Electronics > Projects, Designs, and Technical Stuff
Looking to (re)build a circuit to discharge my lipo batteries to like 3.9-3.7
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gilligan:
These guys make a pretty slick little device that drops the voltage of batteries down to their storage level.

RC stuff comes in 1S, 2S, 3S, 4S.... configurations which are 1 in series, 2 in series, etc.  So the battery drain level is different for each configurations.

$5 is not a bad price for these guys but I also have a bunch of 1S batteries that I need to discharge to storage voltage.

Can anyone look at this and napkin up me a schematic and/or explain how they are doing this?

That's either a shunt resistor or more likely a higher wattage load resistor (I have a few of those), with a small driver circuit for an LED.  Are they basically using that LED as the driver for the transistor running to that load resistor which once it drops below a certain voltage (set with the other resistors) the LED goes off and the drain stops happening?

not1xor1:
the 3 terminal device might be a TL431 or similar shunt regulator
as soon as the supply voltage gets below the value programmed by the resistors divider it goes off and let just 0.1-1µA current through it

here is a simulation with a capacitor used as a rough battery model


the green trace is the battery voltage dropping to about 7.6V (60% charge) at the end and the red one is the discharge current
eblc1388:
not1xor1 has shown you a 2S circuit and I'll show you the 1S discharge circuit, similar to his. I added the LED for indication. The LED will go off when the discharge process is completed.

The main discharge current is through the 68R resistor and the LED. This will continue until the cell voltage has dropped to the desired cutoff level, selected by user as per the value of resistor R5.

There is is still some small current drain when the LED turns off so the circuit is not meant to be connected to the cell for storage purposes as it will slowly drain the cell down.

 
magic:
A single transistor could perhaps replace the TL431 if you can live with a few % of discharge voltage variation over typical room temperature range. Also, a low voltage TL431 variant like TLV431 or many others could be useful.

It is interesting however that the original circuit clearly uses some other method, because resistor values just don't make sense.

These are, as anyone can see: 31Ω, 5100Ω and 3.6/5.1/8.2MΩ, depending on battery voltage. They must be using some clever obscure IC here.

edit
Let's do some math.
Reference input current of TL431 is 2µA typical, flowing into the chip. This times 3.6MΩ plus 2.5V makes 9.7V, 3.2V per cell.
TL431 output saturates down to 2.5V, leaving between 7V and 10V across the resistor, for 30~40mA of discharge current and up to 400mW dissipation in the resistor.

For 4S it's 3.17V / 33~23mA / 460mW and for 6S 3.15V / 20~15mA / 450mW.

Could it be? :scared: :-DD
Siwastaja:
It takes ages (days to weeks) to discharge through the small resistor and TL431, so you need to leave it mounted. Then there's the risk you forget it connected, and because the quiescent current is fairly high with that resistor divider in there, there's a risk of destroying the battery by overdischarge during storage.
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