Upgraded my North Trail to AGM batteries from CostCo for $400! I was replacing them in the campground, but CostCo assured me they were fully charged and ready to plug and play. They both went dead and the heater stopped working the first night! When I got home and put my charger on 2amps it went to zero quickly and auto shut off. Maybe they were fully charged after towing 8 hours, but I won’t know until my next dry camping weekend. Did I make a mistake buying AGM batteries?
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What were the batteries you bought? Deep cycle or starting batteries?
Did you replace the batteries with equal or greater capacity than the originals?
Did you check the voltage before installing? After installing? Were you plugged in and using the onboard converter?
If your batteries were dead when you left camp, towing for 8 eight more than likely did not fully charge them.
Peukert's law is only an approximation and does not take temp into account, or the age of the battery, it does require a 'constant' in the formula depending on the type of battery since multiple battery types of today were not in existence 100+ years ago.
If the new batteries are equal to or greater in capacity then the batteries that came with your RV, and you were able to get longer run times with the old set, skip the equation and get the batteries properly fully charged and load tested.