I am searching for a schematic or design that would help me charge a very small 1mAh 3.7V LiPo battery. I've done a pretty thorough search and the closest off-the-shelf IC I can find provides 10mA minimum current. My LiPo is only rated for 0.5C-1C charging, so ideally I'd like to charge it with 0.5mA max current to be safe. I've already tried using a really large resistor attached to the standard PROG pins of various LiPO charging ICs, but it doesn't work because 1mA is so much lower than the minimum 100mA. I have not tried with the 10mA charging IC, but I imagine similar behavior could be expected.
One thought I have, is potentially using the 10mA minimum current LiPo charging IC I found here and connecting a constant current sink to the output, set to sink 9mA or so. I believe this may interfere with the chip's auto shut-off feature though and cause risk of overcharging. There may be a way to add some shut-off logic based off of the CHARGE output of the IC, where I could shut off the current sink thus shutting off the entire circuit.
I really hope that creating a LiPo charger from scratch is not necessary. However, I have minimal experience with electronic design (primarily software engineer) so I am not sure how difficult or not this would actually be. I am able to create and route PCBs, but to do so I need help with a schematic to accomplish this 1mA peak LiPo charging current and this is what I am struggling with. Any help here would be greatly appreciated!
Best Answer
You can try to make your own charging circuit based on LM334 constant current source. Maybe. It's just my first idea. You need to sense battery voltage all the time to avoid overvoltage. And LM334 is, as it stands in its name, provide constant current. When battery reaches 4.2 you have to go to constant voltage mode, where current must fall, which is a tricky part. Don't have a solution for it.
Maybe you can do something with LM317 in current limiter mode. Again, you need battery voltage control. But since LM317 is not a constant current source but a current limiter, it has to be able to do both CC and CV if you can manage to limit it 0.5ma.
And you can always have LM317 having higher current limit if you can't set it that low, but you can load it with constant current source LM334. For example, you can have LM317 set for 1ma max current and connect 0.5ma LM334 to it. So battery current will go up to 0.5ma. This is inefficient of course, so depends on how critical it is.
These are just random ideas straight out of the head, there probably are smarter ways.