Fallon
Super Member
Most of the rolling ones are "smart" electronically controlled ones. I had to look hard for my dumb one. It has the knob for the couple 12v or single 6v range & another knob that is the mechanical timer. You have to calculate how long it will run before it starts boiling the battery. For that reason I use one of my 2 smart chargers or couple of trickle maintainers for most stuff. The big unit gets used for starting stuff right now (which my 70@ smart charger usually did fine after a minute or 3) or for electronics projects like testing a starter or running a 12v motor or something. I can't be bothered to calculate the timer stuff properly.Most of the old-fashioned big ones will just keep on trying to charge after the battery is full. As this can boil off electrolyte, they generally should not be left connected continuously.
The older big "dumb" chargers do have a couple of advantages: the high charge rates (such as 25-50 amps) can help start a vehicle with a dead battery quickly, and they will try to charge just about anything. I've not really followed whether the newer big rolling chargers have incorporated "smart" features in their design.
If you want to leave it connected long-term, it's best to make sure you have a good quality smart charger that regularly assesses battery condition and charges accordingly. It's also important to make sure they are maintaining the proper voltage when left connected. I've run across a couple of cheap, off-brand chargers that were significantly over or under the recommended voltage.
Most people should just get a modern smart charger, unless you plan on using it for other non battery charging things. No difference in price.