voltage got tripped with the aftermarket pack and ever since then the bms is dead. It's not my bike so it didnt happen under me but the battery is kinda empty now so i guess the LVC got triggered in it.
Crappy BMS, not uncommon. Lots of threads about dead ones from various completely normal usages, because insufficient parts were used, or design flaws, etc.



I thought about adding mosfet diodes but i decided against it for i thought two common port bmses must work in parallel no matter what. Also who knows if the reaction time of these pseudo diodes are good enough...i
The BMS FETs are the same as any other FET as far as the diode behavior goes (different FETs have different specs, but...same idea).



I'm thinkin about bypassing the discharge on both pax and only use the bms as charge protection. The owner has two chargers anyway. I'll give him some handy small voltmeter that he can plug into each battery to see if there's no voltage diff before paralleling them. I dont wanna experiment anymore and kill another bms tbh. Or the other option is these pseudo diodes...
Given the previous behavior of the owner regarding the "repair", I would never want to give them unprotected batteries. They'll probably run them down to totally dead while riding, damaging cells and eventually causing themselves a fire, probably while charging it unattended.
 
Meeeh i was catastrophizing superfluously. The friggin switch was the problem it went to shutoff. Either the bimetal temp switch was fkin around or there was some contact problem on the tab...

I have a bms test pack from very bad cells that surge and sag very badly i can immediately shoot up or down the voltage to cutoff levels to see if the bms reacts...so i can test bms-es easily and very well also...

This bms seems fine it also balances nicely so i left it in. However i sooo fkin hate dumb bms-es these days...i try to avoid them like the devil it's just not wort it to save 25 bux. JBD has very thin nice new bms-es these days.

I just have another pack that uses the typical 35A 13S ali bms. Guess what it fkin lets the cell go up to 4.3and then cut...i also had an imbalance in the pack and i just saw one cell at 4.29 i'm like fkin eh....i chuck this 💩 out and buy proper smart bms costs what costs...
 
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