I’m adding a second battery to my car for camping purposes. The second battery will be a deep cycle SLA. It will be connected to my starter battery/alternator via a voltage-sensitive relay (cuts in ~14V to parallel the two batteries and they both charge while car is running, cuts out ~12V to isolate secondary battery so it can drain in peace when car is off)
I got to thinking, how does a car alternator charge a car battery? Doesn’t it just constantly output 14.4V or so for as long as the car engine is running? My question is, then, what is it doing to the battery if a constant voltage is applied indefinitely?
My curiosity is in the context of any other kind of battery- what would happen if you applied a constant 4.2V to a lithium cell indefinitely?
A lot of battery chargers advertise that they cut off charging when batteries are full, some switch to a trickle maintenance function, but I’m not understanding why some special mode is required. Isn’t the system inherently at a standstill when battery voltage equals voltage coming in?