Does a LiPo balancer board need switching for external power supply

Question

I have 4 18650 LiPo batteries in a battery holder that I am connecting a to a charging balancer. According to the description it comes with charge protection and does proper balancing for the 4 cells. Sadly it does not come with a datasheet or even a model number, all I know is that it is typically used for power drill chargers. Since all the pins are properly labelled the wiring is not really a problem. See below an image of the module.

I am going to use an external power supply (generator driven by a small combustion engine) that supplies 17ish volts to the module. The pins in return are connected to the poles of the batteries inside the holder.

As far as I understood how those modules are working they are basically capable of "turning off" the charging of a cell, if it is full before the others are full and "wasting" that energy by generating heat in a resistor. Does this mean if all batteries are full it just starts "burning" power through the resistors for all cells? Apart from longevity and heat, are there reasons why I would need to switch off the power to the module when the batteries are full? Also how would I typically notice, that the batteries are full? I am assuming the 16.8V on the module always supplies the 16.8V independent of the charge of the batteries? Typically my batteries are only used shortly to power the system before the ignition is activated, then some power is drawn to engage the starter DC motor, after that the external power takes over. So usually the external power supply will run much longer while the batteries are charged than charging them.

In addition to that I am also trying to use the external power supply to power my outside system while charging the batteries and use the batteries when the engine is not running. Do I need to create a system like in this question: Li-po Charging circuit question (switching between battery and power supply) or can I "siphon" directly from the 0V/16.8V connections on the module?

edit: I think my second question can be achieved by 2 diodes, I can get significantly higher voltage out of the generator, so using one diode directly from the generator to the buck converter and one diode from the 0/16.8V outputs of the LiPo module should work. When the generator is offline the 0/16.8V are outputting from the battery, when the generator is running the power is taken from the generator diode. Since I am aiming for 5V the voltage drop from the diode should be irrelevant.

Answer 1

Base on comercial website comercial website it give this data

1. Module will discharge cell which exceed 4.2V
2. Module will cut-off charging when any cell exceed 4.25V So for your question

Does this mean if all batteries are full it just starts "burning" power through the resistors for all cells?

The answer is no because it cut-off charging current.

are there reasons why I would need to switch off the power to the module when the batteries are full?

No, the swith already implement in BMS module.

Also how would I typically notice, that the batteries are full?

Of cause, by detect battery voltage. Battery full when it reach 4.2V per cell

I am assuming the 16.8V on the module always supplies the 16.8V independent of the charge of the batteries?

Yes, just like normal battery it supply voltage. If you want to charge its, you need more voltage to force current back in.

Do I need to create a system like in this question

Yes, when you have more than 1 power source for 1 load. 0/16.8V therminal are just like battery lead. You can directly connect when you need to switch power supply.

For second question. Yes diode solution is working but it highly lost and heat up.

Anyway for charging battery you nedd to control charging current. The module not limit charging current. When you put generator directly to battery It's might supply overcurrent and damage battery. If you planned to use buck convertor I strongly recommend to use CC/CV type and set CC to match charging current of battery.