Nice job taking care of your car so you aren’t unexpectedly stranded! A typical car battery should hold about 12.6 volts of charge or more.
If you measure with the engine running, this number increases to between 13.6 and 14.6 volts. You can measure whether your car battery is weak by starting the engine and turning on the headlights.
If they’re dim or struggle to power on, then there is a problem with your battery or charging system. Try revving the engine to see if it makes the headlights brighter. If it does, this means the alternator is producing some power, but it may not be enough and you should have your charging system looked at by a mechanic.