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Author Topic:   Battery 1, Battery 2, or "All" while running
Foulweather Jack posted 06-01-2015 09:38 AM ET (US)   Profile for Foulweather Jack   Send Email to Foulweather Jack  
Recent posts in the "Repairs" section re the heavy current draw required to run the electric Verado power steering pump got me thinking: which battery setting should we use while running if we have two batteries?

I had always thought that battery 1 should be used as the "start" battery and battery 2 was the battery to use while listening to the radio at anchor, etc--the idea being not to drain the vital functions of battery 1 with other trivial, non-essential electrical loads. I recall that my Whaler manual states somewhere not to use the "All" setting with the battery switch, but I don't understand why. So in any case, I typically start with battery 1 and use that one while running, and switch occasionally to battery 2 to charge it up.

What's the best strategy to use, and why?

Jefecinco posted 06-01-2015 01:54 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Switch to ALL before starting.

Leave at ALL while running.

Switch to either 1 or 2 after shutting off the engine.

Switch to ALL before starting again.

Switch to OFF when shutting down for the day.

NEVER change the switch position while the engine is running.


jimh posted 06-01-2015 08:40 PM ET (US)     Profile for jimh  Send Email to jimh     
It was common for decades in small boats to have two batteries as sources of power in a single circuit primary power distribution system. The configuration of the batteries was controlled by a simple switch with positions OFF-1-BOTH-2. Either battery 1, battery 2, or both were selected as a source to power all loads.

Today, with boats having many more accessory loads such as electronics, pumps, and other electrical accessories, this legacy arrangement is not used as much as it once was.

Today on small boats a more typical configuration for two batteries is to maintain two completely isolated electrical power distributions, one for each battery. One battery, called the main battery or battery one or the engine cranking battery is connected only to the outboard engine. This battery supplies all the electrical power to the outboard engine for starting the engine and for operating any engine loads that require more current than can be supplied by the charging system of the engine.

The second battery, called the house battery or battery two or the auxiliary battery, is connected only to non-engine loads. This battery supplies all the electrical power to loads like engine instruments, chart plotters, depth sounders, radios, GNSS receivers, other boat electronics, house loads such as house lighting, navigation lighting, pumps, anchor windlass, motorized gear like down riggers, and so on.

By separating the loads into two groups, battery one (or the main battery or the cranking battery) will always be available for engine starting without being run down in charge by other loads. Battery two (or the house battery or the auxiliary battery) stays isolated from the engine loads, and supplies power to other devices without being affected by engine starting sags or spikes.

The problem with isolating loads and batteries comes in distribution of the engine charging current. Modern outboard engines, such as the Evinrude E-TEC (and others), provide for auxiliary charging outputs from the outboard engine's alternator. Having two charging outputs permits the two batteries to be maintained at charge without having to combine the batteries in parallel.

On modern engines that lack the desirable feature of dual charing outputs, the two batteries can be temporarily and automatically placed in parallel for charging by an automatic combiner relay (ACR) which is operated by a voltage sensitive relay (VSR) circuit. This is not quite a nice as having two alternator outputs, but can work well in many cases with outboard engines lacking the dual alternator output feature.

So the "best" configuration is to abandon the old-fashioned one-source switch, and re-wire the boat to use a more modern approach. If your outboard engine does not have two charging outputs, add an ACR/VSR device.

If you don't follow that approach, then manually operate the OFF-1-BOTH-2 switch as desired to distribute both the load current and any charging current as appropriate.

jimh posted 06-02-2015 09:05 AM ET (US)     Profile for jimh  Send Email to jimh     
'JACK writes:

I recall that my Whaler manual states somewhere not to use the "All" setting with the battery switch, but I don't understand why.

I don't see any particular reason to avoid starting in the ALL position. It might be necessary at some time if neither battery alone has enough charge to provide the current needed.

I typically start with battery 1 and use that one while running, and switch occasionally to battery 2 to charge it up.

That sounds like a reasonably good plan.

Foulweather Jack posted 06-02-2015 11:26 AM ET (US)     Profile for Foulweather Jack  Send Email to Foulweather Jack     
Thanks for the replies. I started the motor last night and ran the both with the switch in "ALL" position. Interestingly, I was having difficulty with the depth sounder on my combined Furuno Navnet (GPS, sounder, radar) system not working. Running the boat in the "ALL" position solved this problem, so evidently, using only one battery did not provide enough juice to service all circuits.

I don't know if I have a dual alternator output on the 250 Verado or not. I'll have to dig into the manual to see if I can figure it out.

jimh posted 06-02-2015 03:47 PM ET (US)     Profile for jimh  Send Email to jimh     
I agree with Boston Whaler on the recommendation to not use the ALL position of a battery switch ALL THE TIME. I can't speak for Boston Whaler's concern, but my concern would be the possibility that one of the two batteries was in poor condition, perhaps even having a shorted cell. When two batteries are in parallel they tend to act more like the weaker battery than the stronger one, roughly speaking.

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