1998 Frontier, 4 cylinder, 5 speed, AC. I've been going through batteries every few months (free replacement) and alternator checks out fine for voltage and amps. I bought this used; does anyone know (or can check) whether this truck originally had a battery "box" (like an insulated box around the battery)? Maybe it's my underhood temperatures that are too high (my God, it's Arizona, 112 in the shade, if you can find any) causing my batteries to die prematurely (they then read 5 volts, and cannot be re-charged). Thanks.

Did you solve your problem? Suggestion - checking cables and confirming clean ground and connection points, beneath plastic cable covering, corrosion can insulate enough that charging and also function can become a draining experience. Experienced a similar issue with a past 90' hardbody.

I think it's OK now; bought two negative cables at Autozone routed one from battery negative to body ground bolt, and second cable from body ground bolt to engine ground. And I put some heat insulation around the battery, don't know if they had any insulation as factory or not, in case underhood heat was a factor. So, knock on wood, it's been OK for over three months now, and temperatures have dropped to 85 F.

Sounds like you attacked it well - I have a 98 Frontier - It does not have an insulated cover or wrap.