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Author Topic:   Dual Battery switching
DeeVee posted 12-07-2005 11:17 AM ET (US)   Profile for DeeVee   Send Email to DeeVee  
I am interested in possibly wiring the batteries in such a way that one is the starting battery and the other is the "house" battery. As I study the diagrams, it doesn't look possible with the switch set up I have.

If anyone would like to comment on wiring dual batteries for this configuration, I would be interested.

Thank you,
Doug Vazquez
1989 22' Outrage
1992 200 Suzuki

Revenge 25 posted 12-08-2005 10:29 AM ET (US)     Profile for Revenge 25  Send Email to Revenge 25     
You do not need a battery switch to wire a "starting" and "house". In my opinion, a Voltage Sensitive Relay ("VSR" also known as a Automatic Charging Relay "ACR") is the best way to automaticaly connect the two batteries while being charged and disconnect (isolate) them when being discharged. Once installed, you don't have to do anything.

With a Batery switch you typically put the switch on "both" while underway and the motor's alternator is charging the batteries. Later, when you arrive at your location and turn off the motors, the switch is then used to isolate the starting battery from discharge. Murphy's Law says the switch will forever stay on "Both" and both motors will be drawn down. This results in both batteries being drawn down and requiring two batteries to start a motor that should start with just one battery. In a worst case sceneraio, you have drawn down both batteries such that the motor will not start even with both batteries combined.

An ACR fully charges the "starting" battery and then charges the "house" battery automatically by opening an closing a relay switch. If the "starting" battery begins to discharge the ACR isolates the starting battery from discharge and preserves it for starting. Blue Sea Systems offer two ACRs. I like Blue Sea Systems model 9112, since it also allows the batteries to be combined for a starting emergency. Blue Sea Systems provides a simple wiring diagram.

You may still want a battery switch as an emergency shut-off, but you do not need it to combine and isolate the batteries. Using a battery switch for that purpose is subject to human error and in my opinion just asking for problems. The other problem with a battery switch between the battery and motor is if it is turned off while the motor is running you can do some real damage to the motor.

jimh posted 12-08-2005 01:38 PM ET (US)     Profile for jimh  Send Email to jimh     
If you want to use an engine/house battery configuration, my recommendation for primary battery distribution and management would be along the lines shown in this document:

The next time I have the motivation and time to re-wire the battery distribution on my boat, I will probably use the approach shown in the document linked above.

Separate engine and house batteries have the advantage of isolating the house loads--which are typically expensive electronic devices like GPS Chart Plotters--from voltage spikes and dips which can occur during engine starting.

Suppose that starting your engine introduces a voltage spike or dip which causes your GPS to lose its lock on its position. This might occur at the worst possible moment in a distress situation.

andygere posted 12-08-2005 08:18 PM ET (US)     Profile for andygere  Send Email to andygere     
Jimh and Revenge 25 are right on the money with VSRs. Here are a few more threads on the topic:

I installed a BEP marine unit on my Outrage last year, and have been very pleased with the system.

DeeVee posted 12-08-2005 11:38 PM ET (US)     Profile for DeeVee  Send Email to DeeVee     

Thank you for the food for thought. I have a lot of reading to do now!

Doug Vazquez

Revenge 25 posted 12-09-2005 01:15 AM ET (US)     Profile for Revenge 25  Send Email to Revenge 25     

What size wire/battery cable did you run from the "starting" battery to the VSR, and then on to the "house batery"?


andygere posted 12-09-2005 01:49 AM ET (US)     Profile for andygere  Send Email to andygere     
Doug, I used 4 AWG wire for both the start and house systems.

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