Battery Box Advice (Sentry 19)

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
quickenberger
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Battery Box Advice (Sentry 19)

Postby quickenberger » Sun Feb 05, 2017 9:10 pm

I am in the process of repairing gel coat cracks, damaged fiberglass, and replacing most of the foam under deck on a Sentry 19. The battery used to be strapped in the splash well but I am thinking about fabricating battery boxes and recessing them under a hatch at the front bow. There will be two additional trolling motor batteries up there for the electric trolling motor. What are the advantage and disadvatntages of this proposition?

A couple things that concern me is having the batteries too enclosed for hydrogen outgassing when recharging. Also water drainage for the battery box. Running the battery cables shouldn't be a [problem] if I build a chase, but I would like to get the batteries up front and out of the way. I have loads of composites experience and an industrial grade vacuum pump so the task is very doable but would like to know others' experiences. You can see up under the console there is room for an under deck hatch. Maybe there would be a good place for the battery box?
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jimh
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Re: Battery Box Advice (Sentry 19)

Postby jimh » Sun Feb 05, 2017 9:38 pm

Re moving the engine cranking battery (and other 12-Volt batteries such as a house battery) to the forepeak of a boat: I have two problems with that proposal, on these general terms:

--the primary load for the engine cranking battery is the engine's cranking motor, and the battery should be located as close to the cranking motor as possible in order to minimize voltage drop in the electrical conductors connecting the battery to the cranking motor. While is is possible to overcome voltage drop in the conductors by increasing the conductor size, there are limits to this. Very large electrical conductors are expensive; passing them through the rubber grommet and into the engine cowling may be difficult; very long conductors should be protected by fuses, which add complexity and expense.

--the motion on a boat making way in heavy seas will be greatest in the bow, subjecting a battery that is located there to higher forces, which could lead to damage to the battery. While it is possible to overcome some of the concern about battery damage from high forces from motion by use of particularly rugged absorbed glass mat (AGM) construction, batteries with very rugged AGM construction usually are more expensive and often must be larger in size and weight to provide the same cranking Amperes. Also, AGM construction will provide a sealed battery, so concerns about venting of hydrogen during charging are mostly eliminated; perpetual overcharging with too much voltage can cause venting from even an AGM battery.

I am sure you could build a suitably strong battery enclosure in the forepeak, and, if properly done, fasten a battery securely there so that it would remain in place in almost any situation, and create a wiring chase or conduit or tunnel to protect the battery cables as they are routed from bow to stern. But a good enclosure, strong retainers, and wire conduit will not overcome my two objections.

Another consideration--outside of the electrical circuits and the battery durability--is the weight distribution. Larger batteries can weigh 50 to 80-lbs each. Moving considerable weight to the forepeak of the boat from the transom splash well will likely affect the boat's static trim and also the boat's handling under dynamic conditions.

It may be instructive to look at the location of boat batteries as employed in the past several decades by professional boat builders and riggers. I do not recall any instance of installation of batteries in the forepeak of a boat, particularly a 19-foot boat.

quickenberger
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Re: Battery Box Advice (Sentry 19)

Postby quickenberger » Mon Feb 06, 2017 5:26 pm

Those are good points. On a boat like this real estate is premium so maybe the engine starting battery could be located aft but the trolling motor batteries could be mounted up front. Are there resources available for how the pros rig the boats? I did a quick google search and found stuff on sail boats but not runabouts.

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Don McIntyre - MI
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Re: Battery Box Advice (Sentry 19)

Postby Don McIntyre - MI » Mon Feb 06, 2017 7:11 pm

Jim's comments regarding problems for the distance of your starting battery up in the bow would also be the same for mounting the trolling motor battery in the aft.

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Re: Battery Box Advice (Sentry 19)

Postby Jefecinco » Tue Feb 07, 2017 9:29 am

If there is space available beneath the helm location that could be a good compromise on cable length vs balance.
Butch

jimh
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Re: Battery Box Advice (Sentry 19)

Postby jimh » Tue Feb 07, 2017 9:52 am

quickenberger wrote:Are there resources available for how the pros rig the boats?


I don't know if any participants in this discussion are employed as professional boat riggers or professional boat captains. However, knowledge of the behavior of electricity or buoyancy is not strictly limited to those two categories of workers.

I'd look at present Boston Whaler boats to see where the engine cranking battery is located as an example of good design. I suspect the engine cranking battery won't be in the forepeak.

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Re: Battery Box Advice (Sentry 19)

Postby quickenberger » Wed Feb 08, 2017 2:10 pm

Don McIntyre - MI wrote:Jim's comments regarding problems for the distance of your starting battery up in the bow would also be the same for mounting the trolling motor battery in the aft.


While that is true the trolling motor has a 60A breaker so the continuous current is much less than a starter motor for a 150HP engine.
I would just like to get the starter battery out of the splash well and out of the way.

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Re: Battery Box Advice (Sentry 19)

Postby quickenberger » Wed Feb 08, 2017 2:17 pm

Jefecinco wrote:If there is space available beneath the helm location that could be a good compromise on cable length vs balance.


There is room just in front of a bulkhead where a battery box for all 3 can be made and mounted above the deck and tucked up there. I was hoping to keep that area for storage and maybe even laying out a pad and sleeping bag for overnighters.
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Re: Battery Box Advice (Sentry 19)

Postby jimh » Wed Feb 08, 2017 3:20 pm

quickenberger wrote:...[re the topic of locating the battery for a bow mounted trolling motor in the stern] the trolling motor has a 60 [Ampere] breaker so the continuous current is much less than a starter motor for a 150-HP engine.


While a flow of 60-Amperes is less than the peak current anticipated when the electric starter motor for a large outboard engine is stalled and drawing its maximum current, 60-Amperes is nothing to take for granted. Assuming that the battery for trolling motor was so far away that 20-feet of two-conductor cable was needed, in order to deliver 12-Volt power with less than 3-percent voltage drop, the conductor would have to be chosen so that even at 60-Amperes the the drop in a total of 40-feet of wire was less than 0.36-Volts. By Ohm's Law we see that the total resistance cannot be more than R = E/I or R = 0.36/60 or R = 0.006-Ohm in 40-feet of wire, or 0.00015-Ohm/1-foot. Scaling up to Ohms/1000-feet, a more convenient way to select a conductor by resistance, we see that we must use wire with a resistance of not more than 0.15-Ohms/1000-feet. Using a table of copper wire resistance as a function of American Wire Gauge rating, we see that the conductors would have to be made using wire of 1-AWG, which really is a very large conductor--and, again, that is just for a 60-Ampere motor. A conductor of 2-AWG could be used if slightly more than three-percent drop was tolerable.

The point of my illustration of the conductor size for a 60-Ampere load at the bow and fed from stern, even on a boat of only 19-feet in length, is to show how much copper is needed to make these circuits work properly when there is any sort of distance between the high-current load and the battery. It is much better design to keep the battery closer to its most demanding load.

By the way, Boston Whaler generally would rig their classic boats, which had very minimal electrical loads at the helm, with a feed to the secondary panel from the primary power of 8-AWG. Even though that feed was only run about 12-feet from aft to helm, 8-AWG was used to keep the voltage drop to a minimum. In 12-Volt power distribution, 12-feet with 8-AWG is good for only 24-Amperes if the drop is to be limited to three-percent.

All this talk of electrical power calculations really belongs in SMALL BOAT ELECTRICAL. I have written a lengthy article explaining the requirements and also invented a simple rule of thumb for choosing conductors. See

Conductor Size for Power Distribution
http://continuouswave.com/whaler/reference/powerConductorSize.html

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Re: Battery Box Advice (Sentry 19)

Postby jimh » Wed Feb 08, 2017 3:33 pm

Earlier it was remarked that the SENTRY 19 would have two batteries in the forepeak for trolling motors. Where will the trolling motor be?

I thought the SENTRY 19 had a closed foredeck, which would suggest the trolling motor must be a transom mounted motor. What is the forward deck configuration on a SENTRY 19?

quickenberger
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Re: Battery Box Advice (Sentry 19)

Postby quickenberger » Wed Feb 08, 2017 5:58 pm

jimh wrote:Earlier it was remarked that the SENTRY 19 would have two batteries in the forepeak for trolling motors. Where will the trolling motor be?

I thought the SENTRY 19 had a closed foredeck, which would suggest the trolling motor must be a transom mounted motor. What is the forward deck configuration on a SENTRY 19?


It does have a closed foredeck. The trolling motor is a Motorguide Xi5 which is currently on my other boat.
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Re: Battery Box Advice (Sentry 19)

Postby jimh » Thu Feb 09, 2017 6:03 am

Where will the trolling motor be installed?

quickenberger
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Re: Battery Box Advice (Sentry 19)

Postby quickenberger » Thu Feb 09, 2017 9:09 am

jimh wrote:Where will the trolling motor be installed?


It will be installed on the front bow just to the side of the bow cleats. I may have to fabricate a locking track to extend the motor forward when deploying.

Here is what I am thinking... The two batteries for the trolling motor can go in front of the forward bulkhead above deck.
The main starter battery can go in a box fabricated in the splash well where it can be out of the way and protected. Basically put the battery back where it was only build a better container for it to keep water off it.

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Re: Battery Box Advice (Sentry 19)

Postby jimh » Fri Feb 10, 2017 4:39 pm

Your plan to have the batteries near their major loads is good.

If you are worried about extra weight in the forepeak, there are now some lithium-ion 12-Volt DC batteries that have a much higher energy density than lead acid. Using them would reduce the weight compared to lead-acid for the same storage capacity. Of course, the lithium-ion batteries cost more.

Here is a link to a vendor with a 40-Ampere-hour-rated 12-Volt DC battery that weighs only 13-lbs, but costs $580:

http://www.lithiumion-batteries.com/products/product/12v-40ah-lithium-ion-battery.php

An equivalent lead-acid battery will be about 33-lbs but cost only $100:

https://www.amazon.com/Power-Sonic-40AH-Sealed-Battery-Terminal/dp/B000V3LG3Q