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Old 11-05-2013, 05:58   #1
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service batteries to start engines on cat

For those who lives on their boat and are always managing electricity, never allowing their service battery bank to fall under 12V, what do you think about get rid of engine start batteries ?

On my cat there is a coupling device that can help in case start batteries are dead...So i am wondering to get rid of the engines start batteries...

(Also i have the generator bat that i can use in the unbeleavable case i would let my service bat (1000 amps) go to a point they can't start one engine anymore)

What do you think about pros and cons of this option, did someone try this before ? (i know racing boat manage with only one battery bank for everything), so why not ?


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Old 11-05-2013, 07:01   #2
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Re: service batteries to start engines on cat

In theory, there is nothing wrong with your proposal at all - our previous boat was that way.

In practice, however, you may have some challenges. The distance between house batteries and engines is quite large in many cats. You would need very large wire to connect them and the voltage drop may give you less starting power than you need.

Also, house batteries are designed for capacity and moderate current delivery over long times. Start batteries are designed for short time, high current delivery.

So your house bank may struggle to start the engines due to the type of battery they are and the voltage drop across long runs. It will all depend on your specific boat setup.

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Old 11-05-2013, 07:10   #3
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Re: service batteries to start engines on cat

I'm definitely a believer in two banks. I had a house battery short out 120 miles out in the Gulf Stream and put my house bank out of business. After disconnecting the house bank, I connected the start bank to the house bank circuits. This all occurred at 2:00 in the morning of course. For a while I was without radar, nav lights, radio, etc. with large ships within 3 miles of me. I tried hailing them on the hand held but they never responded. I had an electronic battery combiner that failed, so I had to rewire the boat to get power to the house circuits. I now have a mechanical combiner switch and an automatic charging relay to keep the start bank topped up. Having two banks saved my bacon, but if you're comfortable with one bank it's your boat.
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Old 11-05-2013, 07:14   #4
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Re: service batteries to start engines on cat

Hello Mark,

Thank you for your reply. Indeed, i did not though about cabel lenght, but my settings already provide big cable for the possibility to connect house batt with engine batt in case of low power engine batt. I imagine the only thing to do is to give it a try and see what happens...
Thanks again for your smart input.
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Old 11-05-2013, 07:16   #5
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Re: service batteries to start engines on cat

This ignores the fact that the internal design of these batteries are optimised for different tasks. The engine start batteryis designed to deliver a very large amp in a very short time - example my own battery is designed to be able to deliver 1000 Cold Cranking Amps.

The house batteries are designed to be able to deliver lower amps but over a very long time, and to be able to deep cycle (i.e. recover from almost flat) up to 500 times. Thus they are rated in amp hours

Of course there are some hybrid batteries that can do both (e.g. elecsol)

I considered getting rid of my generator start battery and just using the engine start, but by having a third battery system, I have good redundancy in my system.
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Old 11-05-2013, 07:18   #6
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Re: service batteries to start engines on cat

Hi Captain Bill,

You are right as well, i did not think about a short circuit in house batteries...I think we can close the thread with the conclusion it was a bad idea !!
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Old 11-05-2013, 07:23   #7
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Re: service batteries to start engines on cat

OR just use redundant house banks. Or use redunant starting banks. Ive grew up on redundant port and stbd systems.
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Old 11-05-2013, 07:30   #8
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Re: service batteries to start engines on cat

I owned a small cat (32 feet) with a very simple electrical system that I lived aboard for more than 12 years year-round. We had one battery bank that all charging sources went to and all loads came from. It was made up of 3 x Group 31 gell batteries, deep cycle. My engine was a 9.9 HP Yamaha, so there wasn't a huge starting load, and I figured I could always hand crank it in an emergency. I think there was only one time in those 12 years where the batteries got down so far that I needed to pull start the engine, but I was glad to have that option. I didn't even have any sort of solar or wind regulation--I just watched a small analog voltmeter and disconnected things if the charging got to be too much.

In any case, it is certainly doable, and the only major problem would be if you let the batteries get down too far and can't start the engine quickly in an emergency. You could always add some sort of low voltage alarm system that would let you know if your batteries were getting down too far. A large enough bank of deep-cycle batteries should have no problem starting your engine.
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Old 11-05-2013, 08:57   #9
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Re: service batteries to start engines on cat

Thanks, it can be doable, but as said upper, if one of the battery failed like short circuit, your are f...!
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Old 11-05-2013, 09:02   #10
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Re: service batteries to start engines on cat

I had another boat that came with a mess of an electrical system, so to get it going quick and dirty I pulled out tons of wire and mess and just hooked all the batteries together in a single bank and it worked fine for quite awhile. I kept a car battery jump starter onboard for emergency starts. Those are useful for other things onboard too.
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Old 11-05-2013, 09:04   #11
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Re: service batteries to start engines on cat

The advantages of two isolated systems far outweigh the advantages of one system.
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Old 11-05-2013, 19:24   #12
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Re: service batteries to start engines on cat

The original battery system on Palarran consisted of a 1000 amp hour house bank, a start battery for each generator, and one for each motor. This made no sense to me so I combined one generator and one motor onto one start battery in the port hull. My house bank in the starboard now starts the starboard motor and generator. So I was able to eliminate three batteries. If the house bank was low, I can start the port generator and charge it back up.

In regards to battery types, I don't believe that to be an issue. First, start batteries are usually starting big motors, not the little puppies we have in sailboats. The load just isn't the same as a car engine. And second, I have 9 batteries tied together, how much can it actually stress any single one of them for the two seconds it takes to start.

This has worked well for me for the last two years. I also don't like the idea of separate banks because I think most people leave the selector switch set to "both" so why? Maybe some do switch them but then your cycling the batteries lower in between charges.

Kettlewells idea of having a battery booster isn't bad. They are light and cheap.
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