Hey LY,
I'm about 6 hrs west of Michigan, and yes it's been quite nippley here... well below freezing for the most part. A few snowy days at or just above freezing, followed by another deep freeze. Normal temps are supposed to be around 25F at night, and around freezing for a high, but it's been much colder... as low as 15 below zero F. This weekend we're having another cold one. (Now that I think about it, that may be what's causing my resting ocv to be lower than last winter? The other day I warmed up my bank and Vbatt actually rose, just from being ~ 30F warmer.)
I'm wondering if he has the portable charger at too high an amp setting, and it's forcing up the voltage too fast, and thus kicking it into float? (It doesn't take much to drive up the voltage on a starter battery at a high amp rate). Just a guess.
