My truck seems to have trouble cranking after 10+ minutes of running just the radio. Had to get jumped the other day and cranked with little charge. The battery is getting old but last time it was checked they said still good. Could it be alternator? Truck is 99 with 101k on it

 

With those symptoms, either one has just as good of a chance at being weak. Just take it to a parts store and they can tell you in about five minutes the status of both of them. You don't even have to take anything off the truck for them to test it.

 

and check voltage with engine running at battery. Should be around 14 volts. If not regulator could be suspect.

If 14 or so volts, leave meter on battery and cut off engine and leave radio on. Watch voltage and see if it drops. Should stay around 12.0 even after and hour or so. If it falls--by a new battery.

By a good battery. Buy the best and cry once, by the cheap and cry all the time.