Connecting Two Different Batteries in Series: What Happens and How to Ensure Safety

When two different batteries are connected in series, several outcomes can occur, depending on the characteristics of the batteries. The correct understanding of these outcomes is crucial for ensuring the safety and efficiency of your battery system.

Voltage Addition

The total voltage of the series connection is the sum of the voltages of the individual batteries. For example, if you connect a 1.5V battery and a 9V battery in series, the total voltage will be 10.5V. This is a fundamental principle that ensures the correct voltage is maintained in your circuit.

Current Flow

The same current flows through both batteries. However, if the batteries have different capacities or internal resistances, this can lead to issues. The battery with the lower capacity may deplete faster, while the higher internal resistance of the other battery may limit the current flow.

Battery Discharge

When connected in series, the battery with the lower charge will affect the overall performance. If one battery discharges faster than the other, it can lead to an imbalance, causing the weaker battery to become over-discharged, which may damage it. This is particularly important to consider in applications where the battery system must maintain consistent performance.

Potential Damage

If the voltages are significantly different, the battery with the higher voltage could force current through the lower voltage battery in a way that could lead to overheating, leakage, or even explosion, in extreme cases. It is important to ensure that the voltages are compatible to avoid such hazards.

Polarity Considerations

Ensure that the positive terminal of one battery is connected to the negative terminal of the other. Incorrect connections can cause short circuits or damage to the batteries. This is a critical step in any series connection to maintain the intended current flow and avoid hazardous situations.

Common Misconceptions

Some common misconceptions about connecting two different batteries in series include the belief that nothing will happen or that the output voltage will simply be the summation of the two battery voltages with the current being the lowest of the two batteries. These statements are simplified and may miss critical details.

If the batteries are connected with the correct polarity, the two batteries add up their voltage, resulting in the combined voltage across the extremes. The Ah (ampere-hour) capacity of the series connection will typically be that of the smaller battery. If the polarities of the two batteries are in opposition, the difference in voltages appears, and the Ah capacity is again governed by the smaller battery. These configurations require careful consideration of the individual battery characteristics to ensure optimal performance and safety.

ESR Considerations

The equivalent series resistance (ESR) of the combination will be the sum of the ESRs of the individual batteries. This is an important factor to consider when evaluating the performance and lifespan of your battery system.

Understanding these principles and considerations will help you connect two different batteries in series safely and effectively, ensuring that your electronic applications run smoothly and efficiently.