I been using Li-Poly Batteries on a GSM based system for some time. My original spec of the battery was 2000mAh 4.2V, with protection circuit.
Recently i upgraded my battery to 5000mAh 4.2V, still with a protection circuit.
My Battery charger IC is stc4054, set to charge at 0.5A
To confirm that i was getting at least twice as much life, i charged the battery fully. And let me system drain the battery for few days to my system was reporting 0% battery capacity.
With the original(2000mAh) version , when the battery reached 0%, even after few days. If i plugged in the USB , the battery would get charged.
But with new 5000mAh, when the battery reached 0%, it would not charge the battery.
After some investigation it seemed the battery voltage reached under 2.8V.
I had to set my desktop power supply to 4V with current limited to 100mA. I then charged the battery directly for about few mins till it reached 3V, after which i then connected the battery back to the system to charger properly with Li-Poly Charger IC.
I know i should not charge the battery directly, but i limited my current to 100mA as a protection. Which seemed to recover the battery from this under voltage stage.
Is this a normal behaviour of Li-Poly battery? Or could the protection circuit not include an under voltage protection?