It's primarily up to the power supply. Cell resistance will show up but it's not the primary limiting factor. This is the problem because a constant 200mah charge when an 1800ma pack is already full is NOT safe for the pack, it will dergrade it's chemistry much much faster than somethign that knows what it's doing. You need an actual intelligent charger, which knows when the pack it's charging is full and stops charging until enough current has passed out of the battery to allow it to again detect when the pack is full when the charge state starts again. The voltage levels and vectors a charger needs to detect a full capacity state only occur at very specific stages near full charge. Approximations can be made, but ALL battery packs need good chargers. A stupid wall wart will shorten the life of the battery pack by 100% or more.