Even some battery chargers hooked to the truck batteries have too much AC noise so you should unhook the truck batteries when charging so you don't hurt the electronics of the truck. I bought a battery charger for 35 bucks that has no ac noise, yet another one for $ 175 that has tons of ac noise and is not recommended to charge batteries while hooked to other electronics. This was tested with a FLuke model 77 volt meter was very surprised to find this results even on what is considered a very good charger.
Thank you Burningman for the additional help and information about the subject of ac ripple testing as is the best way to check the health of your alternator. It can also tell you if you are placing too heavy demands on your alternator...Either too small alternator or too big of cables running between truck and exhausted house batteries.
You will need a good volt meter to read the ac ripple as lesser ones generally do not have this ability. Reading the amount of ac noise going into the system is best read from the B+ wire at the alternator, but you must be very very careful about having long hair or getting your hands/ arms close to the fan, belt or pulleys while testing over a running engine. I did say be very careful, right?
