Fast Charging is a rage these days on mobile phones with companies claiming mad charging rates of nearly 240W. How do they pump this amount of power into batteries?
Most consumer phones that you see out in the wild will be a single-cell type mostly a variation of Lithium based battery as it packs the highest energy density. The battery’s nominal voltage will be in the range of 3.5V to 3.8V depending on the type of chemistry of the cell. The battery capacities can range from 3500mAh-6000mAh. Batteries can be charged at only certain current rates at a voltage slightly above nominal voltages. A higher charging current means more heat. Heat is something really bad for batteries. So phone designers cannot keep increasing the charging current on batteries with a single cell. So what’s the workaround here? Dual Cell batteries…
These are multiple cells wired in series or parallel. Now for series cells, the overall battery voltage is higher so effectively you can charge the cell at a higher voltage and a lower current to reduce your current ratings and losses. Now in series configuration, the internal battery resistance does add up to give a bit more loss. In the parallel config, the voltage remains the same, but you can drastically increase the current as you have 2 parallel paths for currents going into each cell. You can even have 2 separate charging circuits and can effectively double the charging speed by charging both cells simultaneously but this is rarely practiced as it is expensive for the circuit’s POV. For charging wattage claimed over 120W, you usually see a dual-cell setup.
While dual-cell batteries charge faster, they need advanced systems for balanced charging and discharging. They also occupy more space than single cells of the same capacity. So as with anything in life, it’s always a balancing game for a design engineer. An ideal goal is always to determine what power I can pump into the battery maintaining a reasonably small temperature rise. That’s the beauty of engineering!
