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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SiliconWizard:

--- Quote from: Kasper on December 30, 2019, 04:29:15 am ---
--- Quote from: SiliconWizard on December 29, 2019, 11:16:42 pm ---A relay... that would be absolutely not accurate, not by a long shot. The added fun would be that it would probably get into an oscillating state when the voltage gets close to the min operating coil voltage... furious clicking... Fun stuff. :-DD

--- End quote ---

The clicking could notify the user the drain is complete ;)

--- End quote ---

True. ;D

I suspect the clicking would actually not be that furious though, it would likely be more like slow cycles.

SiliconWizard:

--- Quote from: not1xor1 on December 30, 2019, 05:18:46 am ---In any case, with just few more components, it is easy to design a few µA leakage circuit (around 1µA in LTspice). It would cost no more than 1-2 euros/USD and might be a good DIY project.  8)

(Attachment Link)

--- End quote ---

Yup. Nice.

gilligan:
The original is designed to drain 6s batteries... Those are typically pretty large mAh... Would this tl431 be able to do that?

I'll likely whip one or two of these up to try... Especially given my batteries are coming in WAY before the rest of my gear lol.

not1xor1:

--- Quote from: gilligan on December 31, 2019, 03:56:36 am ---The original is designed to drain 6s batteries... Those are typically pretty large mAh... Would this tl431 be able to do that?

I'll likely whip one or two of these up to try... Especially given my batteries are coming in WAY before the rest of my gear lol.

--- End quote ---

That's not rocket science, just basic physics and math.

If you do not feel confortable with that old brewer Lord Joule, let's just speak about A * h.

A 10000mAh battery can provide 10A for an hour.
If you discharge that at 100mA (the limit for a TL431) it would FULLY discharge in 10000/100 hours. That is 100 hours.
But we need to discharge them up to 60% of capacity so if there were no losses in the conversion between chemical energy and electrical one that would be 40 hours.

Now taking into account the energy conversion losses and a safer (for TL431) current of 80-90mA the discharge time would still be around that, likely 35-45 hours.

And do not forget the original post. The circuits portrayed in that photo are quite likely in that range of discharge current, 50-100mA.

magic:
They claim 500mW discharge power.

https://www.hobbyrc.co.uk/lipo-suction-4s-xt60

So I wasn't that far off but they also claim overtemperature protection so perhaps it's not TL431 after all. And 3.9V rather than 3.2V termination so I'm not sure how they are doing it with so few components. And low leakage.

Maybe OTP is a polymer fuse ::)

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