What Happens When You Connect 2 Identical 1000 mAh 12V Batteries in Parallel

In the realm of electrical engineering, particularly in the context of battery management, connecting two identical 1000 mAh 12V batteries in parallel can lead to various outcomes, such as voltage equalization, current flow, heat generation, and capacity sharing. This article will delve into each of these phenomena and explain the implications for users and professionals alike.

Voltage Equalization

When you connect a fully charged 12V battery with a nearly depleted one in parallel, the voltage of the fully charged battery (12V) will gradually equalize with the voltage of the nearly depleted battery, which is below 12V. This process of voltage equalization occurs due to the flow of current from the fully charged battery to the depleted one. The goal is to balance the voltage between the two batteries until they reach the same level.

Current Flow

During the process of equalization, current will start to flow from the fully charged battery to the depleted one. The amount of current depends on the difference in their voltages and the internal resistance of the batteries. Initially, this current can be significant, especially if the depleted battery has a significantly lower voltage. For example, a 3 stack Li-Ion battery with a 3V difference (12.6V for a fully charged stack and 9.6V for a fully discharged stack) and an internal resistance of 300 milli-ohms would initiate a current flow of 5A. This current flow can be understood as a high 5C charge rate for a 1000 mAh pack, which is a considerable amount of current.

Heat Generation

The flow of current can generate heat due to the internal resistance of the batteries. This heat generation is a critical consideration, as a large difference in voltage can lead to overheating. If the difference in voltage is significant, the batteries might experience a rise in temperature, which could potentially damage them. Proper cooling mechanisms should be in place to prevent overheating and ensure the longevity and safety of both batteries.

Capacity Sharing

Once the voltages of the two batteries equalize, both batteries will share the load. However, the overall capacity of the parallel connection remains the same, at 2000 mAh, as long as both batteries are healthy. It may seem counterintuitive, but the performance of the entire system can be affected by the state of charge of the weaker battery. A depleted battery can hinder the performance of the entire system, reducing efficiency and potentially leading to underperformance.

Risk of Damage

It is important to exercise caution when connecting a nearly depleted battery in parallel with a fully charged one. If the depleted battery is significantly discharged, especially below its recommended voltage, connecting it in parallel with a fully charged battery can pose a risk of damage to both batteries. The fully charged battery could over-discharge the weaker one, leading to potential failure or a reduced lifespan for the weaker battery.

Conclusion

The process of connecting a fully charged battery in parallel with a nearly depleted one can lead to significant current flow, heat generation, and potential damage if not managed properly. It is generally advisable to avoid paralleling batteries with significantly different charge states to prevent these issues. Ensuring that both batteries are close to the same state of charge is crucial for maintaining optimal performance and extending the lifespan of your battery system.