I am building a device which I would like to be able to charge with several different types of external battery chargers (different Icharge within spec of battery, of course). If I understand everything correctly, I won't need an extra charge IC on my board with this setup. Now, while this device is charging I would like to still be able to power the loadside logic. What is the best way to do this? Should I add a source selector and only use current provided by the charger to put in my on-board DC/DC during charging? Should I add a load sharer? Or is it sufficient to just power everything from Vbat even with the charger plugged in and pull the 20W required for the logic from the charger like that? Either way, isn't the charger constantly detecting current draand "thinking" the battery is not full yet potentially detrimental for the battery? Thanks!
Depending on the charger and battery type it will probably work ok.
Chargers are usually very dumb and just supply a current/voltage limited output. The charger shouldn't care at all if a load is connected in parallel. Although you should note that the "float voltage" that the charger supplies is higher than the normal battery voltage. The float voltage on a "12V" lead acid battery is usually 13.8V for example.
Most chargers have an output that is voltage limited to 4.2v per cell, as well as current limited. More complicated chargers have time and mAh cutoffs which could trip you up. But in practice that would just mean you have to start the charging cycle again.
These have complicated charging logic. The load likely would trip some threshold on the charger.
If the external power source is a battery charger then it is likely to be confused if there is a load on the battery while it is charging.
Most devices that can run from external power or battery use a simple power supply, and put the battery charger inside the device.