There's a discussion for replacing the cells for robomower batteries at
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You can buy these cells at many battery stores and they're only about $35 each. If you decide to take the DIY approach and replace them without buying from friendly robotics, do it at your own risk!
Robotic Lawn Mower Forums
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"pklo> Does anyone know if you can use a diffrent battery for robomower?
I have the 500 and my battery says "fully charged" even though its not.
> Once you take it off the charger it tells you it needs to be recharged.
> Or does anyone know of a robomower dealer in/near SW Florida?