It depends why do you carry on board charger.
You didn`t mention opportunity charging, but my main reason for carrying on board is a chance to plug in quick without unpacking charger. Here in Calgary, Canada live receptacles are common and everywhere.
It is heaven for stealth charging.
I carry Soneil 10A charger for my 36V10AH LiFePo pack, 2A charger rather doesn`t make sense here, since what to do for 3-4 hours??
I during my average 30 min charge have a meal.
MC