Connecting Two 48 Volt Batteries with Different Ah in Parallel for Inverters

Is it safe to connect two 48 volt batteries with different Ah ratings in parallel for use with inverters? The short answer is yes, but with a few important considerations.

Can I Connect 48 Volt Batteries with Different Ah in Parallel?

Under the condition that the batteries are of the same chemical type (e.g., lithium, lead-acid), it is safe to connect them in parallel to provide a combined AH (ampere-hour) capacity. However, there are critical points to be aware of to ensure the batteries function correctly and safely.

Safety Concerns and Preparation

To ensure safety and avoid potential sparks when connecting the batteries, it is essential to charge them separately to very similar voltages. Uneven voltages can create damage or even more serious issues.

Additionally, use suitable fuses or circuit breakers to protect the system. It's recommended to use cable sizes suitable for the battery; thicker cables won't harm the battery, but make sure the fuses are sized to handle the load appropriately.

Historical Advice and Modern Practices

Many years ago, it was advised not to use batteries with different AH ratings in parallel. However, modern testing and practical experience show that if the batteries are in good condition, it can work correctly. One potential issue is a dead or underperforming cell, which can bring down the entire bank if not properly managed. Regular checks on individual batteries for correct voltage are crucial.

Theoretical and Practical Considerations

Theoretically, a battery is like a large capacitor. When placed in parallel, they offer a combined AH capacity, which increases the total current capacity proportionally. Each battery can supply a set number of electrons per second, and when connected in parallel, the total current and energy supplied increase, leading to a higher total AH.

Practically, when batteries operate in parallel, their charge status tends to equalize as their voltage goes down simultaneously during discharge and steps up during charging, leading to a balanced performance. This self-balancing mechanism ensures that if one battery has a different AH rating, it can still function within the system without major issues.

A Real-World Example

One example involves a setup with three Lithium Ferrophosphate (LFP) batteries of 200Ah, 100Ah, and 50Ah. During a test, the current flow under load and while charging was monitored. The 200Ah battery provided twice the current of the 100Ah battery, which in turn provided twice the current of the 50Ah battery, effectively creating a proportional current distribution. The battery management systems (BMS) cut off charging almost simultaneously in all cases, and the charging rate was also proportional to the available capacity.

Conclusion

Connecting two 48 volt batteries with different AH ratings in parallel for inverter use should be approached with care, but it can work well if the batteries are of the same type, in good condition, and regularly checked. While theoretically safe, practical considerations like voltage matching and monitoring ensure a reliable and safe power setup.