Misbehaving 15s3p 48v 20Ah, not the BMS(?)

[gfkBill]

Hi all

Just picked up for 48v 20Ah "Polinovel" battery, seems a nice unit and got a good price, but it does turn out to have a fault. Seller, to his credit, was happy to refund, but I'd like to try to fix it.

Voltage across all 15 cell groups is a nice even 3.3v, including group 1. But the BMS wiring is reading only 0.8v from 1 to 2. I suspect it's to do with the circuit board, which has a bit of extra protection, see photo. This is battery -ve end of things. Looks like a 10M resistor? No resistance between BMS connector #1 and the PCB contact pad visible below so wiring is fine. All battery groups have this circuit on their -ve end. Resistors on #1 appear practically open circuit, others all measure closed. Although voltage from 15 +ve to B- charge cable side of #1 here is 11.8V (vs ~50V at the actual #1 battery bolt), so a bit getting thru.

Previous owner was mucking about using this on a e-bike. Suspect he's blown/damaged these resistors? Are they important, or could I just short them?
(Given the history, there will be several charge/discharge cycles out on the concrete before I trust this unit!)

EDIT: Clarity

 

[gfkBill]

Sorry - could Moderator move to Electric Technology? Somehow ended here by mistake when in the process of registering and I can't see any way to move it

 

[neptronix]

hey, i moved things, this is the appropriate section!

 

[gfkBill]

(Thanks Mod)

Finally pulled those batteries so I could properly test them and the resistors. Not really sure why the resistors are there, and they are open circuit on that 3P group. Trying to think why I shouldn't just short them, only reason I'm not is I can't work out how having 10meg resistors there works or how they would cope with 20A/3 sustained that this battery is rated for. A normal circuit would just short the -ve terminals to the charge terminal, along with the #1 bms volatage sense wire

Is it actually a surface mount fuse? Because that would make a ton more sense.

 

[Gruesome]

I would definitely not short them. As you are correctly stating, 10 MOhm is very close to open. I'd guess they are just there to drain any vagabonding rogue charges drifting around, without putting a load on anything. From where to where do they connect?

 

[gfkBill]

Thanks Gruesome. I've pretty much concluded they're actually fuses. They appear to be in the main current flow. If you look at the photo, the only way current can flow is through those SMD's. I wasn't certain of that until I disassembled and could see there were no traces on the other side of the PCB - there aren't.

Also desoldered one of the SMD's to make sure there was no trace under it, there's not. The other places they exist in the circuit those SMD's read as almost zero resistance. There's no way you could put 10A thru a resistor that size

A normal battery pack design would just have a direct connection there from battery -ve to charge cable, so I think the manufacturer has just gone belt-and-braces and put fuses thru the pack in case of a cell shorting. Next step will be to load test those batteries to make sure they are behaving themselves and there wasn't a short, but they all show 3.3V as expected, not sure a a shorted cell would do that.