Connecting Batteries of the Same 12V but Different AH Ratings

When considering the connection of two different types of batteries that are both 12V but differ in AH ratings, it is crucial to ensure that both batteries are at an identical state of charge before connecting them. This is particularly important to avoid any unwanted current flow between the batteries, which can be damaging.

Different Types of Batteries

It's important to note that if the batteries are of different chemistries, connecting them parallel is generally not advisable. This is because different battery chemistries can result in significant voltage differences, leading to undesirable current flow. Even a minor difference in voltage can cause a power surge between batteries, potentially causing damage to both units.

Same Type but Different AH Ratings

On the other hand, if the batteries are of the same type but have different AH (ampere-hour) ratings, some experience indicates that connecting them in parallel can be done, albeit with certain precautions. In the case of automotive starting batteries, experts have reported success in connecting units with different AH ratings, provided that they are all of the same chemistry and relatively new.

Method to Equalize Voltage

Before connecting the batteries, it is essential to equalize their voltages using a method such as a small flashlamp bulb. A 2.5V flashlamp bulb can be used to monitor the voltage difference between the two batteries. Initially, if one battery's voltage is higher, the bulb will shine brightly, indicating a significant voltage difference. Gradually, if the differences are corrected, the bulb will turn red, indicating that the batteries are within about 0.5V. At this point, a smaller bulb can be used to ensure a more precise voltage match.

Precautions and Considerations

Even with same chemistry and relatively new batteries, monitored connections should be implemented. It is advisable to have a system in place, such as relays, to separate the batteries if one of them is in poor condition to prevent it from discharging the others, thus negating the purpose of parallel connection.

When two different types of 12V batteries are connected in parallel, the main reason for concern lies in the inherent voltage differences due to different chemistries. Even though both batteries are labeled as 12V, the actual voltage can vary slightly. This variation can lead to one battery discharging into the other, potentially causing damage. Therefore, it is crucial to monitor and control these differences using appropriate devices.

Conclusion

In summary, connecting 12V batteries of the same type but with different AH ratings can be done, provided they are of the same chemistry and are in good health. However, equalizing their voltages is a necessary step to avoid any unintended current flow that could be damaging. If the batteries are of different types, the risk of voltage imbalance increases, and parallel connection should be avoided.