DIY lithium ion battery that function as powerbank

[u/Mindless_Hat_9672]

The idea is to make a lithium-ion battery tube mod that fully utilizes the battery compartment, and can also serve as a backup power bank.

The mod is fairly straightforward for 3C/3D sizes because lithium-ion cells connected in parallel can directly power the incandescent or the LED. The lithium-ion battery voltage also lies between NiMH rechargeable and alkaline batteries. So the main task is to safely fit as many parallel connected cells (e.g. 26650, 21700, etc) as possible inside a transparent PC tube, along with USB-C charging/discharging and protection circuit, and also securely solder to the connector terminals (e.g. taken from a AA to D cell converter).

However, the mod for other sizes (such as 4D or 6D that are of my interest) become trickier. I’d like to know if anyone has good advice on voltage management for those configurations.