They'res a a whole bunch of Gas Gauge chips for Li-Po batteries... about 136000 results in .35 seconds (I have a slow internet connection) Here is one.
TEXAS INSTRUMENTS - BQ27210DRKRG4 - IC, BATTERY GAS GAUGE, LI-ION, VSON-10 From Newark and I am wondering why re-invent a wheel that is already invented.
Take the results from any of these IC's and set a lower limit for warning.
Doc