If you only get 13 volts and you drive short trips your battery will never charge properly and you will have to replace it more often. Depending upon temperature, a battery charger will put up to 'about' 14.5 volts initially and will taper off as it charges. It should only take a few minutes to recharge as you are only pulling a few amps out of the battery.

CCA makes it harder to figure out, but for an example, if your battery is rated as a 800 CCA battery, the standard requires it to put out 800 amps for 30 seconds at 0 degrees f and to not drop below 10.5 volts.

Your battery requires a proper session with a battery charger, or it may be toast!

A fully-charged battery should be 12.6 volts after it has sat for a while (24 hours, if I remember correctly) and 13.8 right after charging.

Your alternator/regulator is probably not working properly.

__________________
1991 300 SEL
218,000 Km

"Xsbank's rules of mechanical intervention: Always go for the easiest solution first; 90% of what ails a gasoline engine is ignition; After that its all a WAG."