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FrozenGate by Avery

Will this work as a DIY a 2S lipo charger?

Diodes and zeners won't work since they have a very wide accuracy tolerance typically 5%. You'd need better than 1% for lithium ion CV termination voltage. Plus, diode / zener voltage drop varies with temperature, current and phase of the moon.
 





djQUAN said:
Diodes and zeners won't work since they have a very wide accuracy tolerance typically 5%. You'd need better than 1% for lithium ion CV termination voltage. Plus, diode / zener voltage drop varies with temperature, current and phase of the moon.
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I wasn't considering that route - it didn't make any sense to me. Somewhere in the equation there has to be an IC with some brains - even if that's just a linear regulator.
 
rhd said:
I wasn't considering that route - it didn't make any sense to me. Somewhere in the equation there has to be an IC with some brains - even if that's just a linear regulator.
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Yes, sorry, that was for The Lightning Stalker since he mentioned using either a string of diodes or zeners.
 
Any thoughts about the viability of this?

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Since this is a little bit outside of my usual wheelhouse, I've held off on laying out a board for the time being (hoping that a second set of eyes can glance at this and give me a yay or nay).
 

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You'd just need a current limited source for the input, also, you'd need to disconnect the pack after charging.

And one more thing, the op amp may not be able to source/sink enough current to balance mismatched cells in bad packs but in a well matched pack, it should be no problem.

Other than that, it will probably work.
 
djQUAN said:
You'd just need a current limited source for the input, also, you'd need to disconnect the pack after charging.

And one more thing, the op amp may not be able to source/sink enough current to balance mismatched cells in bad packs but in a well matched pack, it should be no problem.

Other than that, it will probably work.
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The OP AMP, is capable of 3A - I chose one that was very high current.

Since I have enough voltage overhead, would it be sufficient to nest everything above inside a current regulating setup of another linear regulator? Ie another 1085?
 
Ah I didn't check the op amp datasheet. If so, that'd be plenty.

How big are the cells that you plan to charge with this thing?

Yes, with enough voltage headroom, a constant current regulator before the voltage regulator will work to make a CC-CV supply.
 
djQUAN said:
Ah I didn't check the op amp datasheet. If so, that'd be plenty.

How big are the cells that you plan to charge with this thing?

Yes, with enough voltage headroom, a constant current regulator before the voltage regulator will work to make a CC-CV supply.
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Around 100 mAh
 
With djQ's inputs I'd say it looks like it should work fine.

If you add series resistance mind there will be a voltage drop, depending on expected current this will or won't be an issue. I'd just go with a front end CC regulator, personally.
 
Looks good - but it may oscillate if you don't stabilize it somehow. I would put a cap from pin 1 to ground, myself.
 
Sigurthr said:
With djQ's inputs I'd say it looks like it should work fine.

If you add series resistance mind there will be a voltage drop, depending on expected current this will or won't be an issue. I'd just go with a front end CC regulator, personally.
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Cyparagon said:
Looks good - but it may oscillate if you don't stabilize it somehow. I would put a cap from pin 1 to ground, myself.
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With this advice in hand, I've revised the schematic as follows:

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Conceptually it bothers me to have a current regulator that I know will (in all likelihood) never actually be in regulation. I can't articulate a reason that this would be a problem though.
 

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Below the current limit threshold, the current regulator simply conducts to max (lowest voltage drop). Voltage drop increases to regulate current when the threshold is reached.
 
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