Supplying 4 V (max 3 A) to my circuit using lithium-ion batteries

Question

I'm designing a circuit for a portable device, and the main component requires 4 V (±5%) up to 3 A (12 W). The 3 A is the max for the circuit- typical load is 250 mA.

As I understand, the voltage range for 3.7 V lithium-ion batteries is about 2.8-4.2 V depending on their charge level. At maximum charge, 4.2 V is within the ±5% tolerance for the 4 V component; However, at minimum charge, I understand I'll need a boost converter to adequately supply the necessary 4 V.

Since the 3 A maximum is quite high, I understand that I'll likely need to use a pair of lithium batteries in parallel to prevent voltage drop or battery damage under high load.

There seems to be no off-the-shelf options for a fixed 4 V boost converter capable of 3 A. I considered using 6 parallel 500 mA fixed 4 V boost converters instead. Alternatively, should I consider designing my own boost converter circuit to achieve this instead? Can I be sure that a circuit I design myself will not fry the 4 V component, for example, whenever I connect a charger? Can I be sure the voltage will not drop below 4 V (±5%) if the load is increased suddenly?

Additionally, will I need to ensure that my charging circuit is capable of keeping up with this demand as well, or can I use a less-powerful charging circuit with the understanding that it will actually deplete the batteries while charging under high load?

Answer 1

Use a buck-boost converter. It can output 4 V when it receives a wired range of input voltages, below and above 4 V.

Answer 2

Another approach would be to use two or more cells in series and then a buck converter, as the battery voltage will always be higher than the output voltage. This may be preferable to buck-boost as the circuit is simpler and typically more efficient. If you’re using 18650 cells then 3A shouldn’t be a problem in any case, but certainly check the datasheet for the cells you want to use.