Lipo battery reversing polarity ?

[Carl E]

Hi,
Something I've never seen.....My hand vac died in mid use. Reading  14v on an 18V pack I found a Lipo battery ( 18650 ) with reversed polarity. It was the last battery in a series string.   I removed the cell, tested it again, .500 volts reversed polarity. Anybody ever run across this? Once I replaced the bad battery the pack was good again.

[BradC]

Charge reversal can happen to the weakest cell of pretty much any battery chemistry in a string under the right circumstances. I've never seen it on a lithium cell, but only because the BMS usually shuts things down before it gets that far. As you've demonstrated, it's clearly possible.

[feedback.loop]

http://liionbms.com/php/wp_cell_voltage_reversal.php

[james_s]

Does the unit have a proper BMS that monitors each cell? You probably already know this, but don't attempt to revive the reversed cell, it has been damaged and charging it could be dangerous.

[amyk]

A lot of cheaper designs don't have balancing and rely on the cells being matched in capacity and charge initially. They tend to last longer if you don't deep-discharge the whole pack before charging it each time.

[magic]

I guess you are also supposed not to overcharge, then

Sounds very cheap and dodgy

[Carl E]

Thanks to all who replied. I have already put the defective cell in the recycle bin.The battery pack does monitor each cell for balance, HOWEVER,  this particular vacuum is a B&D Pivot that has a design flaw that shut off the vac after a few seconds. Once it did that and the switch was turned off then on again, it would run a little less time before stopping again. This could be repeated several times. I put a jumper from ground at the battery to ground at the switch and that solved the problem. Apparently there is some circuitry that broke the ground from the battery pack to the switch and once it shut off, the connection was restored but the switch had to be turned off and back on to get it to run.. I could never catch the open with a DVM. I originally thought there was a thermal cutoff in the motor but that was not the case. It will be interesting to see if another cell reverses polarity. I wish I could see a schematic for it.
Will let you know what happens in the future.