GMC Repair: problem starting, electrical diagnosis, battery problem

Question
I have a 98 jimmy 4x4. I had the battery replaced about 4 months ago. Friday, I had a problem starting the car. Lights worked and radio, no problem. However when I turned the key to start, nothing. one small click then nothing at all. After two attempts, it started. I ran it the rest of the day. Again yesterday the same thing. It took four attempts. My father tested it with some device and told me the battery was charged while the car was on. However when I turned it off it only read half charge, 10.5! Please help if you have any advice. I really appreciate you help.

Answer
Hi
Sound to me like a battery problem. With 10.5v it sounds like a dead cell or an internal open inside the battery.
My adivse would be to find a local shop in your area that can preform a "Load Test" on the battery. You may be able to find one that may have like free Electrical diagnosis, or free battery checks. (Some Parts store offer this service free). If not then either find one that your familiar with OR find a Starter/Alternator shop. They'll be able to help you isolate the exact problem.
If they all fail, your if you are not comfortable doing any of these follow up and I'll help you/or dad go through some diagnosis.
Just to clarify, your dad tested the charging system while the truck was running?
Just advise from experience.
I Hope it helps
Paul