Solar-Facts Logo

Charging and Discharging Lead Acid Batteries

The voltage of a lead acid battery when at rest (not supplying current or being charged) will vary according to how fully charged the battery is. Graph of lead acid battery charge level against voltage

The graph shown to the right represents a typical 24 volt lead acid battery which has not been charged or had current drawn from it for a couple of hours.

Battery Discharge Characteristics

A full charged battery will have a voltage of around 25.5 volts. As current is drawn off and the level of charge is reduced, the voltage will fall quite quickly at first (again it would be necessary to stop drawing current for a couple of hours to be able to measure the true voltage of the battery).
With further drawing of current, the rate of voltage drop slows down and will reach around 24.0 volts when the bettery is at half capacity.

As the battery approaches the fully discharged state, the voltage starts to fall more quickly again.

It is important for a battery to never be fully discharged, so your inverter will normally disconnect the supply when the voltage is around 22 volts.

An interesting point to note here is that when an inverter or other power load is drawing a high current from the battery, the voltage will drop. This may mean that the battery needs to be somewhere over 50% charged to avoid the inverter cutting out due to low voltage.

The larger the battery, the smaller this voltage drop will be, and the greater the % of the charge will be useable when drawing high currents.

Battery Charging

If a voltage is applied to the battery which is greater than the battery's voltage, a current will flow through the battery in the reverse direction to when it is supplying current, and the battery will charge.
The rate of charge or current that will flow will depend on the difference between the battery voltage and the voltage that is applied to it (from solar panels etc). Solar panels intended for a 24 volt system are likely to be capable of producing over 30 volts. This voltage ensures that the panels are capable of charging the battery fully.

While it is beneficial to a battery's performance and life to be fully charged on regular occasions, however once a battery has been charged to it's full capacity, it is important not to continue charging as this will damage the battery. A Charge Controller is necessary to ensure that the battery is not over charged. BR>

Battery Efficiency

The Lead Acid battery is not 100% efficient at storing electricity - you will never get out as much as you put in when charging. Overall, an efficiency level of 85% is often assumed.
The efficiency will depend on a number of factors including the rate of charging or discharging. The higher the rate of charge or discharege, the lower the efficiency.
The state of charge of the battery will also affect charge efficiency. With the battery at half charge or less, the charge efficiency may be over 90%, dropping to nearer 60% when the battery is above 80% charged.
However it has been found that if a battery is only partially charged, efficency may be reduced with each charge. If this situation persists (the batteries never reaching full charge), the life of the battery may be reduced.