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Old 24-12-2007, 12:48   #1
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Two Engines - How Many Starting Batteries ?

Cat Tales, our 35' FP Tobago, has 4 house batteries and two starting batteries. Total battery room is 6 regular batteries. I'm considering switching when I buy new batteries to having banks of 5 and one instead of 4 and 2. While long-term cruising, I'm sure we'd be better off with 5 house batteries, but wonder how you folks would feel about starting both engines off one battery.

Each engine has a 55 amp alternator that would supplement the charging of the two banks by the two solar panels and the wind generator. The option would always be there to switch batteries or use the house bank for emergencies if the starter battery fails, and I intend to have at least one solar panel available on a diverter switch to trickle-charge the single start battery to optimum charge to control sulphation.

So, what's your opinion?
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Old 24-12-2007, 12:59   #2
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Why not? Makes a lot of sense as long as you have a backup and your starting battery is big enough and in good shape. Not knowing where the batteries are located I'm guessing there might be some issues with gauge of wiring to the engine farthest from the starting battery?
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Old 24-12-2007, 13:18   #3
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No problem. So long as your start battery is located near both engines so you don't have to run inordinately large cable and of course, and I am sure you know this, you don't try starting both engines simultaneously.
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Old 24-12-2007, 15:14   #4
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I am redoing the elects in my cat but I like the back up of two independent start batteries & their independent charging systems. Have been using 2x6V golf buggie batteries each side up to now. Intent to fit smaller 55AMP AGM for the starting batteries & relocate the 4X6Volt as stand alone house batteries charged with 400w solar panels. Have thought of using magnet clutches on the alternators to get longer life out of the AGM (float charge from the house batteries) can't find any reasonable low cost regulators for the alternators on a 1GM + I might get more power for the props?? (7.5hp of which 1hp is taken by the alternator!)

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Old 24-12-2007, 18:09   #5
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Sono - with your 4 + 2 setup, can you cross connect your house bank to one or both of your engine batts? If so, keep this setup so you can still cross connect to your single, dead starting batt if you go to 5 + 1. JMHO

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Old 24-12-2007, 21:02   #6
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I have one engine battery and 3 house battery's. I can cross connect with a switch should the need ever arise, so I don't see the point of wasting space and adding extra weight by having more than one engine battery. A set of jumper leads is handy to have on board.
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Old 24-12-2007, 21:15   #7
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Bill-
"Have thought of using magnet clutches on the alternators to get longer life out of the AGM "
On a true alternator, as opposed to a generator, you don't need a clutch. If the regulator is working properly, it cuts back power to the field coils or actually shuts power to them, at a very fast rate. So effectively the alternator is not consuming any significant horsepower unless it is actually being used to produce electricity. (It still takes some power to keep it turning, even when it isn't making power, but that's minimal compared to the complexity of clutches and needing to spin it up to speed, etc.)

"can't find any reasonable low cost regulators for the alternators on a 1GM " I confess, I don't have any idea what a "1GM" is, I'm guessing that's a small outboard engine? If these are not conventional "belted on" alternators, your options might be limited, but the internal regulator from a conventional alternator probably could be adapted for the job. Or something else made up. It sounds like you have a car-sized 55amp rated alternator on each 7.5hp engine ???

Or are your alternators actually generators, where power is regulated by dumping it and there's always a full load on the generator?
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Old 24-12-2007, 22:48   #8
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Sono,
I am assuming you are using all 12v batteries, because you wanted to have 5 house batteries.

My opinion is keep the 2 separate 12v starting batteries with the ability to start either engine from either battery or both.

This way you should never be caught with both starting batteries going bad at the same time.

Figure the total storage of the 5 12v batteries you were shooting for, and buy 4 6v deep cycle batterys with the same capacity.

You should get better performance and life with the series/paralell 4-6v battery system than the 5 12v batteries.

If your house batteries become discharged, then you probably would not just run off the starting batteries. You would start the engine and recharge the house bank.

The chances of running down the house bank are much greater than having a hot house bank and using it to start an engine because your starting battery went bad. If you had 2 starting batts then this wouldnt be necessary.

You can place the 3 banks close to their normal loads.

No doubt in my mind 2+4.
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Old 25-12-2007, 03:45   #9
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BI don't have any idea what a "1GM" is, I'm guessing that's a small outboard engine? If these are not conventional "belted on" alternators,
The Yanmar 1GM is a 1 cylinder, 4-cycle, water cooled, inboard diesel engine, rated 6.4 HP (continuous) at 3400 RPM.

YANMAR 1GM SERVICE MANUAL:
Yanmar 1GM Service Manual Index
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Old 25-12-2007, 16:50   #10
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Thanks, Gord. That explains a lot. Sounds more like an Onan genset that wandered off the farm into a marina. (Which is not a put-down, Onan gensets are robust beasts.) Shouldn't be any problem to bolt up a conventional alternator and regulator then, should it?

On the batteries, I have to think two starting batteries PLUS a house bank is simply overkill. One starting battery, dedicated and isolated, and one starter button for each engine, should be enough. The first engine to start would be the one that normally recharges the starter battery--because only one alternator would run back to that. Heck, they started up 4-engine prop aircraft with only one engine being used as the "starting" engine, and then powering up all the others, so why should a small boat need to be more robust than that?

Besides, there will be a huge house bank to use as the "spare". Two starting batteries...I'm a belt-and-suspenders type of guy but even to me, that goes beyond redundancy and into obsessive compulsive. (Just one man's opinion.)
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Old 26-12-2007, 00:39   #11
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Thanks Hellowsailor, (& others)
It came with the 2 X 1GMs which is a bit underpowered for a 36' cat 6.5kts v 7.5kts for a pair of 2GMs (double the power & fuel flow for 1kt) but we do mainly sail!!
It took me 12mths to resist starting the engines when the speed dropped to 6.5kts. after I took the boat over. (And the council of some mono friends who were very happy if they could get 6kts under sail)
I just joined in as there is a bit of a common discussion going on. The problem of the AGMs has not occured yet as I still have to do the rework (new year ) but the general specs for the "hybrid" AGM I have selected gets two specs one for cycle use (14.4V-15V) & one for float use (13.6V-13.8V) with up to 8yrs in float. The alternators used limit 14.3V. When they were connected to 2 banks of 200AMP wet cells on each engine then the voltage was more 13.8V. I suspect the use of the smaller batts will result in having the potential to get the 14+Volts on the AGMS. The price of over $500.00 each to add 3 stage regulators to such small alternators left me thinking of some alternatives. The thought of being able to cut out the alternators after a period seemed to be one way of reducing the time with 14+ Volts being applied to the AGMs but I am open to learning of other ways of achieving the 13.6-13.8Volt range.

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Old 26-12-2007, 09:05   #12
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"The alternators used limit 14.3V. When they were connected to 2 banks of 200AMP wet cells on each engine then the voltage was more 13.8V. "
Ah, cheap obsolete old-fashioned alternators. I say tongue only half in cheek, since there are some common designs (like Delco's Delcotron's dating back to the 1970's) that will put out exactly and only 14.3-14.4 volts and the only variable is that they put it out as pulsed DC, some 100-30,000 pulses per second if I recall correctly. Pulsed DC is also apparently better at charging batteries, by the way, because there's less internal gassing caused by it.
AGM's do have different specs, or did have them, but many of the AGM's designed for automotive use these days have been "tweaked" to work at 14.4 volts in typical use. From an AGM battery maker here, I was told that their spec on flat/vs/bulk charge was really given just to keep old-fashioned people satisfied with numbers, and that in fact the AGMs didn't care about the voltage as long as they didn't get too much amperage while charging. Bottom line, he gave me amperage limits for the battery and said as long as they weren't exceeded during charging, not to worry about 13.8 or 14.4 or whatever their spec sheets actually said!
So...when or if you've gone to AGM, it might pay to contact the maker and confirm that your charging system is suited for them. Apparently as long as you keep the amperage down, 14.4 shouldn't bother them when you are applying it as an "intermittent" charge from the engine alternators, rather than from a constant charging (mains power) installation.
You might want to look at some Delco alternators, cheaply sourced from GM cars at the junkyard, or from rebuilders, to get some efficient models that will hold 14.3-14.4 exaclty, with up to 100A output from them. Or at least something with a similar regulator, after all these years the technology shouldn't be hard to find.[g]
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Old 26-12-2007, 11:29   #13
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We have one starter battery for both engines. I was concerned with this setup at first, but no issues! I use battery switches to enable my house bank to start the engines. The switches let me use my starting battery to run the house. I can also use both batteries for either house or starting. I figure if both batteries somehow go dead, I can use either solar, wind or generator to bring a battery back up. My alternators, by default charge the starting battery, Wind and solar charge the house bank. Since I can us the switches, any source can charge any battery group. I use manual switches because I don't like the voltage drop accross the automatic "battery minders"

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Old 26-12-2007, 13:25   #14
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"I use manual switches because I don't like the voltage drop accross the automatic "battery minders" "

With automatic switches like the West Marine (Yandina) Battery Combiner, or the solid state version from Hellroaring (Hellroarer?) there's no voltage drop to be concerned about. No isolation diodes. Once you've got 13.8V coming out of the alternator for about 30 seconds, it cuts in the second battery and allows both to be charged.
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Old 26-12-2007, 14:43   #15
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The alternators used limit 14.3V. When they were connected to 2 banks of 200AMP wet cells on each engine then the voltage was more 13.8V.
Hey Bill

I have almost the same setup in my cat. I have a pair of 1GM10s with the stock Hitachi 35 amp alternators. I have two battery banks, one wired to each alternator. When charging my batteries those Hitachi's never reach 14.3 volts. I've put a high quality digital voltmeter to verify and confirmed that both alternators top out at 14.1 volts.

Is that enough for AGMs?
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