Whats the proper way to add a third battery?

[muskie hunter]

I have a 1996 7.4 Bravo II. It currently has 2 batteries with a switching dial for batt #1, batt#2, both batteries or off. I want to add a third for more capacity to handle the refigerator, stereo and other odds and ends. I know I could just series into one of the other 2 batteries but Im not sure this is the best way to handle it.
Would an isolator be apropriate? I think I want to arrange it so each batt can be used individually or all at the same time.
Any Ideas or thoughts would be appreciated!!

Steve

 

[muskie hunter]

Re: Whats the proper way to add a third battery?

I guess no one uses three batteries in their boat?

 

[bruceb58]

Re: Whats the proper way to add a third battery?

This is what I would do.

Assuming battery #1 and #2 are your existing batteries, I would put a plain on/off switch between battery #3 and the common terminal of the exist 1/2/Both switch. This would be only used in an emergenct start situation. It will need to be a high current switch. I would also put a on/off switch between battery #3 and all of its loads(fridge, radios etc). I would put combiners between batteries #1 #2 and another combiner between batteries #2 and #3 with a switch to disable the combiners if needed. I would not use isolators as the voltage drop will prevent the batteries from getting fully charged.

 

[PondTunes]

Re: Whats the proper way to add a third battery?

You could also buy another battery that is equal to say battery #2, and wire them in parallel, giving you much more reserve and just crank using battery #1.. If you want to be able to use the three independently things get a little more complex. Since you already have a 1/2/off/both switch you really don't need to change the way everything is already wired. I would recommend just adding another in parallel.

 

[bruceb58]

Re: Whats the proper way to add a third battery?

I would recommend just adding another in parallel.

I wouldn't. One bad cell on either battery will bring down the other.

 

[PondTunes]

Re: Whats the proper way to add a third battery?

Adding combiners will still drag the batteries down when they're combined if one of them has a bad cell.. They might be getting charged from the alternator but it could mask the problem and he could end up with the exact same problem.

Since a combiner will connect the batteries in parallel just like a 1/2/off/both switch in the both setting will if hes paranoid of having dead cells in his batteries he's probably better off with isolators, because the bad battery could just keep dying and never be able to draw off of the good batteries regardless of the voltage drop.

Also muskie you want your batteries in parallel not series! Series would give you 24 volts so be sure you know the difference!

Parallel is Batt #1 pos, to batt #2 pos and batt #1 negative to batt #2 negative. = 12v dc with more reserve

Series is Batt #1 pos to load, bat #1 negative to Batt #2 positive, then Batt #2 negative to load. = 24v dc

 

[Zero Balance]

Re: Whats the proper way to add a third battery?

Since a combiner will connect the batteries in parallel just like a 1/2/off/both switch in the both setting will if hes paranoid of having dead cells in his batteries he's probably better off with isolators, because the bad battery just keep dying and never be able to draw off of the good batteries regardless of the voltage drop.

I would consider this system. It will switch to the battery bank that needs a charge at the time not combining the two banks. The Voltage Sensitive Relay is the key to this system.

http://www.bepmarine.com/Clusters-180.html

 

[bruceb58]

Re: Whats the proper way to add a third battery?

The combiner will be disconnected while the engine is off so the battery with the dead cell will not be connected. While the batteries are connected and charging, the good battery will still get charged as long as the peak capacity of the alternator has not been reached.

Isolators are a poor solution since the batteries will never get fully charged in the first place.

 

[PondTunes]

Re: Whats the proper way to add a third battery?

According to BEP's website their switches parallel the batteries whenever the voltage on the starting batteries reaches 13.7 volts. The also offer a model with an emergency parallel function but it still functions as described previously..

If they do offer a relay that selects the bank based on the level of discharge wouldn't a relay that did such keep trying to dump all the available charge in the battery with a dead cell neglecting the other batteries until they were dead too?

The combiner will be disconnected while the engine is off so the battery with the dead cell will not be connected. While the batteries are connected and charging, the good battery will still get charged as long as the peak capacity of the alternator has not been reached.

Isolators are a poor solution since the batteries will never get fully charged in the first place.

I'm not a big fan of isolators either because of their voltage drop, but forcing charge on a battery with a dead cell isn't a great idea either regardless if it's done via an automatic relay or a manual switch.

I think we're getting a bit off topic here anyway; there are several ways to accomplish what he's trying to do and I don't really think the random possibility of a dead cell in a battery should be his primary consideration. You should always check your batteries out from time to time as leaving the dock with a battery with a dead cell is a bad idea regardless if you can disconnect it with a switch.

 

[bruceb58]

Re: Whats the proper way to add a third battery?

I would consider this system. It will switch to the battery bank that needs a charge at the time not combining the two banks. The Voltage Sensitive Relay is the key to this system.

http://www.bepmarine.com/Clusters-180.html

The voltage sensitive relay is a combiner

 

[bruceb58]

Re: Whats the proper way to add a third battery?

RV's have been using combiners for years. They have also gone away from 12V batteries in parallel to get greater capacity and gone with two 6V batteries instead because of all the problems of hooking up 12V batteries in parallel.