How can I build a power bank which charges to 16.8V and outputs 12V?
&quot;Where I am a bit lost then is, how can I charge the battery? Do I just wire an input path and output path in parallel&quot;
First off, I'm new to this part as well. But this is the idea yes.
Your li-ion charger will do it's constant current (CC) and constant voltage (CV) charging. But it is not just charging the battery it is also powering a &quot;Parasitic load&quot;. So especially during the CC phase it will charge the battery a bit slower depending on how much current the motor is pulling. Turning it off during charging is ideal.
The charger should be a lithium charger configured for your 4S pack. And the charger is wired to the main battery positive and negative terminals. Use a BMS with both balancing and over and under voltage protection.
The other parts of your circuit don't have to change.