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Author Topic:   Second Battery Connection to Charging System
jay18 posted 03-09-2006 09:23 AM ET (US)   Profile for jay18   Send Email to jay18  
I am going to be installing a [second] battery in the console of my [1989] 18-foot Boston Whaler boat. I will be running electric downriggers this season, plus some additional electronics. A friend suggested not to tie in this battery with the engine, but rather keep it independent, and install a battery charger that I can plug in and charge when not being used, similar to a shore cord on larger boats. The concern was that having two batteries charging from the motor, a 1989 Johnson 140 VRO, would strain the charging circuit on the engine.

Any input?


Plotman posted 03-09-2006 10:21 AM ET (US)     Profile for Plotman  Send Email to Plotman     
The West Marine catalog (as I assume the website too) has some very good technical articles in this area. You could also get a copy of a book called the 12-Volt Bible.

You can isolate the batteries as far as loads go, but charge them off of the same engine as long as you use the proper smart regulators. They will make sure each battery gets the proper amount of current/voltage for its state of charge.

You aren't going to stress the charging circuit of your main engine. Worst case is it will take a little longer to charge two depleted batteries than it will one.

The alternator on the engine can put out a given amount of current and no more. It doesn't care if it is feeding one battery or two (or six).


Roarque posted 03-09-2006 10:27 AM ET (US)     Profile for Roarque  Send Email to Roarque     
I connect my T8 Yamaha trolling motor to the battery I use exclusively for the downriggers. This battery sits in the transom well of my 18 Outrage. I also have two batteries connected with a Perko rotary switch in the console and charged by the main engine, an F150.

Trolling with downriggers using a 140 VRO is VERY expensive as I learned last season - thus the acquisition of the T8 over the winter.

I ran your set up last year and pulled the downrigger battery once a week to charge it with a battery trickle charger I keep in the garage at home. It worked fine but you do run the risk of losing juice while your downrigger balls are at 150 feet. Not fun.

jay18 posted 03-09-2006 11:59 AM ET (US)     Profile for jay18  Send Email to jay18     
Can you explain what you mean by VERY expensive? Are we talking price of fuel, or damage to engine?


pglein posted 03-09-2006 05:27 PM ET (US)     Profile for pglein  Send Email to pglein     
If I were you, I would simply buy a two battery switch where you can run on "1", "2", or "All". Avoid using the "All" selection, as you can cook a battery. The main factor for you is that your primary electric draw (the downrigger) will only be used when the engine is running, so you don't need to worry about accidentally running your starting battery dead. Having dual batteries is still a good idea, Your main goal is simply to isolate them while charging and to keep one fully charged in reserve. I don't see any reason for you to invest in fancy electronics to accomplish this, when a simple switch will do the job. But if you want to spend $2000 so you don't have to think about the switch and have it all happen automatically, you can do that. But that really only makes sense when you're dealing with a boat with significant "house" draws, where the engine is off yet significant amounts of electricity are being consumed.
jimh posted 03-09-2006 08:03 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't think it is a good idea to operate with two batteries in parallel at any time, except perhaps very briefly if needed to boost voltage for starting when a single battery lacks the necessary power to crank an engine.

Older engines often do not have charging circuits that are designed for high current output. If you ask them to continuously charge at their maximum rated current you will often find that they are not up to that task, and you will incur some expensive repairs of the charging circuit.

Isolating the downrigger load from the starting battery is a good idea.

Chuck Tribolet posted 03-09-2006 08:30 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Does a volage sensing relay cost $2000.


seahorse posted 03-09-2006 10:38 PM ET (US)     Profile for seahorse  Send Email to seahorse     

You can purchase a 35 amp alternator kit for your 140 from a dealer and you can also get a dual battery charging system kit. That kit splits the charging system for dual batteries so you do not have to switch them all the time.

Roarque posted 03-11-2006 02:49 PM ET (US)     Profile for Roarque  Send Email to Roarque     
Jay 18, last year I was spending $60 to fish for 4-5 hours because my Evinrude 140 VRO used a lot of gas ( and oil )at trolling speed. This year, I troll using an 8 hp engine and I have noted a big difference in fuel consumption. Too soon to be accurate on the savings but going out for a typical 4-5 hours of fishing last year meant filling the tank every other trip. Now it appears to be every 5-6 trips. That;s a 3:1 ratio of gas consumption.

It's easy enough to mount a kicker on the Outrage - you don't really have to be attached to the rest of the boat's machinery or electrics. When I fish with downriggers, I'm spending the day in the stern, so I can steer with the kicker. If the kicker has wires for a battery, hook them up to a seperate battery that runs the downriggers. The kicker is always running when the downriggers are in use so you don't have to worry about draining the battery. The main battery can sit in the console waiting to start the 140 at the end of the day.

If the kicker doesn't have pigtails for a battery, they can be added for about $50.

Revenge 25 posted 03-18-2006 11:10 AM ET (US)     Profile for Revenge 25  Send Email to Revenge 25     
I would consider using a Voltage Sensitive Relay (VSR). Blue Seas makes a nice model as does BEP Marine. This allows you to fully charge your "starting" battery on the main motor and then charge a "house" battery for other electronics, downriggers, lights, etc. The "starting" battery never has its charge drawn down.

If you use a battery to start the kicker, I would not connect it to anything. If it has no other draw on it, and it is charged up when you leave the dock, you know that it will be ready in case of an emergency.

The cost is around $400.00 for the VSR and related battery cables and misc. stuff.

jimh posted 03-18-2006 11:33 AM ET (US)     Profile for jimh  Send Email to jimh     
A system with a voltage sensitive relay may be too complicated for some owners to self-install.

Keeping the engine starting battery isolated from other loads is the best way to insure that the battery will have enough charge to start the engine. Starting the engine is the primary electrical function, particularly on boats where the engine cannot be started by pull-starting. Current from the starting battery must not be used for other purposes which would drain it of charge for starting.

Jakka posted 03-23-2006 05:28 PM ET (US)     Profile for Jakka  Send Email to Jakka     
I'm also looking at setting up dual batteries on my Dad's boat and ahve been doing some research into the subject. VSR are good but the simplest method and much cheaper is to install an isolator so the batteries do not run in parallel and a switch for emergency situations.

Even this way is moderately expensive so I came up with another way which I'm thinking of trying but was wondering if any auto electricians oput there can verify my way of thinking.

An isolator is basicaly dual diodes that can handle the high voltage and Amps created by the alternator. These are moderatly expensive approx $100. Instead I was thinking I could use a bridge rectifier which can be picked up for $3 or $4, and are capable of 100V and 35A, more then an alternator can pump out. They are easy to swap in a circuit if they fail.

Basically you are just using 2 of the four inbuilt diodes and 1 of the pins will be made redundant.

Any thoughts?

Chuck Tribolet posted 03-23-2006 09:16 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Better check the specs on the alternator.

And remember that there's a voltage drop across the diode
which means that the the battery is going to see a bit less
than the alternator is putting out, which in turn means the
battery won't get fully charged.


Jakka posted 03-23-2006 11:04 PM ET (US)     Profile for Jakka  Send Email to Jakka     
Well the alternator only puts out 6Amps at aprox 14Vso the diodes should be plenty powerful and the voltage drop over a diode is generally 0.7-0.9V so the batteries should still get charged at about 13.1V
Chuck Tribolet posted 03-23-2006 11:08 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Which is 0.7-0.9V too low.


jimh posted 03-23-2006 11:44 PM ET (US)     Profile for jimh  Send Email to jimh     
The rate of charging current depends on the voltage difference between the battery terminal voltage and the charging current source voltage. The current will diminish as the two voltage become more equal.

The effect of dropping the charging voltage (as occurs when an isolator is used) will be to lower the rate of charge. Thus it will take longer to re-charge the battery because the charging current will ALWAYS be lower in this situation.

The simplest way to charge a battery is to do it one battery at a time, and use a simple switching arrangement.

andygere posted 03-24-2006 11:32 AM ET (US)     Profile for andygere  Send Email to andygere     
Regarding the installation of a VSR, the BEP Marine model I installed in my Outrage 22 Cuddy was relatively easy to mount and wire. This unit is integrated with the battery switches, and the wiring diagram is very clear and simple. Perhaps the most difficult part of the project was making up the battery cables, but in a small boat like the Outrage 18, off-the-shelf cables could be used. Overall, these are much easier to use than the typical 1-2-Both-Off switches, and are no more difficult to install. My latest Boater's World Catalog had a nice selection of the BEP Marine switches with integral VSRs. The battery distribution cluster (including individual on-off switches for each battery, plus an emergency parallel switch) is listed for $149. I highly reccomend these systems for dual battery conversions in small outboard powered boats.

A good discussion on VSR systems for small boats can be found here:

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