I have two batteries on my boat with no battery switches (winter project #982734892349783254).. i got to the boat today to get her ready for LOTO and both batteries were dead.. left the bilge pump switch on and the stereo and drained both. My onboard charger is toast (winter project #19) so I only have one charger. If I turn my charger up to 15 amps and hook it up to the port battery and run a set of jumper cables to the starboard battery, will both charge up overnite?

Sure, even a trickle will do both overnight.

Dont fry them at 15amps.

thanks ron p

Also if they are hooked together and you have two different rated batteries or one that took a harder hit being ran all the way down to dead, they will work against each other to try to equalize the voltage and could cycle down to nothing without having an external voltage draw. Moral of the story is disconnect them if they both recharge okay (since you don't have a switch) so they don't cycle down by themselves when not in use.

So what setting should i put my charger on? 2, 8, or 15 with jumper cables hooked between the two..

What size batteries?

Truth be, you shouldn't. Every battery will accept a charge ad a slightly different rate. The regulater in your charger doesn't recognise the two batteries, just an average so one could be charged too hot ( fast/current/voltage) and one to cold( slow/current/voltage). Multi bank chargers ( good ones ) read and charge each battery seperately. Charge em seperate at 15 amps and you will be fine.