I have a 3 bank onboard charger. I had a battery go down today while on the lake. I unhooked the dead battery and replaced it with a spare. I left 1 of the chargers banks unhooked and plugged it in to charge the other 2 batteries. I was testing the old battery, when I smelled something melting. I found out the hard way that you can't leave one of the banks unhooked. You must hook all 3 banks to a battery. You can hook 2 (or all 3) banks to 1 battery. It didn't charge any of the batteries. What it did was fry the wire's to the sonar. Lucky for me, the sonar was safe and sound in it's storage bag.