The Smart Dual Battery Charger/Recalibrator  is a great option for people who have extra concentrator batteries, as it allows all batteries to be rotated and kept fully charged at all times. This charger also acts as a calibrator, which means the batteries will be kept at optimal health to ensure longest life.
The Smart Battery Charger will recharge up to 2 batteries at a time as well as recalibrate the battery's fuel gauge. Fuel gauge recalibration enables the battery to calculate a highly accurate estimate of remaining battery life, shown on the battery fuel gauge, even as the battery ages.