Best is to use an external charger as shown here. After the second green "Charging" LED goes out, I flip the battery over as shown on the right charger. Lacking that, the Internal charger can be used. Just re-insert the battery into the Scooba core and plug the core in.

On the 5900 and some of the 5800 models, there was a "Bug" in that the "Done Charging" voltage level was set to a level that the battery couldn't reach and many batteries were wrecked because of overcharging. This can be avoided by removing the battery from the charger after about 4 hours of charging.

Once a NiMH battery has been placed into use, it is best to cycle the battery at least once a week. Do NOT let the Scooba sit for long periods of time in storage.