Originally Posted by Pigman1
Huh? Just what do you think the voltage regulator on any vehicle does? Damage a battery and shorten it's life? Since you're bound and determined to spend your money on something that's not needed, go right ahead, but don't try to justify it by claiming some of the manufacturers propaganda is The Gospel.
Uh, what propaganda. Read anything you want to and you will find excessively high charging currents hurt batteries. That is why jump starting a battery shortens it's life even if it gets you going short term.
The voltage regulator has little to do with it. The major current source would be the battery with the higher charge bleeding current into the battery with the lower charge. The current limiting is the resistance between the two batteries plus any other drop like a diode in series. Basic electricity 101.
In this case if the towed is very low and the towing vehicle is high the diode drop and wire resistance help limit the current so the towed battery gets less current for a longer time resulting in a less damaging recharge. Normally there would not be much current flowing in the charge line so the drops reduce and the battery comes close to full charge. Heavy short term use like a Brake Buddy would mostly come from the local battery due to the wires acting as balancing resistors. The battery then recharges between brake usages.
That is based on circuit theory I learned when I was around 12, reinforced several times since and have been using to make a living for more than 50 years. That includes significant time working with high current low voltage circuits like we are talking about. In short, I don't use no steenkin propaganda. ;-)