Put on the drained batteries without polarity. The eneroid robotic battery charger is a fully motorized automatic rechargeable battery charger. This battery charger can fit 20 batteries AA(AAA) at a time with ease. When the charging starts, the batteries in the waiting stand by-room will channel to the recharging slots to be quick-charged. About 4 batteries will be charged at once and it will take and an hour. The charging will be continued until all batteries in the battery storage are completely charged. The fully charged batteries will be carried to the battery stroage automatically.