I would look at the BQ26231. This chip you install to measure current flow across a very small resistor that is in series with the negative terminal of the battery pack. The chip contains counters that track the charge state in real time. Your 8051 would then communicate with this chip to access this information and thus manage turning the charger on and off accordingly. This chip is available at DigiKey in qty 1 for about $3.

There are other more expensive solutions around that will fully manage the charge process and not involve the 8051. Look at other offerings from TI or Maxim.
Mike Karas