Bought a 15 watt solar battery maintainer yesterday. I have a 2 battery setup with the batteries hooked up in parallel on a 1 or 2 or both switch. Can I hook the charger up to just 1 battery and charge both at the same time? Or do I need to hook the + lead up to the + side of #1 battery and the - lead up to - side on the #2 battery?
You need 2 solar chargers to charge both at the same time.
Geronimo! Why would I need 2? When I use the charging system on the motor through the both switch it goes to both battries and has only 1 charger system!
The panel will do what the panel can do hooked up to one battery with the switch on BOTH, but it won’t do much. Hooking it up the other way means your 12v charger will be charging into 24v circuit, probably not good … 
Reelly Old! I have the battries hooked up in Parallel not series! It’s only 12 volt. The solar system will trickel charge at 7amp.
If you feel confident in splitting 7 amps between 2 batteries for only the max sun of a day to keep your batteries 100%, then go for it. Why bring it up here? But to maintain a deep cycle battery you should trickle 7 amps to it. and to maintain a small motorcycle battery you push around 3 amps to it.
Why not just maintain your main cranking battery and then switch it to both when you crank the motor to charge the backup? I would feel better knowing I have one full battery on go, than 2 batteries at 75%.
Your batts will die quicker than not if you don’t keep them charged up and even boost them every week or so with the engine running or a higher amp boost then cool down period. They are basically going to last the longest if you keep them conditioned. If they get out of shape, they don’t last as long. For wallyworld type batteries, you’re definately blowing your money by not keeping them maintained. You do have enough to maintain a little with the 7.5A to each for maybe 3hrs on sunny days. What about the rest of the time though? Also, when you put your switch on both, you will kill both batteries if either of them has a bad cell. You’ve turned them into a single battery when you run the switch on “all.” Don’t ever run on all unless it’s an emergency and you need the extra amps temporarily. I have been stuck 40 miles offshore before because of running the switch on “all” and assuming “all” were getting charged up when the engine was running. Bad charging system on the motor. Both batts got discharged, and I did not have an isolated batt left to crank up at the end of the day. If you leave the switch on “all” with your maintenance charger, you will have similar results if a single cell in a single battery goes bad. Don’t do it. Manually change the charger between batteries and keep them isolated. Don’t short cut it unless you have time or money to throw in the toilet. A 2 bank automatic marine charger is what you need. Use the solar charger as some insurance on the starter battery maybe. Keep it hooked to the starter circuit when you’re on the water or even install a selector switch on the solar charge lead so you can put the solar amps into whichever battery you want without manually changing it out.
Although a cool idea, solar chargers are usually meant to be on sailboats that aren’t going to have a generator or engine running all that much and may be between shore power for longer periods. The charger you’re describing isn’t really enough for two marine batteries. Better than nothing though because you are g
Phin! Would it work better if I had the charger, only charging 1 battery at a time? I could alternate which one I charged with the switch. That way I could keep at least 1 on trickel and charge the other when I ran the motor. If I switched them on trickel, I was thinking it had to help extend the life of each battery. I leave the boat sometimes for long periods where I can’t use any charger at all. Phin, I went over your answer more closley and you are saying what I was thinking! The switch will work to help maintain the battery that does not get the most use for a period of time.
My bad, misunderstood your alternative set-up … 
Take a look at an automatic charging relay. As long as there is enough current flow it will charge both batteries but if there is no current flow then the batteries will be isolated and you do not risk one battery killing the other.