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Simple lithium battery charger by solar:

Status: Proposal
September 3, 2015
For another project (pollution detector), I tried to build WITHOUT dedicated IC (which are by the way very convenient) a lithium battery charger from a 5V solar panel. I decide to use a standard regulator LM2931CT (adjustable). I planed to use the low consumption version LP but I could not find it where I'm living. At beginning, the LM2931CT was directly power by the solar panel. It was use in 4.2V max output voltage. A Schottky diod to avoid the battery to be drain when no sunshine. First, it was working well when I came from inside to outside and sunshine. So, I put it outside (it was with the pollution detector) but after few days, no more battery and never restart, even with sunshine. I took all back and check. In fact, I found that when the sun was not bright, the panel voltage was below 5V, between 2v to 4v. Using a power supply, I found that at this value, until 4.9v, the LM2931CT was draining all possible current. Only if the input voltage was immediately above 4.9V, it has no drain current. This problem, with the solar panel makes that with low or more sunshine, all the current was drain by the LM and the voltage never increase. The solution tried and found is to use a simple NPN transistor, with a base which will put to GND the enable pin of the LM when the input voltage will be above 4.9V. I could use a more precise voltage reference AOP but so far, the NPN works. Please find the schematics. Quite simple. 2 adjust resistances, P1 and P2. P1 need to be adjust with 5V in input (instead the panel) and find the position for the LM just enter in regulation. P2 is to have exactly 4.2V. On the pollution detector, I add a LED just after the diod for visual the regulator working. Laurent
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