Battery Matching

If your system uses four 6-volt batteries to create a 12-volt battery bank, the batteries should be matched as closely as possible.

The best option is to replace all four batteries at the same time using batteries that are:

• The same type
• The same capacity
• The same brand and model
• A similar age
• At a similar state of charge
• At a similar voltage when installed

This helps the battery bank charge and discharge evenly.

Problems can arise when a new battery is connected with older batteries, or when batteries with different capacities or charge levels are used together.

In a series connection, the same current passes through each battery. If one battery is weaker than the others, it can limit the performance of the whole string. The stronger battery may become overcharged while the weaker battery remains undercharged. The same issue can occur during discharge. A weaker battery may be pushed too far while the stronger battery still has usable capacity remaining. This can shorten battery life and reduce the overall performance of the system.

Batteries connected in parallel also need to be closely matched. If one battery is stronger than the others, it may take more of the charging and discharging load. This can create uneven performance and lead to premature failure.

Where possible, replace batteries as a complete set. If that is not practical, replace them in matched pairs and make sure the batteries working together have similar voltages before they are connected.

If a single battery must be replaced, the whole bank should be checked carefully. Each battery may need to be charged separately before being reconnected, and the voltages should be monitored during the first charge and discharge cycle. 

Small differences between batteries may correct themselves during normal charging, but large differences should not be ignored.

Do not mix different battery types in the same bank. Flooded, AGM, gel and lithium batteries all have different charging requirements and should not be connected together.

It is also best to avoid mixing batteries from different manufacturers, even when they appear to have similar specifications.

For the safest and most reliable result, use batteries of the same type, capacity and brand throughout the system.