you can use a lower rated charger but only when it is off to charge the battery. It will still charge it, providing both voltage and polarity are correct. It will be at a proportionally slower charge rate though, depending on the power rating of the chargers output.Do not use a smaller power supply when the laptop is running. It wkll most likely overheat everything and also may not even be able to run it.If the other charger is close to the originals output you could possibly use it for powering the lapgop if you took out the battery. That would at least take some of the load off of it, but I wouldn't reccomend it unless you had no other choice.