Most heavy vehicles use 24 Volt electrical systems because of the very high power needed to start large engines. A 24 Volt system is made up of two 12 Volt batteries connected in series and is known as a pack. The voltages of each battery are added together whilst the Ah capacity of the pack remains the same as that of a single battery.

Pairing replacement batteries

When replacing batteries in a 24 Volt series circuit, it is essential both are replaced at the same time.

Both batteries must be of the same technology, have the same amp hour capacity, cold cranking specification and voltage.

If only one battery is replaced, then the differences in capacity when being charged will result in the overcharging of one battery, while the other remains undercharged. When not being charged, the older battery will also drain electrical energy from the new one due to the voltage drop between the two. This reduces the total amount of power available across the two batteries.

Replacing in pairs ensures that the battery pack is balanced and eliminates most of the problems associated with cycling batteries connected in series.

24 Volt battery pack maintenance

Over time it is best practice to check each individual battery voltage and recharge as required.

At the end of a charge cycle and after leaving the batteries to stabilise for a minimum of 4 hours, it is important to check each battery voltage as this this will indicate the biggest difference in voltage, if there is one.

There are two ways to equalise the voltage difference if one is detected:

1) Fully charge the pack with a 24 Volt charger, then bring up the lowest battery with a 12-Volt charger. There is no need to disconnect the series connections to do this if you can access just the battery with the lowest voltage.

2) Charge each individual battery with a 12 Volt charger.

In each case, check the individual battery voltages after charging and stabilisation.

Next time you charge and check the voltages, you should see the batteries are closer in voltage at the end of the charge cycle.

NOTE: Direct connection of a non-OE 12 Volt consumer to a single battery in the pack is not recommended as it can lead to charge imbalance problems.

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