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add a battery wiring

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hey guys,

i've just installed a second battery on my 06 lsv 23 using the blue sea add a battery switch and charging relay.

i think i've wired it wrong with regards to the alternator. when the engine is on is shows 12v on my dash (instead of 14v)

when i use my stereo it drops to about 11.9v, and when i use my electric pump for the tube it drops to 11.7v.

At the end of the day of boating once all the house electrics are off (engine on) the voltage still shows 12v.

Is it possible that only the starting battery is being charged by the alternator and not the house battery? hence the dash is not registering the alternator voltage supply? or simply i am drawing more current than the alternator can supply?

thanks for your help

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yes it did read 14v with a single battery, so it is not an alternator fault.

i have wired it accorded to the pdf


is it possible that i have reversed the starter/alternator wires on the battery switch? or something along the lines of the wires feeding the house loads? (everything works fine, no short circuits, just not getting the 14v on the dash, and i believe the batteries are not charging)

shawndoggy, thanks for the diagram, but i do not have an onboard charger setup on the boat.

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You don't need an onboard charger, I don't have one either. But it you wire it up as I've depicted you can add one later and the system will work correctly (charging each battery independently and not combining them when you have the switch set to off).

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Those supplied diagrams are very generic and are impossible to apply accurately to all boats. The theory of the power flow is accurate, but you have to be able to translate what something in the diag means and apply it to what is on your boat.

You need to get a volt meter and start testing. To me, it sounds like you have 2 problems. 1) the helm BUSS B+ is currently connected to the house side of the switch 2) the ACR seems like its not closing.

With a volt meter, you can probe all 4 posts on the back of the switch. 2 are connected directly to the batteries, so voltage all the time and the other 2 are combined with the batteries when the switch is in the ON position. Next, test for voltage on both sides of the ACR 1st with the engine off, then with the engine running you should see alternator voltage. There is also a green LED on the ACR to indicate it has combined. If the hose battery is low, it may take a few minutes of run time or raising the RPM a little to get the ACR to combine.

You can post a diagram of how you've wired it, but a volt meter will be the best tool to determine if its wired correctly and also operating correctly.

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Hi Guys,

I am about to wire up a Add a Battery Pack too. Quick question. From the switch (number 2 side) Top mount we take a positive to the starter that all makes sense to me. But where in this diagram do we get the alternator charge from. In the manual I have it has a separate diagram discussing inboards and says. "Alternator wiring may include the following:

1. to Starter

2. to Engine terminal of battery switch and so on.

Now Im guessing when I put this setup in Ill find a line coming from the alternator and seeing the diagram above I dont have an onboard charger either so where do I connect the alternator line too. Sorry if this is a terribly stupid question. But any help would be great.



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The positive to starter is the alternator wire. If you follow the wires from your alternator, the charging wire goes right to the starter (which is where the pos to starter wire that's hooked to your batteries terminates).

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What size of cable is recommended from the selector switch to the charging relay (383 HH)? Also, should there be a fuse between the relay and the switch and should that fuse be based on the output of the alternator? If it's the alternator, does anyone know the rating on it?


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  • 2 weeks later...

Hey guys,

I have done my installation as per the diagram above thanks to shawndoggy, with the exception that I put the house loads and stereo on the same battery and left the starter just to do the starting. I also have a brand new Alternator. I took the boat out the other day and when I started it I was getting 13.4V at idle which I thought was alright (tested from the gauge on the boat and a separate voltmeter directly on the alternator). As the day went on that voltage reading seemed to drop to around 12.0 and then it would go to 12.9 and back down again. Sometimes on the ACR the green LED would stay on and sometimes it would be off.

Has anyone had this problem is it wiring? The only other thing is I bought one new battery and the other one is really old and has previously been fully drained and needed a fill recharge. Any help would be great.


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I am betting your alternator is on its death bed. Last year mine would provide good voltage then stop charging the start charging again. I had a local shop rebuild it and was good to go for the summer. This year it started doing it again. I left it that way for a while until it died - wasn't a big deal since I have a 20amp onboard charger and plug her in after every use. I bought a new alternator and that fixed it...though the bearings are making a bit of noise (nothing bad, just annoying), so I'll be getting that warranty replaced.

As a result, I bought a voltage regulator for my last alternator to see if that fixes the issue with it. I will be trying that alternator while my current one is in for repair.

Back to the point, I don't believe my issue is wiring related as my first alternator died pre-rewiring most of my boat and the 2nd post re-wire. I didn't replace the alternator wiring but it still looks good at the visible parts of it at least (most of it is in wire loom).

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I was thinking the alternator was the issue too. But ive just installed a brand new 75Amp Alternator. Should I have gone for the upgraded version the 90Amp? Could my old faulty battery be giving me these odd readings?

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