Ay disadvantage of this setup of charging Li-Poly battery with small solar panel

Leelin

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 and 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 , 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.

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

For knowing more about solar cells, I found a news posting about solar cells in Kynix Semiconductor Electronic Blog.It helps me a lot.  I hope it can offering help for you if you need.

Photowhit

I think the native fully charged voltage of lithium polymer batteries is 3.7, perhaps 4.2 is the charging voltage. I don't deal with lithium very much, but if;

 "when the sun is low this would mean no charge at all as the current draw from battery would completely drop the input voltage"

I would suggest there isn't meaning full charging current available. 

I've used a 6v nominal solar panel with a simple 5v voltage regulator, to fee many usb devices and charge them. Including a very picky Nokia N800 which rejected all the wall wart usb regulators I tried other than the kindle's original. 

I just allow the nokia's charge controller to regulate the charging. The same should work with a commercial battery bank. I have one that is spec'd the same as your battery. I'm sure it would work. It seems to charge off anything I use with it.