Even though I have some TP4056 modules for charging Li-Ion cells, those small PCBs generate a lot of heat and the charging current decreases with temperature increase. Since I have parallel pairs of cells from notebook batteries, I would like to charge them with a higher current. Another limitation of those modules would be that the maximum charge current cannot be changed unless I replace a small SMD resistor. Therefore I'll build my own Li-Ion battery charger, with LM317 (LM338). Using a linear regulator does not solve the heat production issue, but at least I can put it on a heatsink.
Li-Ion cells need to be charged in a two-step process. First, until they reach a threshold voltage the charger behaves as a constant current supply. This current is cell-specific, but usually the supplied energy (in Ah) should be less than 80 percent of battery capacity (Ah). When the voltage increases enough, the charger should switch to constant voltage mode, maintaining a stable 4.2 V (or 4.1 V for some cells) until current drops.
