Charging Li-Poly Battery with Small Solar Panel


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I have Li-poly battery that is 4.2V 2000mAh with a protection circuit. I have solar panel that is 5V 250mA.

The aim is to charge the battery to maintain a fully charged battery, so it really preforms as a top up charger. So if the battery is completely low the first step would be to use a Li-poly charger ( to charge the battery and then use the solar panel.

I would like to know if there is a disadvantage of my setup as my tests have found it to be the most efficient way.

Setup I have not used a li-poly charge IC, as i have found they require a minimum voltage. When the sun is low this would mean no charge at all as the current draw from battery would completely drop the input voltage, as my panel size is very small. I feel as my current charge is very low, as it is below 300mA, i can get away with this setup. But if i had a much larger solar panel, I would need a dedicated charger IC.

But what i have done is the following:

1) I have connected my solar panel via a diode and load switch to the battery.

2) Using a ADC and voltage divider circuit (via 2nd load switch), i am continuously monitoring the voltage so that once it reaches 4V i would switch off the charging process. This is done using a micro controller.

I have noticed even when the sun is low, and therefore the solar panel is producing only few mA, the battery still get some charge. The current draw on battery never effects input voltage by dropping it to low as the voltage drop on solar cell never drop below the battery voltage. Where as with a Li-poly IC i have noticed, that there would be no charge in these conditions.

Any disadvantage of this setup? As it seems to be working well.