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  Second Battery electrical question for somebody smarter than me!

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Author Topic:   Second Battery electrical question for somebody smarter than me!
nrathbone posted 04-17-2012 12:43 PM ET (US)   Profile for nrathbone   Send Email to nrathbone  
I recently added a second battery to my 170 Montauk, that otherwise had a pretty stock electrical set-up from 2006.

Between the two batteries, I installed a BEP Voltage Sensitive Relay.

(I did not install a battery switch to combine the batteries against the wishes of the VSR, as I figured I had plenty of ways to combine them in a pinch if I needed to, like jumper cables!)

I attached the big positive battery cable that runs to the engine to the start battery. I attached the small positive wire (which presumably heads to the main breaker, then off to power electronics, bilge pump, etc) to the house battery.

In theory, the only time that there is continuity between the positive posts of these two batteries is when the start battery is receiving sufficient charging voltage that the VSR opens a path from one to the other.

However, in reality, it shows continuity when I check it - all the time. Why is this? It seems I haven't really isolated the batteries.

I checked the VSR unit itself - it seems fine. There is no continuity between its two sides when it is off.

So I guess that the little positive wire must have contact with the big battery cable by some means, somewhere in the boat? For no particular reason, I'm suspecting something to do with the bilge pump?

Has anyone run across this problem? Or have any suggestions? It seems stupid to install a separate battery if it isn't really separate.

Thanks!!!

whale posted 04-19-2012 02:30 PM ET (US)     Profile for whale    
I have blue sea ACR, pretty much same thing.
I cant remember why but I did hook my little wire to the starting battery. That little wire is hooked up to running lights and all, I wanted to keep it to the main battery so I could still navigate at night with a dead house battery.
I was going to separate the bilge pump to the house battery but my boat sits on trailer most of days anyways.


What I would do is to remove the little wire off the house battery and test the connection of two batteries. if off, your wiring of the VSR is collect and there might be some kind of connection between the engine power and tach/volt meters.

if you still cant figure this out, you might just want to isolate the bilge to run from house battery.

Ricky posted 04-20-2012 02:45 PM ET (US)     Profile for Ricky  Send Email to Ricky     
I have Blue Seas ACR myself. Seems like no one is putting any load on the house batt. Lights, bilge, electronics should all be on house. Ok to run at night because acr will be closed and house batt will be charging. The way they are wired now the batts may would be better off wired in parallel.

All engine wires should be connected to start batt.

The relays will stay closed until they sense a draw on the battery,ie a voltage drop. The relay instructions tell you the voltage they open and close at, also the time delay. Because of no load, relay may not be opening.

On Blue Seas model there is an led that is lit if the relay is closed.

These things are fairly complicated but if wired correctly work great. The ability to run stereo, electronics without worrying about draining starting batt is why you install a relay. One other benefit is that your electronics will not be affected when you start engine from low voltage during starting process. Sometimes electronics are fried because of low voltage.

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