Chargers I've head this many times before "My batteries just keep getting better, they store more power the older they get! ... Well that is not quite what is going on, although it would be nice. Things getting better as they age is just not the way of the universe, nor the second law of thermodynamics. People get confused by their chargers and the readout of Mah used to charge a pack. Most charges simply calculate the total Mah input based on total charge time multiplied by the charge amps applied until the battery peaks.  As a batteries internal resistance increases so does the amount of energy that’s needed to charge (peak) the pack. You will often see the Mah charge readout (on the charger) of a pack increase as it ages due to this internal resistance increase. It is not because its storing more energy in the pack but because its taking more energy to get the pack to what is the same (really lower) charge state.
You really need to run the batteries through a good matcher so you have the IR and Mah rating of the battery, not just what it took to charge the pack. | 
