Lithium cells voltage protection

Thread Starter


Joined Jun 4, 2019
I need a simple cheap effective circuit that can be placed across a lithium cell such that when a voltage goes over 4 volts, a relay trips and turns on a light. When the voltage drops below 4 volts it turns off the light.
This circuit could then be placed across a lithium cell when charging with a dumb charger.
When the light goes on you know the battery is charged, plus the relay that turns the light on disconnects the charger so the cell is safe.
I was thinking of using a 6.8k resister on series with a 2 volt zener diode to cross the positive negative of the battery
then parallel another 2 volt zener diode and in series with another 6.8 k resistor across the same poles of battery but in different order.
the voltage between the zener and resistor should be 2 volts when the battery is 4 volts.
when the battery is 4 volts then both zener voltage references would be exactly 2 volts and equal
when the battery is at 4.1 volt then the references would not be the same
and when the battery voltage is 3.9 volts the references would not be the same but opposite.
run a lm339L (low voltage) from the battery source
tie the inputs to each reference
and it should work
trouble is I have to order the parts to see if they would work if not can you make a diagram that would work?
What is needed with desperation in this lithium world, is a simple battery disconnect cirduit for charging lithium batteries, they get destroyed if over charged and the commercial cahargers all overcharge the batteries, I want to take mine to 4 volts which will triple their lifespan.
When making this circuit it is not important to get the voltage accurate to within only 5 % or so it doesn't matter if the batteries disconnect at 3.9, or 4.0 or 4.1 or if they are all different. it justmatters that they disconnect with some hysteresis
So I figure a 10k resistor into the LM339 and a feedback of100k from output to the correct input
I want to make this CHEAP so I can make a lot of them since I have 4s batteries I will put one of these on every 4 volt cell to keep them all save then charge the battery in series.
sounds like a plan?

2nd challenge
Make the cheapest possible device to do the same thing
I was thinking of a relayreed typethat latches at 4 volt and when voltage drops it unlatches but I see this is not as accurate...
anyway, any thoughts appreciated I don't have a lot of money and I have to order parts, could you test it so then I can order parts? thank you.


Joined Mar 14, 2008
since I have 4s batteries I will put one of these on every 4 volt cell to keep them all save then charge the battery in series.
So, if I understand correctly, you want to charge the batteries in series, and have each cell bypassed so the other cells can keep charging when each cell reaches 4V?


Joined Jun 5, 2013
You can pick up a proper battery management board on EBay for practically nothing. The make them for various cell counts and currents, and they do real balancing.

Messing with charging Lithium batteries is dangerous.

As a bonus, they do discharge protection as well.



Joined Sep 7, 2010
You want something like this ....

Sounds like you wisely want to keep the voltage down ... all BMS charge to 4.2 ensuring a life of only 400 cycles , by charging to 4V you should get over a thousand cycles , with only 20% reduction in Capacity ...

For the device above you set the two voltages , in your case 4V and perhaps 3.9V you can wire it to charge the battery until voltage reaches 4V , it then switches off and stays of as the battery voltage drops , at 3.9 it switches and charges again.