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post #11 of 36 Old 11-28-2006
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Golux, I think the difference is that a starting battery in being asked to put out about one or one and a half kilowatt (for engines up to "car" size) for about 3-5 seconds, no more. With a bow thruster you may be wanting to use it for more than 3-5 seconds. That puts you into deep cycle territory.
You also don't have to run monster cables up front, if there's a deep cycle battery up front. The battery can run the thruster, and the thinner cables will just recharge it a bit slower. Compromises.

Dave, I think the devil will lie in the details, as usual. If a single Group31 battery (large car battery) can suply 3500A into a dead short, it can certainly supply the kilowatt to run a starter motor for a few seconds without being drawn down much. A 50-100A load for four seconds still represents a tiny fraction of what the battery has stored--if you have a sufficiently large battery/bank. It's kind of like saying fleas can kill a dog by sucking all of its blood. Well, yes, they can. But one bite once a day, won't be intolerable much less fatal.
Carrying a totally isolated separate starting battery? Great if you can afford the dedicated inflexible resource, and the space and weight penalty, I guess. Since the engineering behind batteries hasn't really changed in the past decades, I have to wonder why "the norm" and the recommendations have shifted over the last 20-40 years, from having an A/B setup to having one house and one starting. Style, more than science, perhaps?
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post #12 of 36 Old 11-28-2006 Thread Starter
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Hello,

Is a dedicated, seperate starting battery really that expensive? You can mount it fairly close to the engine(s) reducing the wire size. The lugs are not that much. Even if you put in a seperate charger and keep it totally independant, you are still out, battery and all, for a few hundred bucks or so(assuming you do it yourself). If you do not want the starter to have its own charger, put an echo on it.

What is a few hundred bucks if you know you can depend on starting your engine(s) when neccessary?? This is one of the few redundant systems that is not that expensive.

Just my thoughts.
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post #13 of 36 Old 12-16-2006
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You are right, bow thruster motors are similar to staarter motors although designed to allow somewhat longer run time. and the batteries that work well for starting, thin plate high output cranking types, also serve well as thruster batteries. AGMs are well suited for this service.Mount the bow thruster batteries near the thruster to minimize large wire run. again AGMs are convenient as they do not require maintainance although care must be taken with charging parameters to avoid thermal runaway. Especially if the batteries are in an enclosed area in warm climates. Most thrusters are 24 volts so a separate 24 volt charging source will be required. There are 746 watts in 1 HP 746 watts/24 volts =31 amps. In reality you should add 25% to account for inefficiency. If you have a 4HP thruster you will draw about 150 amps. Consult a voltage drop table and select the proper wire size for the run from the batteries to the thruster. Be sure to fuse the system close to the batteries. Since the charging current will be less than draw, the charging source, alternator, or AC powered charger can be located further away and smaller wire can be used.. Again consult a voltage drop table and be sure to measure both directions.
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post #14 of 36 Old 12-16-2006
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DC starting, thruster, and windlass motors can draw high currents.

It is probably better to power these intermittent loads with batteries that are designed to deliver such amperage.

Deep cycle types, as I understand them, are optimized to deliver more modest current over a long interval between charges.
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post #15 of 36 Old 12-16-2006
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Hellosailor does make a valid point that seems to get right to Dad's point. If you can crank your engine over, in cold weather, with one of your house batteries why even carry a conventional battery for engine cranking? I would say that this is a good example of: as a general rule, but each situation differs, consult your electrical engineer.
Now if you have a high compression engine, and you can hear each and every one of those little juice cans hit top dead center, all the while praying for just one little controlled explosion. you might want a conventional discharge battery. I would.
On the other hand, if you are in warm environs, and little denny diesel or atomic adam spin easily, probably well broken in. why not use a deep cycle?

We used to jump start an old Ford 8-N tractor with an electric golf cart. The Ford was a 6 volt system and the golf cart ran on 36 volts. We could never get enough juice out of one of the 6 volt batteries to crank her over. Jumping uo to two batteries, 12 volts, helped somewhat but we usually ended up going the full monty-all 6 batteries! That starter motor sounded like a jet turbine when it engaged! In the interest of staying alive I should note that this is very hazardous, a two man job, and we only jumped long enough to spin her over. And, of course, it wasn't our tractor!
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post #16 of 36 Old 12-16-2006
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I think this is an issue of "the very best" vs. "good enough". A lot depends on your specific situation and where you want to put your money. I have separate starting and house batteries (and separate alternators) and really like the setup. But they were installed by a previous owner - would I install this system on a new(er) boat, I don't know.
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post #17 of 36 Old 12-18-2006
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I love theese theoretical disussions about batteries that seems o be a never ending story during off season.
What I have never seen is the yearly costs of all the different solutions.
Add up the costs of the batteries, 'superchargers', regulators etc, divide by lifetime and what is the result?
I dropped 4 70Ah 'cheapest' type into my SO 37 (came in fact from factory) in 2002. The boat was lying more or less unused for a couple of years, then went 20.000 miles from Norway to Malaysia. One month ago it was time to replace the 'package' .
Close to 5 years 'lifetime', no extra gadgets, no 'super++++', just ordinary wet cell 'leisure' batteries. No problems, original factory charge set up.
I do no deny that there are systems and setups that may be more 'technical' correct, but sometimes I find the simpliest the eaysiest, and when it works at low $$$ why not?
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post #18 of 36 Old 12-19-2006
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Perhaps "cheapest" is the best solution when you are going to leave the batteries unused for a couple of years. Why invest money in paperweights? Who needs batteries at all when the boat isn't being used?

I don't think anyone would disagree with you.
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post #19 of 36 Old 12-19-2006
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My experience. I bought my boat in 1993. It had a house bank of 4 Surrette 6v batteries that were installed in 1974. But they died a few years later. With a history like that I decided to replace with new Surrette batteries. But by then the company had changed hands (and quality). I only got 4 or 5 years before they stopped holding charge. This time I replaced them with generic golf cart batteries and they have been working ever since. Early on I did install an Ample power monitoring system and one of their regulators. I think careful monitoring and care is critical to battery life.
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post #20 of 36 Old 12-19-2006
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"But by then the company had changed hands (and quality)."
I thought Surette was now a division of Rolls, who are still considered to be a or the top name?
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