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Lately, there seems to be a trend in cell phones where they have 2 separate batteries, and this is claimed to increase changing speed. See "Oppo suggests that because the two batteries can be charged at once, charging can go faster than if the device only used one unit." And "With this new setup, both batteries can charge at more than 30W simultaneously while staying cool even when using the phone,"

As I understand it, lithium batteries have charge rate typically defined in C, where C is the capacity. For ex. a 4Ah battery with a 2C charge rate can charge at 8A. This leads to the idea that the battery charge time is 1/C hrs (ignoring the constant voltage segment at the end).

With this understanding, splitting the battery into 2 cells shouldn't increase charge speed at all. Is there some electrochemistry explanation I'm missing for why smaller capacity cells have higher charge rates?

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When you split a certain capacity battery into two separate (half size, half capacity) batteries, you might be able to have better thermal performance.

That means, it gives you more flexibility in where the batteries are located and that could potentially help in keeping the batteries cool(er) during charging.

During very fast charging (maximum current you can get away with) batteries get hot. If you have two separate batteries it might be easier to spread that heat and/or you could charge only one battery at a time while the other one cools down.

Do realize that companies like Oppo prefer a short charging time over battery lifetime so they will employ all kinds of "tricks" to lower the charging time. That is their choice but realize that the faster charging increases the battery's temperature and that in turn increases wear on the battery. I would advise to charge the batteries slowly when possible and only when you really need it, charge them as fast as possible.

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