The Rascal 600/300, remove the seat, lift off the cover, unplug a battery, replace the old battery with a new one, putting the cable connections on the same way, red is +, black is -. Do the same with the next battery, reconnect the anderson plugs,reinstall the cover and seat and plug in the charger with the key on watch to see the needle raises up toward the full mark, allow to charge fully, overnight is good. keep the bats charged up not allowing them to drop below half way for the first 20 charges to break in the batteries. as a rule charge the scooter after every use!
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I’m happy to assist further over the phone at https://www.6ya.com/expert/bill_bcbb89783b08c489