I'm using a CX-10 3,7V lipo battery to supply a circuit. To charge the battery I have a USB wire that provides 5V to the battery, but If I charge the battery when is connected to the circuit I would damage it (it is working at 3.3). I tried using a step down converter between the battery and the circuit, but the power consumption is too high for my application (5mA). Is there anyway to isolate the battery from the circuit, or even better, stabilize the voltage at 3.3V when charging the battery?
And the second problem:
I would like to switch on a led when the battery reaches it's maximum capacity when charging. I'm currently reading the voltage of the battery and from it I can know it's current capacity, but when charging the battery the voltage will be always 5V isn't it? How could I do it?
Thank you very much.
Connecting 5v directly to a LiPo battery is NOT okay; you must use a proper charging circuit that limits the charge current and prevents a voltage greater than 4.2v from ever being applied to the battery.
Applying a voltage in excess of 4.2v to a LiPo battery will damage the battery and potentially start a fire. LiPo batteries burn almost explosively.
The battery you have been testing with may be damaged already; it would be wise to discard it.
I would use a charging IC - something like the MAX1551 or MAX1555 would be my pick; dead simple to use - just needs 2 external capacitors. One of those two models has an indicator pin that pulls low when charging.
If the rest of your system will work off of any voltage from 3.7v through 4.2v (instead of 3.3v), I would run it directly off the battery, thus saving the cost, complexity and quiescent current of a regulator.
Otherwise, use a 3.3v linear Low DropOut (LDO) regulator spec'ed for dropout of 0.4v or less and low quiescent current, like the AP2114 (which has the same pinout as common 1117 regulators, but much lower dropout). Be sure to follow the guidelines in the regulator datasheet regarding external capacitors; many regulators, particularly high-spec LDO ones, are picky about the caps.