Connecting Unequal Li-ion Batteries in Parallel

Question

I'm working on a battery-powered project and came across something that seems simple but feels more complicated the more I think about it. Suppose I have two identical 3.7V Li-ion cells, both with the same capacity and chemistry, but one is sitting at 4.1V and the other at 3.9V. If I connect them directly in parallel (positive to positive, negative to negative), what exactly happens?

I know current will flow from the higher voltage cell to the lower one, but:

How much current are we talking about?
Is there a risk of damaging the cells or causing overheating?
Why doesn’t the higher-voltage cell just “wait” until they equalize gradually?
Would internal resistance limit the surge, or is it still unsafe?

I’m also curious how BMS (Battery Management Systems) handle this situation, and whether any passive or active balancing is required before connecting cells in parallel.

If anyone has experience or insight (especially real-world examples or best practices), I’d really appreciate it!

Sebastian · Answer

When you connect two Li-ion cells with different voltages—like 4.1V and 3.9V—in parallel, current instantly flows from the higher to the lower voltage cell. Because Li-ion cells have very low internal resistance, even a 0.2V difference can cause a surge of several amps.

This surge can stress or damage the cells and create heat. The cells don’t “wait” to equalize—the voltage difference drives immediate current flow, limited only by resistance. While internal resistance slows the surge slightly, it’s not enough to make the connection safe.

A Battery Management System (BMS) helps balance cells over time but doesn’t always protect against mismatched voltages during connection. To stay safe, always match cell voltages within a few millivolts before connecting them in parallel.