You may end up buying a "Battery charger" for RS1000 which does not disconnect charging once battery is full. More then 6 hours of over charging will actually damage a battery dead. My advise will be to go for any UPS smallest KW you can acquire (i guess RS2500 for a 500W which typiclly have charging rating of 10A) and use it as charger. With out load connected at socket it just acts as charger! Also you may want to find out the varistor on this PORTABLE charger PCB,control board, in order to throttle the charging current to slow charging for smaller batteries. If this controlled charging is not a need then you can use your home UPS and just parallel a drained battery to be charged for 4-6 hours for charging.