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Author Topic:   Perko Battery Switch #8501
electricaldoctor posted 05-06-2008 07:45 PM ET (US)   Profile for electricaldoctor   Send Email to electricaldoctor  
I bought a boat last year and it has two [PERKO #8501 battery] switches. I am installing three new batteries, and I want to have everything wired up properly. I want to have one battery for starting both engines, and I want to parallel the other two batteries to look after all of the typical house loads.

Both switches and wiring have been removed as it was a very sloppy installation, and it was almost impossible to trace which cables went where. Is it neccessary to have both switches for this configuration? How would I wire these three batteries up to the four-way switch?

Regards ~ Ken

jimh posted 05-06-2008 08:27 PM ET (US)     Profile for jimh  Send Email to jimh     
Give details about the battery switch.
jimh posted 05-07-2008 01:51 PM ET (US)     Profile for jimh  Send Email to jimh     
Ken sent me an email describing the PERKO #8501 battery switch. From his description it sounds like it is the common battery switch, an OFF-1-BOTH-2 design.

I do not like the proposed arrangement of the batteries. I do not recommend operating twin engines from a single battery. When you connect two engines to one battery you will be connecting the charging systems of the two motors in parallel. This could cause problems. It is also an unusual configuration. Generally in a twin engine installation each engine has its own starting battery, and each engine recharges that battery. The two battery and charging systems are kept isolated.

I do not like the configuration for the house battery in which two batteries will be connected in parallel. There is a general preference against connecting batteries in parallel. When two batteries are connected in parallel they tend to behave like the weaker of the two rather than like the stronger of the two. However, if the batteries are closely matched in terms of their size and state of charge, you can parallel them.

The fundamental problem to be overcome in a system in which the house loads are connected to a separate battery from the engine starting battery is to provide some means for charging of the house battery. The best way to accomplish this is by use of an automatic charging relay (ACR), also known as a voltage sensitive relay (VSR). The ACR will provide charging current for the house battery when the starting battery is fully recharged and surplus charging current is available.

There are many possible configurations for battery, switch, and motor interconnection. It is awkward to try to describe them in a narrative. There is some good literature available from BLUE SEAS:

A common configuration used with two basic battery switches, two starting batteries, and two engines is shown in my article in the REFERENCE section. See:

under the sub-heading "Boston Whaler Factory Installation."

If you have house loads that require a very high capacity, beyond what can be provided by a single battery, you will have to parallel two batteries. Make sure the two batteries you connect in parallel are as alike as possible, that is, of the same type, size, manufacture, and age. Then you can treat them as a single battery. Use an ACR to provide charging current for them from one of the motors.

seabob4 posted 05-08-2008 10:10 PM ET (US)     Profile for seabob4  Send Email to seabob4     
Unless you are planning on running a 12V Air-Conditioner unit on your other two batteries, why do need two batteries to run your house loads? The important [concern] is that you can fire up both those motors. Each with a separate battery with the ability to parallel if need be. I want to get home more than I want to listen to music! The third battery is your house battery. Get a good one, and you'll be good to go. Get an isolator and run it off of one of your starting battries to your house battery for charging purposes, when you shut the motors down, the isolator will automaticcaly disconnect the house from your starting batteries, play your mp3 all you want, run your spreader lights until the cows come home, when you go to fire up those motors, you'll be glad you've got fresh batteries to do it with!
electricaldoctor posted 05-11-2008 08:03 AM ET (US)     Profile for electricaldoctor  Send Email to electricaldoctor     
Thanx for the excellent information jimh. With regards to why I wanted to parallel 2 house batteries, we anchor out most weekends and I just wanted to have lots of reserve power to keep the fridge operating efficiently as well as general lighting, music, etc. We also must have our anchor light on over night. I have purchased a solar panel to help out in this regard.

To recap our existing system components, we have (2) 4.3 litre Merc-Cruiser engines (2 alternators with noise filters), (2) Perko 4-way switches (1,Off,2,Both), (1) charger/rectifier with 3 charging circuits, (1) solar panel (15 watt/1 Amp) that is used to keep the house battery topped up & (1) 3,000 watt inverter.

I could always cap off one of the charger's leads, but I remember reading something that said that the charging leads should always be connected to a battery to avoid damage to a charger stator coil.

To satisfy our boating syle, I feel that we should parallel (2) new new deep cycle marine batteries for the house requirements (750 cranking amps, 205 minutes reserve). I would then purchase (2) new batteries for the engines. Should these batteries be deep cycle or gegular cranking batteries?

regards ~ Ken

seabob4 posted 05-11-2008 02:15 PM ET (US)     Profile for seabob4  Send Email to seabob4     
Well, now that I'm a little more informed about the appliances you have onboard, I can see your 12V requirements are somewhat different than your typical Whaler. Using (2) Deep Cycles wired in parallel for your house is an excellent idea. You can run the third leg of your charger to one of the tandem and the charge will flow through both. It will take longer to charge, but eventually it will. A charger needs to sense a returning voltage to have an output. Put a meter on a charging leg that's not hooked up to a battery, and you won't see an output. If the circuitry inside the charger doesn't detect a load, it won't put out any voltage.

As far as your engine batteries go, get a pair of good Marine starting batteries, and that should take care of the motors. Of course, as Jim said, an isolator wired in between your engine battery charging circuit and your hose charging circuit will make sure the engine batteries are untouched by your house loads.

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