I keep a car parked in a lot in another state. I get out there every few months or so. The battery under the hood sometimes is dead due to the small system draws while the vehicle sits idle for a few months. Rather than find a jump my solution was to buy a spare battery which I keep in the passenger compartment and jump the car with that. Here is the question. Can I trickle charge my spare battery using the cigar lighter while driving it around? I bought a cigar lighter plug in with red and black alligator clips running from it. I've used it for several trips now hooking the red lead to the positive post of the spare battery and the black to the negative post of the spare battery. (I can't tell for sure if its charging but the spare has never failed to jump start the car). Last time I did this my check engine light came on. I stopped charging the spare battery and the light didn't come on next time I started the vehicle (once I drive it for a while the main battery charges enough to start the car). Am I harming my car by charging from the cigar lighter? Are the amps coming from the cigar lighter ok for charging? Do I need to be concerned about polarity? Right now I put the red lead to the positive post and black to negative. Thanks for any thoughts you may have.
Yes, but use a DC to DC "smart" battery charger.
Directly connecting the leads won't damage the wiring because the cigarette lighter is fused, but you may damage the battery by over-charging it. Best to get a proper DC to DC battery charger to protect the battery, and be sure it is a "smart" charger, not a "trickle" charger as the latter will also over-charge the battery.