This is an FYI for others.
I tried to be cheap and tried a $6 one-amp charger and ended up returning it.
While bench testing it I noticed that when the charger is not plugged in, but still connected to the battery it is draining the charge from the battery.
I guess extra diodes would have doubled the price.
My larger charger doesn't drain when unplugged, but it's also not practical to mount in the car for on-board use.
Back to looking at the expensive marine units.