Discharge Voltage – the amount of battery voltage available at any given point while the battery is discharging.
The voltage of a battery gradually decreases as it discharges. The rate of this decrease depends on the device it is powering and the battery chemistry. The voltage in sealed lead acid batteries, for example, tends to decrease gradually, but visibly. In a lithium ion battery the decrease is extremely small until the unit is almost flat at which point the voltage falls off very quickly.
In the above graph we can see how the voltage decreases in one particular 12 volt 26 Ah sealed lead acid battery (note every battery model has its own discharge characteristic). The left most blue line shows what happens if the battery is connected to a 78 Amp device. In the first minute the voltage drops to just below 12, then gradually decreases over the next 4 minutes before rapidly falling away as the battery’s power is exhausted.
In this example the discharge voltage after 5 minutes would be just over 11.
The right most blue line shows what happens if the battery is connected to a 1.3 amp device. Here there is a small drop in voltage during the first minute, but then the voltage remains fairly constant for the next hour before gradually declining over the next 19 hours.
In this example the discharge voltage after 1 hour would be just below 13.