Ygtjki

I am using NCR18650 batteries with the tp4056 charging module.

When I connect the charger at 2.8 V (fully discharged), the tp4056 IC is getting hot at a 1 A charge current. Is it OK to reduce this charge current by changing the charge current selecting resistor? So the tp4056 IC heat would be reduced, wouldn't it?

For my application I don't need a quick charge. With a low current, charging time will be increased, but it does not matter for my application.

Will reducing the charge current affect the battery's lifetime? Or will it damage the battery?

I am trying to reduce the heat of the tp4056 IC by reducing the charging current, maybe from 1 A to 500 mA or less. The 18650 battery never heats for a 1 A charge current.

I use TP4056 charger boards a lot, both on their own and embedded in some devices. In addition to existing answers, I'd like to mention this - it's OK for TP4056 to heat up, it's a linear charger and it heating up doesn't impact its performance/reliability all that much. As long as the board has proper heatsinking, you should be fine - i.e. the popular "blue PCB with USB port" chargers from China have good enough heatsinking, as they tend to connect the ground pad of the TP4056 to copper-filled areas on both layers (sometimes only one, bottom one, with vias).

If it's a custom board that you yourself designed and it doesn't connect the ground pad to anywhere useful, only then I'd be worried - in that case, swapping the current set resistor for ~2K (that's what you need for ~500mA IIRC) should do the job. Otherwise, you should be fine as you are now, with 1A charging.

Yes it is OK to reduce the charging current and that is what the TP4056's charging current setting resistor is for. If you halve the charging current, charging will of course take longer. Charging slower (but not too slow) should actually increase battery lifetime. But for an 18650 cell, 1 A charging current is reasonably normal.

2.8 V is a very low voltage for a Li-Ion cell, if you discharge often to such a low voltage the battery's lifetime might be shorter. I would prefer not to discharge below 3.4 V.

The regular NCR18650 Panasonic battery has the recommended charge current at 0.7C, or about 2A. And the discharge threshold is 2.5V, according to manufacturer's discharge curves. So you are charging the battery already at half permissible rate. Formally yes, charging a Li-Ion battery at slower rate doesn't do any damage and might be even better for battery's SOH (State of Health). It will just take longer.

The TP4056 is a LINEAR charge controller, so it will dissipate a lot of heat and will be hot, if the board is designed with improper heat sink. If you are concerned with IC overheating, you should either make a better heat sink (at least over the top if IC), or reduce input voltage to, say, 4.5V. It will still dissipate 0.3- 0.5W and should be warm. Or get a different TP4056 charger board with a better constructed PCB and with bottom heat slug soldered down to it. Or get a better charger system. Reducing charging current of TP4056 will be the least optimal solution.