Looking after the Lead Acid Batteries in your Solar Power System

Many people rage against Lead Acid batteries and claim that they don't last more than a year or two. This post is about looking after your batteries and get 5 to 10 years of service out of them.

The key thing to remember is not to allow then to discharge completely when in use. The recommendation is only to use half the rated capacity of a Lead Acid Battery. This means that when using a 100 Amp hour battery, only use 50 Amp hour. 

For example, see the warning label on this battery. The 4 year warranty is invalidated if it is fully discharged,

See the table below for an approximate relationship between battery voltage and capacity.

If you are using the output on your solar charge controller, then make sure that the cut-off voltage can be set to make sure that the battery does not fully discharge. 

With this solar charge controller, unfortunately I could only go up to 11.5 V for the cut off voltage, which is around 20% of capacity. The switch back on voltage is higher, and I set this to 12.4 V which is around 80% of capacity. So when the battery discharges down to 20% of capacity, this charge controller cuts off power to the output and then switches it back on when the battery is charged and it's capacity is back to 80%.

Of course, switching off at 20% of capacity is too low as the recommendation is to do this at 50%. There are devices available that will fit between a battery and a load and achieve this. They are generally known as Low Voltage Disconnect Modules. A few examples are shown below. Before buying one, make sure that the voltage setting and current capacity is suitable for your needs.

A simpler solution is to use a Battery Monitor with an audible alarm when the capacity has dropped to 50% or less. Have a look at the range here.

When charging Lead Acid batteries, the voltage used for a 12 V battery must be controlled to around 13.7 V - this is the float charge voltage. Most Solar Charge Controllers will be set to this - but please check.

Finally, Lead Acid batteries must not be left in a discharged state - on standby they need to be charged so that the voltage is always above 12.3 V ie at least 70% capacity.

In summary, looking after the charging and discharging of lead acid batteries will ensure a long service life in your solar power system.