I am designing a portable battery powered device and trying to decide on my battery configuration. It will use an ESP32 chip with a few additional ICs, which will draw about as much current as an ESP32 dev kit, I'm guessing about 200mA peak. It will be constantly running Wifi and will not use Bluetooth. It will also have a Nextion 3.5" LCD which runs on 5 volts and draws 150mA. So we'll say the device will draw between 350mA and 500mA total. I am deciding whether to use a one cell lithium ion battery or a two cell.
If using the one cell, I would use a charging IC such as MCP73832 and charge with USB input, then use a boost converter to boost the 3V-4.2V battery output to 5v for the LCD. Then a linear regulator from 5V to 3.3V for the ESP, or otherwise just go from the battery to a linear regulator to drop to 3.3v, and the ESP should be able to run on just 3 volts when the battery voltage drops down below the 3.3V.
If using the two cell, I would use a charging IC such as MCP73842 and charge with 9V wall adapter input, then use a linear regulator to drop the 6V-8.4V battery output to 5v for the LCD and take that and use a linear regulator to drop to 3.3v for the ESP.
I would also prefer to use linear regulators as opposed to switching regulator to keep ripple noise down since I will be measuring small signals with an ADC, but if using one cell I would have to use the boost converter of course to get the 5V.
I will probably use one or two 18650 batteries at 2500-3200mAh each, I would like to get at least 5 or so hours use from the battery and more would be better. If using the one cell maybe a 5000 mA block style battery to get more capacity. What I'm worried about is being able to power everything once the voltage starts to sag with the one cell. I've tested approximately what my load will be when connected to one 18650 Ultrafire, when it was fully charged at 4.2V the device powered on, but at 3.85V it wouldn't power on. I didn't test any voltages in between. I'm aware that the Ultrafire 18650s are not the best, I would use a Panasonic or similar name brand.
Should I go with the one cell or two cell? Right now I'm leaning toward the two cell. And does my design sound right or need improvement?
Thanks a lot for any help.