Start an a/d conversion, count the number of 50hz cycles and when you know you've done enough for the a/d module to have finished (depends on clock speed etc, but its probably only going to be one cycle, if the processor clock is fixed then the time per conversion should be the same, 1024 atod clock cycles x the division ratio you've programmed) then read the battery voltage and process it between 50hz o/p's.
As Nige says with interrupts its easy.
Another way if you dont like interrupts, setup timer1 and poll it to see when 10ms has passed, if you poll it quick enough it'll be fine.