Homemade Ebike battery randomly failed.

[05silgto]

Okay, I made a battery pack a couple months ago by buying 2 cheap Chinese 48v battery packs on aliexpress and stripping them and then reconfiguring the cells to 20s 5p to make a 72v battery pack. I bought a 72v BMS off of amazon.
https://www.amazon.com/gp/product/B...n_title_o06_s00?ie=UTF8&psc=1#customerReviews
Anyway, battery pack worked well at first, I didn't test it that much initially because I didn't have much time when school started. However recently, I went on a few rides back to back without any issues. Then I go on a ride with my friend and at one point early in the ride we stop to take a break, while stopped I noticed the battery symbol flashing on my display and it was only reporting 60v. So I head home and test everything. Plugging in the charger it turns green immediately and reports 84v both on the display and a multimeter. Unplugging the charger and putting any load on the battery including just powering the display causes the voltage to drop rapidly. I took apart the battery pack and while removing my electrical tape I believe I undid a few tabs that I had soldered(Note: the entire battery pack was not soldered, just enough to join the 2 battery packs and a group of cells). So I started to solder these tabs back on and halfway through doing so I noiticed the bms started to smoke, I unplugged the balance wires as quickly as I could which seemed to stop it but the board was already damaged pretty bad. I wired up the battery to the bike without the bms and it ended with the same results, any load causes the battery's voltage to immediately drop and charging it causes to immediately jump to 84v. It seems like it won't hold a charge and the cells are in my opinion are ruined at this point.

Anyway, I have a few questions about this.
Does anyone think the bms might have caused this?

Are the cells for sure ruined or is there a way to restore them?

Does anyone know of a place to get quality battery cells/36v battery packs for cheap? I would prefer to buy 2 36v battery packs to make a 72v battery since I don't have a spot welder.

Lastly, does anybody know of a reliable 72v BMS I can buy?

 

[fechter]

Sounds like either a bad cell connection or a bad cell group.

If you put a voltmeter across each cell group while applying the charger, you should be able to find which one is drastically changing voltage and track down the bad spot.

If you are lucky, you may find just a bad wire connection somewhere and the cells might be OK. If you're not so lucky, you may find one or more cell groups is toast and the cells need to be replaced.

 

[999zip999]

Cheap hear is cheap https://batteryhookup.com/products/hp-43-2v-module-with-24x-30a-lg-18650-cells
Or I'm battery Texas USA for new.
Put a gasket on POS end.
How cheap a Chinese battery pack do you have a link. And add how many amps are you using this 72 volt battery ?

 

[05silgto]

I don't have the link, the seller/brand is viset and at the time they were a brand new seller with no reviews. I paid about $86 each for 2 battery packs which were 52 cells each, price went up shortly afterward but their description did too so possibly also their cells.

My motor initially ran on 45a as its default rating but I was only running 30amps to try and be easier on the batteries since I didn't know what the actual output rating was.

 

[05silgto]

Sounds like either a bad cell connection or a bad cell group.

If you put a voltmeter across each cell group while applying the charger, you should be able to find which one is drastically changing voltage and track down the bad spot.

If you are lucky, you may find just a bad wire connection somewhere and the cells might be OK. If you're not so lucky, you may find one or more cell groups is toast and the cells need to be replaced.

I can give it a shot but I don't know if this will work since the voltage only drops under load, not when disconnected from the charger, it's like there is no capacity at all left.

 

[05silgto]

I measured the battery pack voltage again and it was 74v. I measured each group in parallel by taking a multimeters probes and touching the top and bottom of the same cell in the middle of each group, each time I got .42v except for two groups of cells which measured 0v, 10 cells in all.
Here is a picture of the damaged bms which might help explain what might of went wrong, https://ibb.co/YQHzzBm
Anyway, since these are knockoff china cells I don't know the exact specifications which would make replacing these damaged cells next to impossible assuming the rest of the cells are still good and I don't think 80 cells could support 30amps. It would be nice if I could somehow use this battery pack as a secondary source of power/charge a primary battery pack, but charging a primary battery pack wouldn't be efficient/too time consuming to be worth it unless I could also have it power the controller but limit the output and have a one way diode.

 

[05silgto]

I measured the battery pack voltage again and it was 74v. I measured each group in parallel by taking a multimeters probes and touching the top and bottom of the same cell in the middle of each group, each time I got .42v except for two groups of cells which measured 0v, 10 cells in all.
Here is a picture of the damaged bms which might help explain what might of went wrong, https://ibb.co/YQHzzBm
Anyway, since these are knockoff china cells I don't know the exact specifications which would make replacing these damaged cells next to impossible assuming the rest of the cells are still good and I don't think 80 cells could support 30amps. It would be nice if I could somehow use this battery pack as a secondary source of power/charge a primary battery pack, but charging a primary battery pack wouldn't be efficient/too time consuming to be worth it unless I could also have it power the controller but limit the output and have a one way diode.

Cancel some of that. I had my multimeter set at 200v when I was measuring the cells individually and they actually measured 4.20v not .42, I just remeasured the cells and only 5 cells, one group is toast. Already removed the strips from these cells just in case to prevent future damage, thinking about redoing the battery pack with 22s 4p since it only puts 1.5 extra amps on the cells and the controllers capacitors are rated at 100v, thinking about avoiding using a bms and just go with some type of charging board. Not sure if the bms might have caused this or not, I'm about to hook up a light to the remaining cells to see if the rest are still good.

Edit: I hooked up a 60w light to part of the battery pack 3s 5p and kept a eye on the voltage, I maintained a steady voltage so I guess the majority of the cells are still good. I have 4 extra cells I didn't use originally when I built this pack so it kinda sucks that I'm one short of what I need. Anybody think it will be a big deal if I assemble the pack with 99 cells and just short one group one cell?

 

[Chalo]

I'd get one more cell, of a larger capacity than the rest. (The four unused cells may well have larger capacity than the used ones. But maybe not.) Having an outlier cell group is much easier to deal with if it's stronger than the rest of the pack, rather than weaker.

 

[05silgto]

My battery pack has very little use but I made sure to charge it every now and then the past few months, the other 4 cells have no use but probably have only been charged once or twice, so it's difficult to say which cells are in better condition.
Thinking of just getting 1 18650 Samsung 25r cell, should be about the same capacity if not better, and definitely enough discharge.
I found this battery charger board for cheap, but I'm not sure if this is what I need, says it lacks balancing function but has balancing protection.
https://www.amazon.com/dp/B07RKX89M...olid=3PS8JYBF5UNR6&psc=1&ref_=lv_ov_lig_dp_it
I didn't like my previous bms and feel like it could have caused the failure, plus the bms cutoffs prematurely at 68v completely before the controller does with no warning.

 

[Jan-Erik-86]

Sounds more like your cell group was bad from the start and the bms only shut down at 68v to protect you and the battery because that group was over-discharged. In other words, it did exactly what it was supposed to. I can't explain why it smoked when you reconnected the leads though, but it seems more likely you have a bad/shorted cell rather then the bms caused the failure.

While a cheap bms might not be too trusty, running unknown china cells without any bms or monitoring is like asking for a fire or explosion. I would strongly suggest not to skip the bms unless you know exactly what you are doing. At least do make sure you balance charge if you skip the bms.