Batteries in my plow truck R dead.

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Okay, so the problem I ran into over the past week was that the primary (starting) battery in my plow truck was dead. I jumped it w/the accord and it ran fine. Bear in mind that the at night temperature here has been 0-15. Wind chill below 0. And a few days last week the wind chill brought it to 15-30 below. If I didn't make it clear, the truck has two batteries, one for starting that is run directly to the starter and alternator, the other is for accessories that is controlled by some sort of automatic relay system.

I took it for a beat run to charge the battery for a while and parked it for 2 days again.

2 days later the muther5ucker was dead again so I jumped it again.

It runs fine, but the only quirk here is that when I move the plow (Meyers) in any direction, the alternator charge gauge drops WAY low, to like 6-8 amps (in the red zone) and then when the load is taken away (not moving plow in any direction) the alternator charge gauge goes back to normal (14-16 amps).

So my question to everyone is this: Is the accessory battery dead, so when I move the plow in any direction, it is forced to grab power from the alternator and starting battery, thusly discharging and effectively killing the starting battery?

How can I test whether the battery is good or not with my lil multitester?

Thanks everyone ! ! !