Battery Glossary – Charge Voltage

Charge Voltage – the amount of battery voltage when the battery is fully charged or the voltage available at any given point during the charging process.

Various sources describe charge voltage in two different ways, so we’ll cover both here.

Voltage when the battery is fully charged

For this explanation its important to know:

• The voltage of a battery gradually decreases as it discharges
• Most batteries actually have a voltage slightly higher than what you see on the label. 12 volt sealed lead acid batteries usually, in reality, are 12.6 or 12.7 volt batteries when fully charged. Manufacturers make batteries this way because producing units of exactly X volts is actually surprisingly hard. Voltages vary even between batches coming off the same production line. As such, it is safer to have a margin of error so they cannot be accused of marketing a 12 volt battery while selling an 11.7 volt unit. The charge voltage refers to this ‘real’ voltage when the battery is fully charged.

Voltage then is a measure we can use to see if a battery is fully charged, but only if we know what the real voltage should be, not what is on the label. To stop charging a 12 volt battery when it reaches 12 volts would be to undercharge it and doing this regularly will reduce its life span. As such, the term charge voltage refers to the ‘real’ voltage of the fully charged battery in order to know when it is really truly charged.

Voltage available at any given point during the charging process.

The graph below shows how the voltage drops in a particular 12 volt 12 Ah battery as it is discharged. The left most blue line shows what happens if the battery is connected to a 36 Amp Hour appliance – the voltage will drop below 11 in just over 5 minutes. However if the battery is connected to a 0.6 Amp Hour appliance the voltage will still be above 11 after twenty hours.

During the recharging process the battery gradually recovers its full voltage. Some refer to the charge voltage as how much voltage the battery has at any given point during the recharging.

See also

• voltage
• float voltage
• nominal voltage
• discharge voltage