My ‘96 Chevy Blazer LS won’t start.

Hi, my blazer won’t start we bought it from
someone online and on the drive home it
continued to lose power as if it was out of
gas, but it had a full tank. We took the battery
to Walmart in which it was purchased and
they tested it and found out it’s completely
fine. It’ll run and start for a little bit if it has
over 12 Volts, but then it sputters and dies. It
died 4 times on the hour and a half drive
home and it’s currently parked outside being
charged on an Everstart battery (we’ve been
charging the battery since 4am and it’s
currently almost 5am the next day) even
Walmart charged it, but the damn truck won’t
start. We need help figuring this out because
nobody seems to be helpful at this point.