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Author Topic:   Power Distribution on Twin Engine Boats
russellbailey posted 03-29-2013 08:34 AM ET (US)   Profile for russellbailey   Send Email to russellbailey  
On a twin engine boat, is a third battery that is isolated from the two starting batteries needed to handle house loads to protect the electronics from voltage fluctuation during engine starting?

The starting wiring on my 1984 Outrage 25 with twin 2006 OptiMax 150-HP engines is in need of replacment. Currently it is setup with dual Perko switches that will allow combining of both batteries if needed. House load is on the starboard battery. The boat currently has essentially no electronics on it, only a single inexpensive Humminbird.

I'm in the midst of significantly modernizing the electronics with a modern chartplotter and radar (likely Simrad NSS8 and 4G), plus likely some other pieces. The new electronics are a lot pricier than the $100 Humminbird on there now and I want to make sure I provide appropriate quality electrical power.

We commonly stop and start the Optis multiple times a day. When we do watersports we do it every 10 minutes or so, as we make it a practice to always have engines off when a person in the water is climbing back into the boat. Thus, the significant electrical load of an Opti is placed on the same battery sharing house load currently.

I am unclear if the starting load will cause problems for the new electronics. If it has the potential to do so, I will simply add a dedicated house battery and likely connect all with a BEP 917 cluster. If the starting load is not a concern for the electronics, I'll likely instead use a BEP 915 cluster.

I have good batteries: Sears Platinum Marine 34m.

Thanks for your assistance.

jimh posted 03-29-2013 09:07 AM ET (US)     Profile for jimh  Send Email to jimh     
With the increasing cost of marine electronics and its increasing sophistication, it seems to me that operating expensive marine electronics from an isolated battery is a good method to avoid problems that might occur if the engine starting battery has voltage sag during cranking or otherwise might cause voltage fluctuations. With twin engines and two starting batteries, providing isolated power for electronics will mean a third battery. Because the two engine starting batteries can provide back-up for each other for engine starting, the third battery for only electronic loads can be a battery with less capacity and smaller size, if desired, which may reduce the weight, size, and cost of the third battery.

I could not locate any information for the BEP Battery Cluster devices you mention. I did find this page of information about BEP Battery Cluster devices: clusters-manual

From that page I found more details about the BEP Model 717-140A device: 717-140a-dvsr-twin-outboards-three-battery-banks

It looks like a nice device for a twin-engine, three-battery set up.

Jefecinco posted 03-29-2013 09:12 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

I wouldn't think a third battery is necessary. Modern marine electronics don't draw a lot of power, with the possible exception of radar and big amplifiers for a stereo system. I assume you are not running the radar while enjoying water sports and you did not mention a stereo system. A VHF transmitter can use a bit of power but how often and for how long are you transmitting?

When you have made your choices on electronics you can determine your worst case load and make a considered decision based on that.

The only negatives of adding a third AGM are weight, space loss and cost. If those things are inconsequential to you go for it.


russellbailey posted 03-29-2013 09:15 AM ET (US)     Profile for russellbailey  Send Email to russellbailey     
I mistyped earlier with a 9 instead of a 7. I meant the 715 (no house battery) and 717 (with house battery) clusters.

I am leaning towards adding the 3rd battery - figure an extra $400-500 for the battery and extra cost of the appropriate switch, which is a lot less than the electronics.

jimh posted 03-29-2013 09:45 AM ET (US)     Profile for jimh  Send Email to jimh     
I recently re-wired my boat so the electronic loads were isolated from the engine starting battery. I had not previously observed any particular problems, but now I am more comfortable with the electronic devices running on their own battery.

The isolation works both ways. The electronic devices will not be affected by engine cranking voltage transients. The cranking battery cannot be drained by house loads.

6992WHALER posted 03-29-2013 05:05 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
I have twin Engines and I have a third battery that I just recently made a totally isolated house battery, with all the navigation electronics wired to it. The house battery is charged by a separate wire from my port engine. I have a battery switch so if the port engine fails I can switch the house load to charge from the starboard engine.

I replaced my three deep cycle/starting combo batteries with Two actual starting batteries and one deep cycle battery.

This was one of the best things I ever did to the boat. My electronics are safe when I start the engines and my starting batteries are not in danger of being drained by the house load.

Hoosier posted 03-29-2013 05:42 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
John, I like that, if the house goes dark, so what? If the engines don't start ...911..?
Jeff posted 03-29-2013 10:16 PM ET (US)     Profile for Jeff  Send Email to Jeff     
While this may seem like a dumb question to most but, how does one wire an isolated house battery? I know how it would be wired to supply power to the electronics but, how does it get wired so it charges off the motor without supplying power to the motor for starting?
russellbailey posted 03-30-2013 07:17 AM ET (US)     Profile for russellbailey  Send Email to russellbailey     
Jeff, in my case it would use voltage sensitive relays (vsr) to allow either start circuit to charge the house battery once the start battery reaches a certain voltage. The house battery does not see load from the start circuit with a VSR.

It sounds like John has something similar.

jimh posted 03-30-2013 09:19 AM ET (US)     Profile for jimh  Send Email to jimh     
Some models of automatic charging relay devices have a special input lead which is intended to be powered from the START circuit of the ignition switch. The relay drops off any connection between the batteries when the START circuit is energized. This is a further protection against the house battery being affected by the starting battery during engine cranking. The model with this feature is called the STARTING ISOLATION or SI model. For information about the BLUE SEA SYSTEMS devices that have this feature, see:

conch posted 03-31-2013 08:06 AM ET (US)     Profile for conch  Send Email to conch     
Some engines have a separate lead for house battery charging. Sometimes the lead is down in the cowling unused,or needs to be purchased and installed.A common ground cable is needed to the starting battery from the house battery.If this is an option on your engine then a vsr is not needed.Very common on large Yamahas for the last two decades.
Jeff posted 03-31-2013 08:55 AM ET (US)     Profile for Jeff  Send Email to Jeff     
Thanks Chuck. I have a 1999 Yamaha Saltwater Series II 150 so I will look fir that connection on my motor.
swist posted 03-31-2013 03:19 PM ET (US)     Profile for swist  Send Email to swist     
Coreect me if I'm wrong but virtually all modern electronic devices have computers and other senstive integrated circuits in them. The effect of any power supply jitter or other voltage misbehavior on these devices is not good to say the least. I believe most power supplies in these devices have very serious regulating and filtering mechanisms. You can get a hint at this by looking at the input voltage specs where it is not uncommon to see things like 8-20 volts input allowable for nominal 12V devices.

I am less familiar with modern AC input power supplies but I believe it is also the case that some of them now don't care if they get 120 or 240V input, and if it hasn't already happened, things like consumer electronics will no longer need different versions for different countries (save perhaps the power plug style).

jimh posted 03-31-2013 05:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Most devices that are nominal "12-Volt" devices will not be pleased to operate if the voltage supplied to them sags to 8 or 9-Volts. Also, I don't want to make a bet that a 100-Volt transient on a 12-Volt circuit is just going to be shrugged off by my $2,500 chart plotter.

As I already mentioned, the isolation works both ways. The electronics are isolated from voltage sags or spikes. The cranking batteries are isolated from being drained by leaving a cabin lamp on all night or a pump that runs too often. It is hard to argue against this arrangement, and the cost of wiring to achieve this is not very high.

Modern switching power supplies can work from a wide range of input voltages. But modern chart plotters and other 12-Volt devices expect to get 12-Volts. They do not expect to get 8-Volts or 48-Volts.

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