I assume you don't want to run the battery down because you might need the battery on the next flight.
However, unless you test the battery, you have no idea if it's working so you might not have it if you need it.
A bit of a catch-22!
The battery will charge pretty quick when the plane is on. If you go for a 30 minute flight (including all the ground running) then you'll have 10-20 minutes of backup. If you feel like you can't depart without a working battery, then you really do need to do the test and let the battery charge before you take off.