I’ve done this with my MacBook Pro on occasions when I mislaid its power supply. An 18W charger is helpful in keeping the battery from draining at its normal rate (in absence of external power) but not enough to charge. It gives you extra time before you have to charge in other words, and it is able to charge the battery when system is in standby (say overnight). I am not a battery expert but I don’t think there is any damage potential. I don’t know if there’s a lower limit on ‘useful’ wattage: does a 5W charger help proportionately less? Or maybe they do not exist...
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