DMcQuin is correct. Most wall warts designed for charging the typical 4 cell packs have an open circuit voltage much greater than the battery to be charged. When you connect it to the battery to be charged, the internal impedance of the charger is enough to limit the current and the resulting voltage measured at the batter (while it is connected to the charger) is in the proper range. The same thing happens when you connect it to the 5 cell battery, it is just that the current is now less. To determine how long it will take to charge the 5 cell battery, just connect an ammeter in series with the battery and charger. The time it will take to fully charge that battery will now be the capacity of the battery devided by the measured current multiplied by 1.6 to account for the ineffeciencies of the charge chemistry and to insure that all cells wind up fully charged and balanced. Check out www.rcbatteryclinic.com for some good advice on how to use wall warts for batteries they were not intended for.