I own a 2008 sprinter/Winnebago ViewJ 24' RV which I store over the Wisconsin winters. I've purchased a 5 watt solar charger. Can I attach it to the + post in the engine and body frame part and feel confident that it will trickle charge the battery over the winter or use the cigarette lighter plug?
attaching a solar trickle charger to the 3500 on 2008 Dodge Sprinter 3500
Your first statement is correct, or see if the cigarette lighter is "live" with the keys out of the ignition if it is you can use the cigarette lighter but if it not live after ignition it will do nothing for you to attach the charger to the lighter circuit.
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I have done that and it works just fine.