Calling all battery builders - is my 8S4P pack beyond repair?

I’m crazy. But I would just check the voltage of each p group. If they are all similar, I would reweld the connections and run with it. I agree that the s connections acted like a fuse.

Yep, super thankful that it didn’t burn the house down, but still cannot help feeling bummed that this has happened right at the end of the build. I have spent a ton of cash in getting to this point and I’ve been building it for an eternity (slowly buy piece by piece). Plus the timing sucks as currently in the process of changing job and moving house so will not be able to spend much time or money on it for awhile. Anyhow I must keep telling my self it could have been a million times worse. Sorry rant over.

Although there are burn marks on the series connections, they all seem to be intact. The only nickel strip to break was the one connecting the BMS to the pack (first picture above).

I think the battery pack is ok as one my colleagues at work was able to charge the pack by bypassing the dead BMS using A PSU.

Unfortunately I don’t have a spot welder and will not be able to get one anytime soon. So I was thinking that I’ll probably take a risk and replace just the BMS.

Do you know of anyway to attach the BMS to the battery pack without spot welding it (first pic above)?

Like it was made before by soldering it on it. Get a 10awg and solder it on the bms and on the nickel on the batterie. You sure there is nobody around you who could remake the Batterie? You already had once luck, wouldn’t risk it once more. Better save than sorry with batteries. Especially if you just moved…take care about your new home…200-300$ are less then thousands for rebuilding :wink:

You need to scrap that BMS.

There’s an issue with them, where if there is a load connected to the battery (like it’s shorted) and a series connection within the battery breaks, then the balance cable portion of the BMS will break.

This is because normally the BMS only sees a Max of 4.2V difference between balance cables. When you short the battery and break a series connection, the new voltage differential between the two balance cables on either side of the series connection break jump to your full battery voltage. This almost certainly fries the balancing function.

BUT this is only the case if the nickel tabs fused out. If there’s still continuity within the pack then you might be lucky.

This is also why I’m not a fan of cell level fusing, because in the case of a fuse out scenario it might set the BMS on fire.

See I think I like cell level fuses and don’t like bms’s. Mine is bestech and seems to be nothing but trouble. I would rather just drop the deck once a month or so and check/charge with a hobby charger to manually balance

They’re definitely opposing solutions to one problem.

Agreed. Bms makes it a lot like a factory build, out of sight out of mind, but we diy so I don’t mind opening it up to check things out every now and then

if you get a chance bypass the discharge function and run charge balance only. we have seen so many packs with good cells in them and bad bms units. we will soon start taking request to rework factory battery packs. we can’t rework every battery pack but the majority of them can be fixed…

I decided to go the simplest route for now and just replace the dead BMS with an identical one. That way I can just plug the existing balance wires into the new BMS and copy the soldering of the power button, display, charge port, usb port and any other wires from the old BMS onto the new BMS. Hopefully that will get it up and running again with little effort. If it works I’ll continue to keep the battery pack in the shed.

Can anyone tell me if I need to reconnect the wires to the new BMS in any particular order?

Yes, in the same way it was before :sweat_smile: Just take a picture, or take the pictures you already made and connect the wires like they where before

Do I need to connect the balance wires after soldering the -ve and/or +be wires, or does it not matter which wires are soldered to the BMS first?