I have a 14s battery made of used cells. I will only charge it to a maximum of 54v (so ideally 3.86v per cell). The pack has a BMS on it already to prevent over discharge but because it never gets to the top voltage of the cells (4.1v) it never balances the pack. My idea was to create a simple cheap circuit to attach to each cell to prevent them going over 3.86v by bleeding off power through a power resistor (design uploaded). My idea is to set the variable resistor such that when the cells voltage reaches 3.86v the MOSFET would turn on and prevent the cell voltage going any higher. I only would want to bleed off a max of 1 amp to keep the voltage going over whatever threshold I've set with the variable resistor. My issue is selecting a suitable MOSFET. I had thought about the irlz44n but am a bit of a newb and am not certain. Any advice on why this MOSFET would or would not be suitable and why?

 

hi Nicholas,
One problem would be as the MOSFET started to shunt the charge current around a cell, wouldn't that increase the effective voltage/current across the remaining cells.?
Battery voltage balancing charging for batteries in series is always a problem.
E

 

I'm not sure it will make much difference based on my own experiments with manually bleeding off current. It seems to work ok. I should perhaps add that it is a 24p pack and the current into each of the 14s blocks never exceeds 10 amps.

 

A problem I see with that approach is that mosfets don't have a sharp on/off action like a switch. They start to conduct at the threshold (which can be quite low) and transition to fully on. Here is an arbitrary graph showing that effect:

 

Wouldn't that actually work better then? The higher the cell voltage is over the threshold the more current will bleed off.

 

By far the simplest way is to use leds ... different colours start drawing power at different voltages ....

So get an led , or series combination that starts lighting up around 3.86 .. don't put a resister in series ... the higher the voltage creeps up the more bright the led becomes conducting more current and quickly balancing ...

I used this method to balance some super caps in series , white start to light about 2.5 ... you may need 2 blues in series I think ... 3W

 

That's an interesting idea, thanks.