minute counter for pic

I am looking for the easiest way for a pic (16F877) to count minutes while doing something else..I use a Microengineering picbasic pro compiler and dont care much for assembler...however if its the only way...I have never used interrupts before so maybe I have to begin now , unless somebody knows some tricks. In fact what I want to do is track a charging or discharging battery voltage and knowing the charge or discharge current (which I maintain constant) I can work out the charge the battery has supplied or accepted...provided I know for how many minutes its been connected. Is it a better idea to connect the pic to a real time clock chip and poll it at fixed intervals using for example an I2C bus ? (the pic basic pro compiler has I2C I/O commands).
All suggestions will be gratefully received.