Thank Goodness For Two Batteries

[Sunswan]

I read in one of the post "to charge both batteries at the same time", Batteries charged like that will only charge to the max. of the bad battery. If the weak battery has a bad cell and will not fully recharge the the good battery will only be charged as the bad ine. A battery isolator, as mentioned several times, is the only answer. MHO is solar, isolator, alternator are the only setup worth having. Me being a new boat owner and electrican, that is the route I'm going in. As I have heard many used boats have a rats nest of wires, I found this is true un my case. Winter in the back yard will take care of the wiring problem.

[osirissail]

Sunswan - quite true as you stated - but - for safety and convenience is is common to have at least two batteries onboard the boat.

One dedicated to the "ship's services/house" like lights, radios, instruments and other electrical equipment.

The other battery dedicated to starting the engine. AND - they are separated by a battery switch or switches that keeps then separate from one another until you decide to do otherwise.

For the easiest and most convenient recharging of the batteries the use of a battery isolator (diode bar) or Battery Combiner (voltage sensing relay) allows the alternator or shore power battery charger or some other source like wind generator or solar to keep the batteries from being electrically connected to each other. So if one battery goes bad or is mistakenly discharged too much, you have the other battery available to start the engine and get you and your boat home.

The exact equipment and choice of such systems is extremely varied and you will find pages and pages of recommendations of which one or more to use. But the bottom line is "keep them separate" and keep one of them available to the engine so you can get home.