He will also be able to fully charge his packs, it will just take longer since it is larger capacity.

You are likely talking about the capacity setting, which really only affects the charge rate (sets it to 1C automatically) or 6.0 amps for 6000 mah battery. If you were to charge a 7000 mah battery with the 6000 mah setting, it would still charge at 6 amps but take longer. Would still completely charge.