# battery charging with charge pump

#### bgk

Joined Oct 11, 2006
1
Hi,

We're trying to figure out how to charge a Li-ion battery 3.7v from a source that's only about 0.4v and able to supply a constant 40-70mA of current. Would a charge pump be able to do this for us? Of course, it would be intermittent, but I could imagine ramping up a voltage greater than 3.7v, applying it to the baterry, reset, and do this over and over again.

Any thoughts out there? Has this been done before? Anybody have an off the shelf chip they know about?

Thanks,
Brian
brian04@(nospam)audionce.com

#### thingmaker3

Joined May 16, 2005
5,083
I've never heard of a DC-DC converter able to use such a tiny input. PN junctions have a finite voltage drop. Of course, the universe is full of things I've never heard of...

#### jtodd5781

Joined Sep 29, 2006
6
I can see a charge pump working, but like the previous post stated. I think your source is too small for it to work. Not many components work in the .4V range.

#### mrmeval

Joined Jun 30, 2006
833
.4 is pretty low, you might be able to come up with a mechanical solution, create a make break circuit (mechanical vibrator) so that the DC is interrupted and boost it up with a transformer.

It will probably not limit itself to 70ma.

#### n9352527

Joined Oct 14, 2005
1,198
Here's an idea, bootstrap a boost converter with the battery voltage that you want to charge and then use the output to charge the battery. You, of course, have to realise that this wouldn't be an efficient way of charging a battery. You'd only probably get less than 4mA net charging current with 70mA input, not really worth it. I've done this sort of circuit before, but the input voltage and current were little bit higher than yours.

This, however, wouldn't work if the battery is flat empty when you charge it.

#### Søren

Joined Sep 2, 2006
472
Hi,

We're trying to figure out how to charge a Li-ion battery 3.7v from a source that's only about 0.4v and able to supply a constant 40-70mA of current. Would a charge pump be able to do this for us? Of course, it would be intermittent, but I could imagine ramping up a voltage greater than 3.7v, applying it to the baterry, reset, and do this over and over again.
Sounds like youre trying to use a solar cell ?
If that's the case, go for a couple more cells, that will make life much easier

That said, a switcher with a germanium transistor should make it possible.
I have made switchers running down to 0.6V with silicium transistors.
A similar project with germaniums is still awaiting some leftover time (and inspiration ), but I assume ~0.3V is possible.

#### Søren

Joined Sep 2, 2006
472
Hi,

This, however, wouldn't work if the battery is flat empty when you charge it.
A flat empty lithium battery is a dead goner anyway, so that shouldn't be an issue.