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Author Topic:   Adding a second battery
SGilliland posted 04-28-2014 02:06 PM ET (US)   Profile for SGilliland   Send Email to SGilliland  
Hi All,

I have a 90 hp Etec with a single battery. I have a couple of fish tanks, navigation lights, bilge pump, fish finder/GPS and a USB port for charging my phone. I would like to add a second battery to the system.

I read thought many posts - there is a lot of good information. Thanks this is a great resource. But I still have a couple of questions. I will use the Blue Sea Mini Kit and I would like to add the second (deep-cycle) in the centre of the boat. This would require a run of just over 10' from the negative bus or ACR. I posted a sketch of my plan on photo bucket (let me know if I did not do this properly - it's my first time):

Primary Power Distribution Diagram

I'm worried about the long run between the deep-cycle battery and the motor. I think the cables for the motor are about 6-feet long. This would make the total run from the battery to the motor about 18-feet. Is 2-AWG wire adequate for the connection between the battery and the ACR?

I plan to use a 50-Ampere Terminal Fuse Block from Blue Sea to connect the deep-cycle battery. Does this sound correct? I'm worried that the starter may draw more than 50-Amperes if the house battery has to be used to start the motor.

From what I read on forum, a fuse is not required between the Start battery and the ACR if the cable is short. Here it is about 18-inches; maybe 24-inches if you count the connection between the ACR and the Switch. Is this fuse required?

I'd appreciate any suggestions.


jimh posted 04-28-2014 08:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Re the circuit from the Deep-cycle battery to the Switch and the negative bus:


WIth 2-AWG cable in a 12-Volt DC circuit, the voltage drop can be figured as less than 3-percent for up to 12-feet for a current of 94-Amperes. That is really quite good.

For more on how to quickly calculate the maximum distance for 3-percent voltage drop, see

Wire Conductor Size for Power Distribution


The outboard engine's electric cranking motor will probably draw more than 50-Amperes at some point in the engine start cycle, probably right at the beginning. To predict if a 50-Ampere fuse will open or not is difficult. Why not fuse the circuit at 100-Amperes? The wire can handle that much current without any problem. Use a slow-blow fuse and perhaps the fuse will hold during engine cranking. The only way to be certain what fuse will blow and what will hold is to try it. I know the cost of the fuses become significant when you get to these high currents. Maybe a circuit breaker is better. It can trip and be reset, and you don't have to replace a $10 fuse each time there is an overload.

Whether or not a fuse is needed or required depends on who you ask. If you ask the American Boat and Yacht Council (ABYC) you will get one answer. If you ask the Coast Guard you get, perhaps, another answer. If you ask your state's legislature, maybe a third answer.

Since the electrical shut off for the circuit is more than a few feet from the battery, in seems prudent to provide over-current protection at the battery, but there are other considerations. If the conductor is large, like your 2-AWG, is protected against mechanical damage, say with a sheath or by being run against a bulkhead, and is run in a way that keeps it in open view, not buried into a tunnel, I don't think 10-feet is too long. I bet on my boat the second battery probably has 8-foot or 9-foot leads over to the switch. They are not fused. I never gave it a thought, really. It's been like that since the boat was first rigged in 1992.

Re the circuit between the starting battery and the ACR:

The ACR installation instructions shows all sorts of fuses. Once I totaled up the cost of all the fuses being recommended and it was greater than the cost of the ACR. A fuse in the circuit between the ACR and the Starting battery is intended to provide protection when the ACR is closed. If the ACR closes and there is a very great difference in the terminal voltage of the two batteries, a high current might flow from the Starting battery into the ACR. A fuse is recommended to protect the ACR from damage if the current flowing exceeds the rated current of the ACR. I bet this fuse is mostly omitted.

By the way, the fuse is needed no matter what the cable length. It is there to protect the ACR relay contacts.

jimh posted 04-28-2014 08:35 PM ET (US)     Profile for jimh  Send Email to jimh     
Re your drawing:

First, it is a great drawing--good job as a draftsman.

Second, I propose a change in the circuit. I'd connect the batteries to the main Switch directly. I'd have two cables on each terminal at the switch (on the battery side), one going to the battery and the second going to the ACR.

You have drawn the circuit so the battery lead goes first to the ACR and then onto the Switch. Did you do that because the layout puts the devices in that physical arrangement?

Third: why is one cable to the ACR from the Switch 6-feet and the other is 6-inches? Is that a mistake?

jharrell posted 04-28-2014 08:48 PM ET (US)     Profile for jharrell    
I installed a Blue Sea Add-A-Battery kit on my Montauk but moved both batteries to the console and used 12" 4 gauge wire to the ACR from the battery. The full sized ACR and large wire can easily handle starting loads if necessary (or large equalizing currents if one battery is discharged). So I did not use fusing on the positive leads to the ACR. I did however fuse the ground wire with a 10 amp inline fuse as recommended.

Any concern I had in this setup was eliminated when I examined the back of the popular alternative to this setup the BEP 716 cluster. The 716 Has no fusing whatsoever and the positive wires to the ACR look to be only 8 gauge!

The Blue Sea mini kit you are looking at has roughly half the capacity of the the full size kit (65A vs 120A) and since you are locating the battery some distance from the ACR most likely passing through various openings I would recommend fusing that lead keeping in mind that the 50 amp breaker will most likely not support starting loads defeating that feature. You may however be able to combine the batteries allowing the house to charge the starting battery at less than 50 amps, then attempt starting after some time has passed. However if the start is discharged significantly, combining batteries may cause a larger than 50 amp equalization current tripping the breaker.

I would recommend upgrading to the full size Add-A-Battery kit and moving the two batteries closer together to avoid expensive breakers. If it is to much effort to relocate then go with the larger ACR and a larger breaker as jimh recommended.

Here is a photo of my setup: WhalerProject?authkey=Gv1sRgCOHiqYL_38ejFw#5609294919527468690

jimh posted 04-28-2014 10:03 PM ET (US)     Profile for jimh  Send Email to jimh     
I'd also go with the full-size switch. The mini-switch looks to me, when I saw one in person, to be too small for the task.
jimh posted 04-28-2014 10:06 PM ET (US)     Profile for jimh  Send Email to jimh     
Re the BEP clusters that perform this same switching function and ACR function:

The BEP cluster approach is also quite interesting, but it takes up more room, However, it does give you added flexibility. If you wired it a certain way, you can completely isolate a bad battery. I figure if I had a bad battery, I'd just disconnect it at the battery and wrap the terminal in electrical tape as a temporary disconnection.

saumon posted 04-28-2014 10:38 PM ET (US)     Profile for saumon  Send Email to saumon     
I'm no electrical expert but if you're already going through the hassle of running the cables from the console to the stern, why not also putting the starting battery in the console (not on the shelf but directly on the deck, with cut-outs in the shelf)?

That's what I've done on my boat (a 1991 Outrage 17 with a E-Tec 90hp) with 2-AWG cables and it work fine. You'll only need a few feets more to wire the starting battery to the switch but you could also save some by connecting the 2-AWG cables to isolated "power posts" in the stern, thus saving to enlarge the engine cables grommet and keeping your OEM engine cables (usually 4-AWG).

SGilliland posted 04-29-2014 06:45 AM ET (US)     Profile for SGilliland  Send Email to SGilliland     
Thanks for al the advice.

Based on jimh's advice I have:

1) revised the drawing. The 6' wire between the switch and the ACR was my mistake - it will be less than 6" like the other side.[user]=140059213&filters[recent]=1&sort=1&o=0

2) upgraded the fuse at the Deep-cycle battery to 100 A. I plan to use the BlueSea 5191 terminal fuse block. The 100 A fuse is a bit expensive ($16.75 CAN) -- I looked for a circuit breaker, but did not find one that was reccomended for starting motors. Any suggestions?

3) 10' Cables between the Deep-cycle battery and the Switch will not be visible. It will be supported along the entire length with cushioned cable clamps - do you suggest running it in flexible tubing too?


Mini vs. full size Switch and ACR. Wish I consulted before I purchased this kit and it was a mail order. I think I might give Blue Sea a call to see what the advise before I return it.


That's a good thought - I will consider this.

Thanks again

russellbailey posted 04-29-2014 10:18 AM ET (US)     Profile for russellbailey  Send Email to russellbailey     
I would not consider a run from the battery in the console to elsewhere in the boat without a fuse near the battery. In the scheme of moving a battery to the console it is a simple addition and greatly improves the safety of the insulation. You are wise to include one.

I used this fuse holder when I moved batteries to my Outrage 25 console. It works well. It also made it easy to use smaller wire in the console with the BEP switch (2 gauge) and larger for the long run to the power hungry Optimax 150s (00 gauge).

Blue Sea 5005 ANL 300 Fuse Block w/ Cover

I was not sure how much the Optimax engines would draw on starting and went with 200 amp fuses, which have worked fine. I'm sure your's would be much less - 100 amp sounds reasonable. You can always try a smaller fuse yet and keep a 100 amp as a spare.

jimh posted 04-29-2014 11:34 AM ET (US)     Profile for jimh  Send Email to jimh     
If you aggregate the two batteries, the primary power switch, and the automatic combiner relay in close proximity, you may not need a fuse in their interconnection, except as the ACR manufacturer recommends to protect his device, and the long run of 2-AWG from the electrical center to the outboard engine could be handled as follows:

--left unprotected because you can shut off the power on this circuit with the main switch, or

--protected by a 100-Ampere (or higher) fuse, downstream of the main switch.

jimh posted 04-29-2014 07:16 PM ET (US)     Profile for jimh  Send Email to jimh     
The recommendation of the ABYC can be found in their document "E-9, Electrical Standards." They have something specific to say about the length of unfused positive conductors in Section E-9.10. DC POWER SOURCES. See paragraph (a), Overcurrent Protection Device Location, at

Ungrounded conductors other than cranking motor conductors shall be provided with over-current protection within a distance of 7-inch of the point at which the conductor is connected to the source of power measured along the conductor.


--if the conductor is connected directly to the battery terminal the 7-inch distance may be increased up to 72-inches.

--if the conductor is between the source of power, other than the battery terminal, and the required over-current protection device, and is contained throughout its entire distance in a sheath or enclosure such as a conduit, junction box, control box, or enclosed panel, the 7-inch distance may be increased up to 40-inches.

jimh posted 04-30-2014 04:23 PM ET (US)     Profile for jimh  Send Email to jimh     
If you look at the sketches that are included with the ABYC literature, and reading the section cited above, you see the cranking circuit conductors are never fused, no matter what their length.

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