New 36v battery dead, won't charge.

So ... I bought 2 brand new 36v 21ah Reention Dorado batteries (hoping to convert our MB to electric) from a local auction house. Both are totally flat. Output voltage on one is 2.5v and the other 1.6v. Charger led stays green when plugged in, so assume not charging, although when plugged in I get 42v at the output plug but drops down to 2.5v when unplugged. After scouring the internet, someone said it might be possible to slowly charge each bank with a mobile phone charger to get the cells up to 4volt ish to maybe wake the BMS and hopefully charge up.

Anyone had this problem (and hopefully sorted it!) ?

Any help/advice would be very welcome.

Charger sounds ok, since the battery is so low voltage the bms is probably stopping a full charge from reaching the cells and my be trickle charging them until a safe voltage has been attained?? (Not an expert)

if no luck after a few days (dont leave charging unattended or overnight) you may need to strip the pack down and look at cells individually.

dropping to such a low voltage is not good for the cells and generally they should not be discharged so low.

2.5v battery output indicates the average cell voltage to be 0.25v!!

It is possible to recover a cell thats drained below 1v i have done it with a single cell a diy electronics project drained, but how much luck plays in that i just dunno..

will keep the digits crossed for ya.

So ... I bought 2 brand new 36v 21ah Reention Dorado batteries (hoping to convert our MB to electric) from a local auction house. Both are totally flat. Output voltage on one is 2.5v and the other 1.6v. Charger led stays green when plugged in, so assume not charging, although when plugged in I get 42v at the output plug but drops down to 2.5v when unplugged. After scouring the internet, someone said it might be possible to slowly charge each bank with a mobile phone charger to get the cells up to 4volt ish to maybe wake the BMS and hopefully charge up.

Anyone had this problem (and hopefully sorted it!) ?

Any help/advice would be very welcome.

The first thing you need to do is open up the battery, pull the multipin connector out of the BMS, then check the 10 individualm cell voltages on it. Any that are below 2v are scrap. You might be able to charge them up individually, but they''l be weak and saggy, so not worth it. Anything above 2v is worth trying to rescue.

One may able able to recover a single cell for low power use like a torch but certainly not a bank of batteries in a larger battery pack, they will simply not handle a deent discharge or hold voltage very well.

One has to do as has been said, open the battery and check the 10 cell groups any under 2v then the battery is scrap.

Looks like a job I would have to pay someone else to do ....

Don't try this .......

A very satisfying end to the video ...

Edited January 21, 20242 yr by FastFreddy2

So ... I bought 2 brand new 36v 21ah Reention Dorado batteries (hoping to convert our MB to electric) from a local auction house. Both are totally flat. Output voltage on one is 2.5v and the other 1.6v. Charger led stays green when plugged in, so assume not charging, although when plugged in I get 42v at the output plug but drops down to 2.5v when unplugged. After scouring the internet, someone said it might be possible to slowly charge each bank with a mobile phone charger to get the cells up to 4volt ish to maybe wake the BMS and hopefully charge up.

Anyone had this problem (and hopefully sorted it!) ?

Any help/advice would be very welcome.

May be a long shot, but I purchased a battery new from china last year - it was DOA until I established a Bluetooth connection to the BMS, and then enabled the charging & discharge port.

The general procedure is to tear down the battery to the point where the BMS and its ribbon cable are exposed. You don't have to tear off all the shrink wrap covering the cell array,

The low voltage at the output is only because the BMS has isolated the cells from the output. You probably have significant charge in the battery, enough to cause damage or a fire if you make a mistake. Don't do it unless you've got the skills, Be careful. Work at your risk.

If you unplug the ribbon cable plug, you'll have access to all 13 cell groups via the pins, A quick check of the two most opposite pins gives toy the total battery voltage. Then checking each pair of adjacent pins gives the individual group voltage. If you can get that info, the forum will know if the battery can be salvaged,

Edited February 1, 20242 yr by harrys

My connector was tiny, it's all too damned small... so I got a big magnifying glass for next time, to make taking measurements less nerve-wracking.

Needle probes, with added heat shrink to avoid a short.

Edited February 1, 20242 yr by guerney