I drove my Subaru 20 miles, stopped at the gas station; and it wouldn't start back up.

So I got a jump. It started up fine and drove about 3 more miles, shut the car off and it started back up fine again.

Why would the battery charge up in 3 miles yet lose its charge in the previous 20 mile ride?:confused:

& is it a safe bet to replace the battery or could this be an alternater issue?

Most auto parts stores will test batteries and alternators for free.

I would remove the battery from the car, place it on a charger, and go from there. Odds are that the battery is bad.

My 1999 Subaru Wagon has been intermittently not starting since 2006. An auto shop recommended we get it looked at. A Citgo station replaced the starter. Later they put in a new Interstate battery. 3 years later it is still intermittently not starting. The battery, now close to 4 years old has some white corrosion on it. At times I turn the key up to 200 times and it then starts. If I get beyond 200 tries, I let it sit and in the next hour it starts like nothing was ever wrong! Or the next day it starts right up, restarts every time, 10-20 times a day, perfectly, then one day it does not start. Last 2 times I got a jump and it started right up! MD mechanic says it had a loose wire on the starter, that I could replace that or replace the starter, as well. I chose the former. Today it does not start after about 10 clicks and has been sitting for about a week.
Diagnosis, please?

Alfa, check the ignition switch for wear or pitting--it's likely bad. Easy job. This link may help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235560

Why would it start, after the last 2 failures, first crank with a jumper battery attached? More voltage to a pitted/worn ignition switch? Thanks...

I've seen Low water in Battery do this?