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Old 02-05-2009, 23:32   #1
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Genset / Engine Battery Bank

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Originally Posted by Rick View Post
William,
If you start with a low-resistance battery bank, correctly sized cables and low resistance power distribution points you will not have voltage dips caused by peak load currents that can allow dips to 8V or below at any other load.

A proper heirarchy of load terminal attachments to a distribution point (a star arrangement is superior to buss bars) the highest current sources and loads stack together in the center and the lighter loads/sources stack on both sides from "center" outward on a stud. This heirarchy minimizes voltage drops caused by parasitic resistance.

One company flew me to a truck OEM to troubleshoot a production problem whereas a power inverter for the sleeper kept shutting down on low voltage. All I had to do was reconfigure the cabling to the individual batteries in the bank and restack the heirarchy of attachment and the problem was solved. Each battery in the bank needs to "see" the same parasitic resistance as the others in getting to the load. Same argument for sources.

One can calculate just how much parasitic resistance is tolerable for each cable, connection and battery for any given peak load so that no other load ever sees a voltage that does not meet its minimum operating specified volage. One does not need to overspecify beyond assumption of normal time degradation due to thermal heating causing annealing of copper in the attachment points. Periodic tightening of all components prevents that.
Assuming the above is not possible without major modifications, and assuming I can have high load equipment like windlass, electric winches, and main-sail furler powered via the Genset battery (as opposed to the separate Engine battery):

How would I go about determining what size (i.e. CCA and/or A/hr) battery I would require for the Genset and to provide power for the above?

Here's why I ask:

I've had until now two 12v 600CCA 100A/hr wet cell batteries: one exclusively for the Genset, and a separate one exclusively for the Engine.

There is a ‘2-direction’ toggle switch which allows me to (depending on which way you ‘’press it”) use the Genset battery to start the Engine, or use the Engine battery to start the Genset. (BTW, as a reminder, I'm now going to AGM-Gel hybrids for the above and house bank the latter which is to be increased from 440A/hrs to 675A/hrs)

Originally it was proposed to me to go with two (2) 150a/hrs, 1300CCA batteries, one for the Engine, one for the Genset, to switch the windlass, electric winches, and boom roller furler requirements from the House Bank to the Engine/Genset bank.

I've determined that two (2) 12v 150a/hrs, 1300CCA batteries would be “size prohibitive” and am wondering whether two (2) 100a/hrs, 900CCA batteries would be sufficient. This as they would provide 50% more CCA (900 vs. 600) than the former current Engine/Genset battery bank, given the ‘toggle switch’ set-up described above

If not, would the combination of one (1) 12v 100a/hrs, 900CCA battery for the Engine, and one (1) 12v 150a/hr, 1300CCA battery for the Genset achieve what I seek to accomplish (i.e. Genset battery to address the windless, electric winches, and boom roller furl requirements)??? A 12v 100a/hr, 900CCA battery for the Engine, with a12v 150a/hr, 1300CCA battery is something there would be sufficient space for.

I realise that perhaps the power requirements of the above mentioned "power hungry" pieces of equipment might first need to be known.
Regardless, thanks in advance for your expertise.
William
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Old 03-05-2009, 07:08   #2
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Congratulations. You, like Jonathan Livington Seagull, have just moved to a higher level of boat ownership. You now realize that you put all the batteries in that you have space for, and then deal with what you have.

Just be sure that you have some way of switching all of your batteries together, because I'm 90% sure that after you have been living on the boat for a year you will leave everything connected into one big bank.
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Old 04-05-2009, 12:03   #3
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CCA vs Amp-hour rating

Donradcliffe is probably correct in his prediction.

The important technical information regarding thruster, windlass, and starter motors is that their maximum power will be equal to 2X the rated peak power (always at a rated voltage). If the source battery does not hold rated voltage at peak power then the current will rise towards the stall current rating (calculated from 2X peak power divided by the supply battery voltage appearing at the motor terminals).

What this means to you is that a battery Amp-hour rating is essentially irrelevant when choosing a battery for dc motor loads. For a minimal sized battery that will reliably deliver a tolerable terminal voltage choose a CCA rating equal to or greater than the stall current rating of your motor loads.

If you have other sensitive loads present that could be affected by magnetic inrush current (which can be 8X or higher than peak rated current) at the beginning instant of a motor starting you can install a low ESR (equivalent series resistance) capacitor in parallel with the battery with short leads (less than 10 inches or #10 AWG wire). Such inrush currents are transient lasting milliseconds or tens of milliseconds (can be longer for transformers in ac applications...not relevant here). Choose a capacitor having 3 milli-ohms or less ESR. Such capacitors are relatively large (the size of a small spray can or more) and have screw connections for high peak currents. It is almost irrelevant what the actual capacitance rating is it is only the ESR that is important here. You will find such capacitors rated from 27,000 micro-Farads and up with 3 milli-Ohm ratings or less. They will be from one of the major manufacturers.
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Old 04-05-2009, 12:43   #4
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Thanks Rick, you've provided the comprehensive answer I would have expected (and yes, still a bit "over my head" ).

Here's what cam out of my meeting yesterday with my electrical system advisor (who is selling me the new batteries and charger(s)):

Because of the size limitations mentioned above and what I forgot to mention the weight prohibitiveness of a 150A/hr, 1300CCA battery, we will combine the Genset & Engine battery into one bank of the two (2) 100a/hrs, 900CCA batteries giving me a bank of 200 A/hrs. (BTW, would that also result in a 1800CCA bank???)

We will also add in a "switch" that enables me to start the Genset or Engine from my House Bank.

Does this solution sound acceptable to you?

BTW, I am also exploring how I might increase the size of my planned new 675 A/hr House bank to 900A/hr (was currently at 440 A/hr) so I don't draw down as low (percentage wise) on the house bank. Of course that will also require another charger. My advisor is suggesting I go with the 675 A/hr solution, take 1 or 2 long coastal trips to see what my loads actually are (as opposed to what my calculations suggest), then decide whethewr I expand to the 900 A/hr (which means making room for 2 more batteries) solution.

Your thought on this as well please

William

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Old 04-05-2009, 14:08   #5
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Yes, if you parallel two 900CCA batteries you may have an 1800 CCA rating. The proviso is that for each additional cable and termination added one adds parasitic resistance which results in a lower voltage under load delivered to the motor terminals. Be sure to make wiring such that each battery "sees" the same length of cable in reaching the motor. For example, if one merely places two batteries side-by-side with motor leads on the right-hand battery and leads from the left-hand battery directly to the + & - terminals of the right-hand battery then the left-hand battery cannot deliver the same current as does the right-hand battery.

To correct that example one could change the motor cable (pick either the + or - lead but not both) from the right-hand battery and connect it to the proper polarity on the left-hand battery. Now the parasitic resistances will be the same for both batteries.

Be judicious when choosing a battery selector switch for use with starting motor currents. Some diesel start motors have locked rotor currents of 900-1800 Amps and most switches will not reliably carry that load for years. Cole-Hersey has good switches that have proven to work over time even with loads above their ratings. Perko switches, however, have failed (in my experience) even with motor loads within their ratings. Other switches also may not fare well over time. Guest makes a 1000A switch that works well (on/off, not for two banks with common). They are large (good ones are) and last longer than other brands that are physically smaller that carry the same apparent rating.

I agree that it would be better to use your existing house bank under actual cruising conditions (if you can) and observe your consumption with a good Amp-hour battery monitor before purchasing more batteries.
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Old 04-05-2009, 14:17   #6
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I have a related question, and I hope it will be regarded as complementary to this thread, and not thread-jacking. It has always been my habit to put power into the battery bank when a lot of power is going out, namely when I'm using the windlass and/or bowthruster heavily, in order not to drain the house bank. I have a 175 amp by 24 volts alternator, which amounts to about 4kW, which should be enough or nearly enough to make up for the 10hp bowthruster which is not usually used continuously, AT LEAST AT NOMINAL OUTPUT.

But what RPM is needed to get meaningful output from the alternator? I am thinking that at idle while I'm messing with the windlass or maneuvering, this will not occur? What do you guys do? Should I use a fast idle, say 1000RPM? Or what do you guys do?

Should I rather put the generator on? I spent some time recently on a friend's boat where the bowthruster would not even work if you didn't have the generator on, but my boat is not wired that way.
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Old 04-05-2009, 15:03   #7
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Thumbs up

Dock:
I don't regard your query as a "hyjack" and am equally intersted inlearning what Rick's view on your sityuation might be.

Rick:
Again thanks for your expertise. While I have on previous postings stated my confidence in my electrical advisor/electrical equipment supplier, as he knows, it's more about my gaining an understanding of this particular subject, which I seem to not have an aptitude for.

Reducing the confusion and/or doubts in my mind, by multiple assurances helps that tremendously ... and I always learn something in the process ... albeit very slowly.

Gracias Amigo....
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Old 04-05-2009, 15:30   #8
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supported system

Dock:
You are describing what is called a supported system wherein the alternator provides the preponderance of current to the load and the battery bank supplies the rest, if any.

The easiest method for determining the rpm for "breakeven" is to use a battery monitor. Slowly raise (or drop) the engine rpm until the net current into/outof the battery is zero.

A more agricultural approach is to use the alternator output curve of spindle speed vs output current, multiply the spindle speed by the pulley ratio of the alternator pulley diameter and divided by the diameter of the engine pulley. Discover the current required by the load and look up the engine rpm from the curve.
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Old 04-05-2009, 16:09   #9
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Quote:
Originally Posted by Rick View Post
Dock:
You are describing what is called a supported system wherein the alternator provides the preponderance of current to the load and the battery bank supplies the rest, if any.

The easiest method for determining the rpm for "breakeven" is to use a battery monitor. Slowly raise (or drop) the engine rpm until the net current into/outof the battery is zero.

A more agricultural approach is to use the alternator output curve of spindle speed vs output current, multiply the spindle speed by the pulley ratio of the alternator pulley diameter and divided by the diameter of the engine pulley. Discover the current required by the load and look up the engine rpm from the curve.
Thanks. But is it a linear relationship? I would have thought that the alternator would get up to its rated capacity at some RPM well short of the engine's redline, whereupon the regulator would kick in. I suppose it probably varies from boat to boat, but what is the usual case? 1000RPM? 1500? 2000?
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Old 05-05-2009, 12:33   #10
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Non-linear curve

Alternator output current versus spindle speed is non-linear. Doubling the spindle speed does not double the output current. In addition, there is a threshold of spindle speed below which there is effectively no output but this is quite low.

Large frame alternators are often limited to 7000 to 10,000 rpm spindle speed. Work the pulley ratio to figure if your engine red line is equal to or below this value (it should be). Small frame alternators with special bearing grease designed for high speed are usually limited to about 15000 rpm spindle speed.

Alternator output curves are independent of any regulator, they are rated at what is called "full field" whereas the field voltage is connected directly to the alternator output (which a good regulator will do when the demand is there).
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