I did not want to hijack the other battery thread, so I will show my ignorance here. Boat is on a lift in a covered slip an hour away. I have two batteries on a switch and an onboard charger. Bilge pump is wired direct to battery 1. Charger is wired to battery 2. There is a cable connecting the negative poles of the two batteries.

My question is....Assuming both batteries will hold a charge, with the charger hooked to battery 2, will both batteries be charged?

Saturday I plugged in the charger to power at the slip and went to work on our lake cabin. Came back about 4 hrs later and the light on the charger had gone from red to green and my assumption was that that both batteries were charged. This was the first time to start boat since December and there was not a sufficient charge to start the boat(Yes I know my lazy a$$ should have pulled the batteries and charged them, but I always thought that "next week" we will have time to take the boat out). Batteries are Duralast brand and at least 3 yr old. It may just be that both batteries were bad and neither will hold a charge, but I want to know if my assumption is right that the charger should charge both batteries the way it is wired. Thanks.

BTW, I bought a new Optima blue top last night at Costco. There is a $25 rebate on Optima and you can register for rebate online if purchased at Costco.

If your switch was in position for battery 2 or off then no. No way the current can reach battery 1 this way. If you set your switch to both and your charger is big enough then yes. It could have charged both batteries.

I would recommend replacing the charger with a dual battery type. I have a Guest that works well. It keeps the two batteries isolated and charges regardless of the battery switch position. Alternatively, you could add a second, on board charger and wire them together on the AC side. Two singal chargers on a shared AC service is really all a dual charger is.

I also recommend having a special purpose charger that is designed to maintain two batteries separately.

Cranking and deep cycle batteries have very different characteristics and are not the same when it comes to charging and maintaining their state.

Also, the two batteries are used differently during the day and thus will have different charging needs.

Dual battery chargers know the difference and are best suited to the application.

As Dave points out, these units are directly connected to your batteries and thus you need not worry about the battery switch position as far as charging is concerned.

My thinking is that with a proper charger, the batteries will last longer and provide more reliable service during their life.

A dead battery can really put a kink in your day on the water. It is too easy to eliminate the risk.

Or get a smart battery combiner. It will charge both batteries with your current charger, if big enough, and you get the advantage of charging both batteries when cruising.

Thanks guys. I obviously need to read up and get more educated in this area. This is my third season with this particular boat and I had a complete tune up and everything checked last spring. This is also my first boat with two batteries and in my ignorance I have just operated on the "both" setting (we mostly cruise and pull tubes, and rarely operate accessories with the engine off). I assumed (yea, I know) that the switch controlled the current to the boat systems and, since the charger was on the same "side" of the switch as the batteries, both batteries were charging when in the "off" position. Apparently only the deep cycle was being charged with not enough juice to start the engine (as that is not its purpose).

As it turned out, the battery I pulled and brought back with me was the start battery. The new blue top will easily serve this purpose and I can check the deep cycle to see if it will hold a charge. I will try to get the details of the current charger and make the needed adjustments. I am sure more questions will be forthcoming.

Although discussed above, I'll summarize; you can charge two batteries by the following methods:

1. placing your 1-both-2 switch into the both position.
2. purchasing a combiner that connects the batteries in parallel during charge (this works for both engine and portable chargers).
3. installing a battery isolator.
4. purchasing a dual charger.

#1 was fairly popular a few years ago, and is still a popular method used by manufacturers. Unfortunately, if you leave the switch in the both position - after the engine quits (and the engine alternator quits charging), or after removal of a portable charger, both batteries can discharge simultaneously.

#2 is a very good method these days as the combiner will allow charging of both batteries regardless of the switch position, and you don't have to remember to switch from Both when the engine stops.

#3 has lost favor since the development of the combiner, but can still be found in some boats. A battery isolator connects the batteries together for charge purposes only, and is done via "steering diodes". Unfortunately, with the diodes in place, a battery will charge slower, or may not completely charge.

#4 is the preferred solution for dual batteries if you want a permanent setup, but does nothing to charge both batteries from the engine.

The current thought is a combination of #2 and #4, as #2 will charge both batteries when the engine is running, and #4 will charge both batteries from shorepower.

While there is some redundancy here (i.e. a single charger could be used), having a dual charger will charge the batteries quicker. And as many boats with dual batteries have significant DC power requirements (for instance a cruiser, with fridge, pumps, lights, etc), one bank of the battery charger will actually take on the task of providing the house current for all of these other needs. If only a single battery charger were used, it would have to do double duty in this regard.

Thanks Al. That is a great summary and explanation. I was frankly somewhat confused, thinking that a combiner and dual charger were the same thing. Thanks for setting me straight.

When I bought this 21 ft Hurricane Sundeck deck boat w/ 150 hp OB, I was surprised that there were two batteries on this type boat. I had a 24 ft Hurricane Fundeck with a 5.7 Merc and it had only one battery. As I said, I have had the switch on the "both" setting when operating, but part of my routine when we leave the boat is to always turn the switch off so as not to drain the battery. My second time out on this boat it would not start. After a few minutes I discovered that some fool forgot to turn the battery switch on.

+1 on setting switch to "BOTH" to charge 2 batteries.

I do it that way all the time. If the batteries are really dead, I charge one, then the other; it keeps the flames from shooting out of the charger.

FYI - I have a setup like #2 and #4 from Al's post above. The dual battery charger is a Guest 'smart' charger. Blue Sea Systems suggested disabling the Combiner when using a dual smart charger to ensure it could sense each battery independently. (A switch on the ground from the Combiner disbles it.)

While thinking about something else, it jumped out at me that the negative cable between the two batteries is likely the only ground for the second battery. It's my understanding that grounding each battery directly to the engine block is the preferred method - you might consider going this route if you are going to make other changes.

I agree with your battery grounding thoughts.