A unique battery fuel gauge that recognizes both the discharge and charge cycle of your battery. It will sense the current charging condition of the battery, display the percentage of remaining capacity and display a charge trend arrow. The D-130 will continue to measure capacity and trend as long as it is connected to the battery. When an additional “load” is applied to the battery, the capacity will decrease according to the load applied. When the load is removed the digital readout will remain at the last shown level and the trend arrow will switch. This rebound indicates the battery is starting to recover, but the digital readout of the capacity will hold at its lowest level unless you reset the unit by disconnecting and reconnecting the D-130 to the battery. When a charger is attached to the system, the digital readout of capacity will remain the same, up to an hour, with an up arrow indicating that it is receiving a charge, before the digital readout will start to advance. Note, with some batteries, the very top rating for a charged battery (12.7 volts) or 100% rating will not stay at 100% even though it might be fully charged. This is normal. Each battery charges differently and may be fully charged but only show 95%, instead of 100%. This is a normal occurrence with batteries since not all batteries will hold the same top-end percentage when fully charged and is not an indication of a poor or defective battery.