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Author Topic:   Charging A Second Battery
carlo2533 posted 03-15-2011 02:06 PM ET (US)   Profile for carlo2533   Send Email to carlo2533  
[I] just bought a 2010 [Boston Whaler 190 MONTAUK] with VERADO 150-HP. [I] want to add a second battery for radio, electronics, and piece of mind. [W]hat is the best way to do this so the alternator charges both batteries?
jimh posted 03-15-2011 05:23 PM ET (US)     Profile for jimh  Send Email to jimh     
I think the best way to maintain charging to a second battery and keep it isolated from the primary battery is to have an auxiliary charging output from the outboard engine. I am afraid that I cannot tell you with any certainty if your Mercury VERADO engine provides that option. My guess--only a guess--is that it does not.

If lacking an auxiliary charging output, the next best way to maintain charging of a secondary battery is to use an automatic combiner relay (ACR)--sometimes also called a voltage sensitive switch (VST). The company BLUE SEA SYSTEMS makes a very nice ACR and has excellent technical literature that describes how to properly install it. The drawback to an ACR compared to having an auxiliary charging output from the engines is:

--the ACR installation is rather complex if you follow the recommendations, which require a multitude of fuses to be used, creating many additional connection points and wiring problems; and,

--the ACR places the two batteries in parallel, thus defeating the purpose of having isolated electrical distribution between engine and accessories.

To help prevent engine starting voltage spikes from affecting the isolated battery and its loads, a further complication has been added to the ACR function, called "starting isolation" or "SI" for short. This adds additional wiring complexity and more fuses, but an ACR with starting isolation is the best way to accomplish more isolation between electronic loads and the engine starting battery.

BLUE SEA SYSTEMS has a good website found at

Read about the BLUE SEA ACR with starting isolation at

deepwater posted 03-15-2011 06:09 PM ET (US)     Profile for deepwater  Send Email to deepwater     
What I did was add up all the amps at max draw from all my electronics I wanted isolated and I got a lawn tractor battery that was way over that and wired everything to a separate buss strip with fuses and run them all from that battery,,I took it home after a long trip and put it on a battery tender and it was ready the next trip,,Battery weight 12lb approx and about $22
jimh posted 03-15-2011 07:10 PM ET (US)     Profile for jimh  Send Email to jimh     
The advice sought is "Best that alternator charges both."

Adding a second battery that is not charged by the outboard engine is not a good solution.

lakeside25 posted 03-15-2011 08:20 PM ET (US)     Profile for lakeside25    
If you dig deep into the Blue Seas website and look for the Application Briefs you'll find some great "how to" articles such as ...

Read the complete series.

Jefecinco posted 03-16-2011 09:44 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

A satisfactory method that is fairly simple and relatively inexpensive is what I use. It requires two batteries, a four position selector switch with the following positions: Off, Battery One, Battery Two, and Both Batteries. Some additional battery cable and terminals are also required.

I recommend the use of AGM batteries and they are recommended by Mercury for use with Verado engines. There are several good brands available but Sears Diehard AGMs are high quality with a good warranty and are priced within reason.

There are wiring diagrams available so I won't try to provide one. If it is not overly complex I recommend dedicating one battery to the engine. Call that battery "One". Dedicate the second battery to the other electrical loads such as electronics and lights. Call that battery "Two".

Because this system does not have any relays to ensure both batteries are charged whenever the engine is running the operator must pay attention to the switch selection used. In my experience this is not inconvenient.

When starting the engine I select the "Both" position. This provides a generous cranking source for the engine. I leave the switch in this position while underway to allow the engine to charge both batteries. When stopped with the ignition switch off I change the switch position to "Two" or "Off". If I will use any power I select "Two" or if not I use "Off". Before starting again I select "Both". After shutting the engine down for the day I place the switch in the "Off" position. With this system battery "One" serves as a backup battery and will serve to provide sufficient starting power if battery "Two" has become discharged by the house loads. Storing the boat with the batteries turned off helps to ensure that no parasitic loads discharge the batteries.

I believe there is much information on this subject in the Reference Section of the site. The information in the Reference Section will be presented in a much more understandable and useful manner. Give it a look.


jimh posted 03-16-2011 10:19 AM ET (US)     Profile for jimh  Send Email to jimh     
See my article in REFERENCE on dual battery configurations at

The article only shows simple configurations, and it does not show any ACR or VST wiring.

For information on charging a second battery with an auxiliary charging output see

Jerry Townsend posted 03-16-2011 11:36 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
The simplist way is to use a solenoid (common automotive starter solenoid) and actuate the solenoid from the engine's alternator. Connect one of the solenoids main terminals to positive post on the main battery - and the other solenoid terminal to the positive post of the second battery.

With only one charging connections/lines from the alternator, install a diode in the charging line to the main battery- so that electricty can only flow to the battery - and not from the battery.

Connect the negative terminals of each battery together with a #2 - 4 cable.

The solenoid coil is wired using the case as the ground - so connect a small wire from a battery negative post to a mounting screw on the solenoid.

This automatically charges both batteries when the engine is operating and isolates the batteries when the engine is not operating. The solenoid is sealed - but mount it in the console anyway. The solenoid might cost $25 +/-.

Should the main battery go belly-up - simply remove the cable from the main battery and put on the secondary battery. ---- Jerry/Idaho

jimh posted 03-16-2011 05:31 PM ET (US)     Profile for jimh  Send Email to jimh     
An ACR or VST is a solenoid, it just is a solenoid with some intelligence in its control system that knows when it ought to be closed.
Jerry Townsend posted 03-16-2011 06:22 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Jim - I might be wrong - but, the only time I see that the contacts should be closed to charge the second battery is when the alternator is operating. Actuating the solenoid from the alternator makes this contact. --- Jerry/Idaho
Jefecinco posted 03-16-2011 06:41 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

Thanks for the judicious editing. Much improved.


SJUAE posted 03-16-2011 07:44 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     

Are you sure about charging both on the simple 4 way switch usually fitted?

I thought it was specifically noted in the handbook not to use both for any extended period and the only way to charge both is to use a ACR unit or similar or with an ETEC when fitted with the split charger cable


Jefecinco posted 03-18-2011 09:16 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

Good question. I have never been told that it is inadvisable to leave the switch in the Both position while running to charge both batteries. I don't recall any handbook on the subject. I have been looking through my "The 12 Volt Doctor's Practical Handbook" but have not found anything on the subject. I've noticed my book is the 1983 edition so perhaps I should invest in a later version. How time does fly. I remember buying this book at Spa Creek Instruments in Annapolis back in the day.

I've been using the Both position while underway to charge both batteries on my boats since 1978 without a problem. If it makes a difference, I use matched batteries put into service on the same dates and over the past several years have used AGM batteries.


Jerry Townsend posted 03-18-2011 10:57 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Butch / Steve - I suspect that it doesn't make much difference - since a battery is a good regulator - it will charge as needed. Think of it this way - with a discharged battery, the voltage will be lower then that from the alternator and current will flow to the battery. When the battery is charged, the voltage difference between the battery and the alternator tends to be very small - which decreases the current flow to the battery. ---- Jerry/Idaho
jimh posted 03-18-2011 11:30 AM ET (US)     Profile for jimh  Send Email to jimh     
Moving an OFF-1-BOTH-2 battery switch to the BOTH position parallels the two batteries. When the solenoid in an ACR closes it parallels the two batteries. There is no difference. An operator who knows when to move the switch to BOTH and when to switch back to isolated batteris can function just like an ACR.

An auxiliary charging output does not place the batteries in parallel,

SJUAE posted 03-18-2011 05:51 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     
From my BW Manual 2006:

"Your 210 Outrage uses a battery selector switch to
control the delivery of the DC power from two batteries.
The battery switch is located on the port bulkhead
in the helm console and can be accessed through
the port console door.
The dual battery switch has four (4) settings:
• “OFF”, you will have no power to the engine
or accessories.
• “ALL”, you will have power from both batteries
at the same time. This parallels the batteries
to assist you in starting the engine(s).
Once the engine is started the battery switch
should be taken off of the “ALL” setting, and
set to charge either battery .
• “1”, you will have power from the port battery
• “2”, you will have power from the starboard
battery only.
When the engine is shut down or not providing a
charge, the system will allow isolation of the starboard
(house) battery. This will allow you to run all
the boats functions without affecting the port battery.
You can run the house battery flat and still start
the engine."

From the 2008 Manual:

"Dual Battery Switch
Your 210 Outrage uses a battery selector switch
located on the interior aft bulkhead of the center
console and can be accessed through the port access
door. The switch allows you to control the delivery
of DC power from the two batteries.

The dual battery selector
switch has four settings,
“1”, “2”, “ALL”, and
“OFF”. “1” gives you
power from the standard
battery only. “2” gives you
power from the optional
battery. “ALL”provides
power from both batteries
and “OFF”, you will have
no power.

When the engine is shut down or not providing a
charge, set the battery switch in either the “1” or
“2” position. This will allow you to run all the boats
functions from a single battery. In the event that
battery discharges completely, you will still be able
to start the engines by turning the battery switch to
the second battery position for engine ignition.
In the “ALL” position you will have power from both
batteries at the same time. This parallels the batteries
to assist you in starting the engine, once the engine is started the battery switches MUST be switched
from the “ALL” setting, and set to charge either
“1” or “2” battery."

Notice how it changes from should to must be switched from the all setting once the engine is running

So is BW advice not to be followed ?


SJUAE posted 03-18-2011 06:30 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     

You may well be correct re charging in parallel with an ACR unit

I recalled some good details by Andygere who uses a BEP switch BEPwiring.jpg IMG_0697.jpg

This switch notes emergency parallel so maybe it's not the charging thats the main concern but something else when leaving the switch in the both position.


Jefecinco posted 03-18-2011 06:45 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

I've never taken delivery of a boat with a dual battery installation. I've always simply added matching batteries and a 1, 2, Both, Off switch in a wiring system very much like the one provided in the reference section for single engine dual battery installations.

It is very difficult for me to understand why it would be undesirable to allow battery charging while underway by setting the switch at Both. OTOH, I've been often warned not to change the setting of the switch while the engine is running as that can blow the alternator diodes.



SJUAE posted 03-18-2011 08:37 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     

I also don't know :) maybe our guru sparkies (Jim/Jerry) can give the answer, all I did was follow the manual.


deepwater posted 03-18-2011 08:49 PM ET (US)     Profile for deepwater  Send Email to deepwater     
carlo,,It might help if you contact the manufacture of the motor and ask if the alternator is capable of charging 2 batteries,,Pulling more out of any electrical item than it was designed to produce seems to be harmful to the unit causing premature failure,,They built it they should know
Jefecinco posted 03-19-2011 07:22 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

Carlo has a 150 Verado on his 190 Montauk. No worries on the charging capacity.


jimh posted 03-19-2011 08:33 AM ET (US)     Profile for jimh  Send Email to jimh     
The concern expressed about operating a boat with dual batteries with the batteries wired in parallel (as occurs when you place an OFF-1-BOTH-2 switch to the BOTH position or when the solenoid of an ACR closes) is for a weak or damaged battery to cause harm to the other battery. When you connect two batteries in parallel, the battery with a lower terminal voltage will tend to draw current from the other battery that has a higher terminal voltage. Another concern is for the charger. If both batteries are deeply discharged they will both tend to draw high current from the charging source. Charging two batteries in parallel may overload the charging source.

The ACR mitigates these problems because is will not operate the solenoid until the first battery terminal voltage has risen to a point which is typical of a battery with full charge. When an ACR puts batteries in parallel, one of the batteries is already fully charged. If the load of the second battery is too great and causes the terminal voltage of the combined paralleled batteries to fall below a threshold, the ACR will drop the solenoid and open the connection. The ACR has some built-in intelligence, and it knows when to allow the batteries to be in parallel.

An operator who moves the OFF-1-BOTH-2 battery switch on his boat to BOTH has placed the two batteries in parallel, and the operator may not be aware of any problems with charging that could result. If both batteries are in good condition, that is, without a shorted cell, and if both are of a similar type and age so that their charging rates are similar, there is likely no great harm to occur from letting them be in parallel while being charged.

When there is no charging current available, it is probably not a good idea to keep two batteries wired in parallel. Any load on the batteries will be discharging both batteries. The principal reason to have two batteries is to keep one in reserve in order to be able to re-start the engine. If the battery switch is kept in the BOTH position, one could find themselves with two batteries that were both discharged to the point of being unable to supply current for engine starting.

Batteries are not self-aware, and they don't know the difference between being placed in parallel by operating the OFF-1-BOTH-2 switch to the BOTH position or being placed in parallel by the solenoid of an ACR or VST closing. The electrical circuit is the same. The only difference is perhaps a few one-thousandths of an Ohm of resistance increase due to the added wires in the ACR circuit.

deepwater posted 03-19-2011 04:52 PM ET (US)     Profile for deepwater  Send Email to deepwater     
Thanks Butch.

[Odd and incorrect attribute to me of something I did not say has been deleted--jimh]

[S]ometimes we ask too much from our electronic gizmos and they over heat or starve for power.

jimh posted 03-19-2011 11:12 PM ET (US)     Profile for jimh  Send Email to jimh     
If anyone is aware of a Mercury VERADO option to provide dual isolated charging outputs, please join our discussion. Otherwise, I think my earlier guess is probably correct. A Mercury VERADO does not have dual isolated battery charging outputs available.
jimh posted 03-20-2011 07:33 AM ET (US)     Profile for jimh  Send Email to jimh     
There is not much worry about the charging output capacity from a VERADO motor. They typically have very strong charging output, and they need it. The battery associated with a VERADO is extremely important because there is no hope at all of the engine operating without a very strong and fully charged battery to support it. The inquiry of Carlo about adding a second battery is really quite a good decision. I am a bit surprised that Boston Whaler sells boats with VERADO motors with only one battery riggeed.
jimh posted 03-20-2011 09:47 AM ET (US)     Profile for jimh  Send Email to jimh     
Carlo--Your inquiry has elicited many responses. You should respond to acknowledge them.
DeeVee posted 03-20-2011 11:59 AM ET (US)     Profile for DeeVee  Send Email to DeeVee     
Steve and Jimh,

I installed the BEP switch cluster with ACR a few years ago. The switche cluster includes 4 switches- two on/off switches (one for the starting battery and one for the house battery), one emergency parallel switch, and the automatic charging relay.

If I remember correctly, the starting battery is always charged first, until it reaches full voltage. Once the starting battery is fully charged, the ACR switches to charge the house battery. I believe the two batteries are never in parallel, unless the emergency parallel switch is thrown.

I have also installed an on board battery maintainer/charger that I connect to 110 volt power when at home in the carport.

I have been very happy with this configuration. I usually run the VHF, the depthsounder, chartplotter, and stereo all day. Raw water washdown pump, bilge pumps, two electric downriggers, running lights, and miscellaneous deck lights are run intermittently throughout an outing. Knock on wood, so far I have never encountered a lack of 12 volt power.

Doug Vazquez

SJUAE posted 03-20-2011 12:44 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     

Thanks seems the BEP seems slighly more intelligent than the simple ACR Jim describes. I think your switch must be the same as Andy's (pictures above)

Jim thanks for the explanation I'm glad I followed the BW manual :)

Is it's possible when having the switch in the both postions that if a device becomes faulty and not only drains the batteries but tries to draw extremly high load and so possible overloading the wiring ?


If the first battery is faulty will it pull all the charging current and deprive the second battery thats good, unless an ACR is fitted ?


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