Automatic Battery Discharger/storage voltage

Thread Starter


Joined Feb 1, 2019
Hey fella's,

In the RC hobby we use a lot of LiPO batteries. They last longer and are safer if they're stored at there storage voltage.

There are some cheap automatic dischargers on the market(pics below) which is just a female xt60 connector with a small circuit board attached to one side, and the circuit is super simple... Just made from passive components since each one is designed and dedicated for a certain series cell count 2s,3s,4s,etc... (Picture included)
I'm trying to figure out the circuit it's using.

Can someone explain to me how the circuit works inorder to disconnect the battery from the power resistor?
I'm assuming that all of the resistors are just for the LED portion of the circuit. And they're using a transistor with a "vgs/turn on" that is close to the batteries storage voltage. So as the power resistor slowly lowers the voltage of the battery it eventually Falls below the "vgs / turn on" of the transistor which opens the circuit... Correct?

I was also thinking that just a zener diode could be used also. I'm just not sure if there's any downsides with doing it that way.

Something like this:
1613682044010_0_New circuit.png
I appreciate any help. I've used some of these before and I know that they work so I trust using a circuit as there's.

I know there is some more complex circuits which can determine the cell count of the battery. And the discharger can be used on batteries of different cell counts. But I'm interested in just making it work with just one type.

Thanks again.
Below are multiple images of the circuit in question



Joined Apr 7, 2016
Ahh OK. So it's using the lm431 /tl431 which is a "adjustable zener diode" and R1 & R2 are setting the value.

Correct? And the LED also only has a pasture ground if the LM 431 is closed
Yes that's my guess, with the number of components on the PCB.

But I forgot the fuse in the + B connection.


Joined Nov 6, 2012
If you want to build your own, super-simple, but slightly bulkier, dis-charger,
you only need an appropriately sized and rated Zener, and a Resistor.

The trick is, getting the Heat-Dissipation calculations right.
Use Ohms-Law, it will give you all the answers.

Under ALL conditions, the Zener Heat-Dissipation will always be calculated by multiplying ......
Zener-Rated-Voltage X Circuit Current = Watts of Heat-Dissipation from the Zener .

The Resistor is calculated differently .......
The Resistor will only have the % of Battery Voltage THAT IS HIGHER THAN the Zener Rated Voltage.
So, if you have a 12V Zener, and the Battery is Charged to 15V, the Resistor will only "see" 3V,
So, 3V X Circuit Current = Watts of Heat-Dissipation from the Resistor.

Total Heat-Dissipation .......... =
The Battery Voltage at the time of measurement,
The Circuit Current at the time of measurement,
Total Heat-Dissipation at the time of measurement.

The Total Heat that must be Dissipated changes from ~"Maximum", to Virtually Zero,
as the Dis-Charging Cycle runs from "Starting Voltage", to "Finishing Voltage".

Roughly ~75% of the Dis-Charge Cycle will be close to "~Maximum" Heat Dissipation,
then, in the last ~25% of the Cycle, the Heat-Dissipation will drop-off to zero rather quickly.

You can get Zener-Diodes and Power-Resistors in High-Power Packages,
These Packages, when Mounted to a Large Heat-Sink,
with, or without, a Cooling Fan,
can Dissipate a substantial amount of Heat,
making the process much faster,
the Battery can power the Fan while it is being Dis-Charged.

Do not discharge your Li-Pos at more than "2-C" on an average, continuous-Time, basis,
you may cause the Battery Cells, or Battery Wiring, to Over-Heat.
Yes, I know ......
You have the latest "Trikk-Wazooo" "10,000-C" Rated Batteries,
but don't go there if you want them to survive long-term.

A 750-maH Battery means 1.5-Amps Continuous Dis-Charge, maximum.
2-C Dis-Charge = ~30-minutes,
you are only removing about ~25% to 30% of the Battery's Maximum Capacity,
the Dis-Charge procedure should take around
~10-minutes, at a 2-C Dis-Charge-Rate, in this example.

This, of course, assumes,
that your chosen components are capable of safely Dissipating that much Heat,
in that period of Time,
without releasing the "Evil-Blue-Smoke",
which all Electronic Components are made of.
.Battery Dis-Charger 730ma Flat .png