5.7 mercruiser add 2nd battery/switch

[firstinfireman]

I'd like to add a second battery for my accessories and leave the starting battery for just that. My question is will the alternator charge both? Second is how would I wire it in while adding a master battery switch.

 

[Bt Doctur]

Re: 5.7 mercruiser add 2nd battery/switch

quick and easy is to wire the isolator across the battery switch. Whatever position you select charges thru the switch and the other one charges thru the isolator.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

Add-A-Battery Kit - 120A - Blue Sea Systems

 

[Walt T]

Re: 5.7 mercruiser add 2nd battery/switch

Hey First in, Colorado firefighter here where you at? 20 years now a Division Chief

 

[JustJason]

Re: 5.7 mercruiser add 2nd battery/switch

Add-A-Battery Kit - 120A - Blue Sea Systems

Agree, and that's the kit most folks go with. However there is more than 1 way to wire a boat, especially batteries. And the switch that comes with it is a simple On/Off switch. And you can use that switch to either combine batteries in case of an emergency (dead main), Or you can use it as a disconnect, but not both.

Another way to do it, and how I did my last boat, is I use the same Blue Seas ACR that comes in that pack, but instead I use a 1/2/ALL/OFF battery switch. And the way I wired mine up is my main battery is #1, from the main there is another wire that runs over to the ACR, and from the ACR to the 2nd battery. And then the 2nd battery is also paralleled to #2 on the switch. (I think I explained that right).

Now the way it works is when your OFF, all batteries are disconnected from everything except for my bilge pumps. (those always stay powered up, no matter what). When I turn the switch to 1, the main battery connects to the engine. And when you start the engine the engine charges the main until its full, and then the ACR kicks the charge over to the 2nd battery. The way the boat is wired, is the engine and the bilge pump is connected to the main. Everything else is connected to #2. I can run all my lights and stereo and GPS until the 2nd is dead, and the main will start every time. And in the event the main does die, I can flip the switch to #2 in order to bypass the ACR and combine the batteries for emergency starting.

That's how I did my last one.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

Another way to do it, and how I did my last boat, is I use the same Blue Seas ACR that comes in that pack, but instead I use a 1/2/ALL/OFF battery switch.

That's also how one of my boats is done. The other is with the add a a battery kit.

He specifically says he wants a battery just for accessories though.

 

[alldodge]

Re: 5.7 mercruiser add 2nd battery/switch

I'd like to add a second battery for my accessories and leave the starting battery for just that. My question is will the alternator charge both? Second is how would I wire it in while adding a master battery switch.

You can also add an isolator and it will keep both charged but they are not connected by any other means. An onboard charger can be connected directly to each battery for charging at home/dock.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

Only problem with isolators is that your batteries will never get fully charged due to the voltage drop across the isolator. You could use remote sense on the alternator but that is sort of a bandaid since you have to choose which battery to put it on.

 

[alldodge]

Re: 5.7 mercruiser add 2nd battery/switch

Good greif, :facepalm: been using an isolator for more years then I can count. My boat has been wired this way for almost it's entire life. Never had an issue with not having a fully charged battery. Your talking about 0.7 of a volt difference for a diode. If 0.7 makes a difference in being able to start you need another battery.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

Doesn't charge it fully. Yes it makes a difference. That'd why people run remote sense lines. Hardly anyone uses these anymore now that ACRs are around. I used to use them years ago and had thr regulators adjusted to compensate for the drop.

 

[alldodge]

Re: 5.7 mercruiser add 2nd battery/switch

Doesn't charge it fully. Yes it makes a difference. That'd why people run remote sense lines. Hardly anyone uses these anymore now that ACRs are around. I used to use them years ago and had thr regulators adjusted to compensate for the drop.

You should have thought of this. This is a Mercruiser which uses a sense lead which turns the alternator on. The feed comes direct from the battery via the ignition circuit. The isolator feed goes in between the output of the alternator to the start battery. The alternator will fully charge the battery and the isolator makes no difference. Your talking about a one wire setup.

 

[JustJason]

Re: 5.7 mercruiser add 2nd battery/switch

I agree with Bruce. And it's not so much your starting voltage you have to worry about, it's your reserve capacity. Most people add a second battery so they have something to draw off of when they are sitting in the water engine off, without worries of killing the main.

Lets say you bought a battery that had an advertised reserve capacity of 20 amps an hour for 2 hours, and then charge it through an isolator, its actual capacity is now going to be more like 20 amps an hour for 1.5 hours. It will of course still start an engine, but it's not going to run your stereo/gps/fishfinder for as long.

The exact math would depend on the specific battery and specific isolator, but that's the gist of it. But with an ACR, you get a full charge every time.

 

[alldodge]

Re: 5.7 mercruiser add 2nd battery/switch

Agree with your most folks comment but makes no difference. After sitting and drawing our second battery down low or only a little, you head for home. While running there is no problem with try to fully recharging the second battery, there is not enough time nor amps. You make it back home, unload the cooler and plug in your battery charger, your all set for tomorrow. Didn't matter if you had it hooked direct to the alt of not there is not enough time to recharge fully. Only battery your worried about having fully changed is the one you use to start the engine.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

Depends on how you use your boat. We would spend all week on ours up in Canada/Alaska or Lake Powell. We would spend a lot of time moving around so there was plenty of time to get the battery charged. We had an electric fridge that we used so it would draw the battery down during the day.

If you are only using your boat for a day and then charging the battery when you return, neither an ACR or an isolator is really even necessary for that matter.

 

[alldodge]

Re: 5.7 mercruiser add 2nd battery/switch

Depends on how you use your boat. We would spend all week on ours up in Canada/Alaska or Lake Powell. We would spend a lot of time moving around so there was plenty of time to get the battery charged. We had an electric fridge that we used so it would draw the battery down during the day.

If you are only using your boat for a day and then charging the battery when you return, neither an ACR or an isolator is really even necessary for that matter.

What no Gen and weeks of on the boat, the inhumanity
Give me a GD brake, quit trying to justify that you have the only answer to any and all situations. I to have spent many days on my cruiser and never had an issue of the house battery not being able to recharge it. My comment as you "should" know (but probably will not admit), dealt with the statement posed by JJ, there is not enough time to charge the battery due to amp hour capacity. Your trying to justify that you have the one and only correct answer and no one else dare refute it. Come on back for your last word, and don't forget to wash your hands.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

Have a nice day!

 

[thumpar]

Re: 5.7 mercruiser add 2nd battery/switch

I used an isolator on my old boat. One battery ran the trim and engine. The other battery ran everything else. We hang out on the beaches on our lake and listen to music and blow up tubes/floaties, etc. I picked up a second battery for the new boat and thinking about going to a relay but not sure yet. The isolator is nice because it keeps the batteries isolated all the time not just when not running.

 

[firstinfireman]

Re: 5.7 mercruiser add 2nd battery/switch

Thanks for all the replies. Let me modify my question now. With your typical 1 2 off all switch I would wire my house accessories to the switch common to make the switch disable them in the off or number 1 battery position. Basically what I want is to be able to disconnect power from my accessories while boat is parked. As well as keep one battery isolated for starting. I.e. Battery 1 start battery. Battery 2 accessories. Will this switch isolate the two batteries while in the all position as far as starting and accessories goes. Or would my accessories be pulling from both batteries. Obviously I'm new to these switches.

 

[bruceb58]

Re: 5.7 mercruiser add 2nd battery/switch

If you wanted to use a 1/2/both/off switch, you could use #1 battery while you start/underway and then switch to #2 battery if you were going to use your accessories for a long time while you were anchored, switching back to #1 when you wanted to start. When you are parked you would turn the switch to off.

Kinda depends on what your usage is going to be. For the Wellcraft boat I currently have, I just have 2 Odyssey batteries so I just leave it on 1 all the time and my #2 battery is basically my backup. I have a 1/2/both switch on that. On my pontoon boat, I have the other type of switch that is basically a switch for each battery connected to its particular load. It has a starting battery for the motor and a deep discharge for the house battery.

 

[JustJason]

Re: 5.7 mercruiser add 2nd battery/switch

Like this

Bat 1 is for engine starting and bilge pumps

Bat 2 is for everything else

During normal operation, you switch from off to 1, and go use the boat.

Alternator charges bat 1 until full, then starts charging battery 2 via the ACR

If for some reason bat 1 dies when your out. Switch to 2 to use batt 2 to start the engine. Or switch to ALL/BOTH to combine both batteries.

Bats 1 and 2 are electrically isolated unless the switch is in the ALL/BOTH position. Which means 2 can never draw from 1. And 1 can not draw from 2.

If you look at the round battery switch in the diagram, the A is your Common output. I just labeled it A.