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Old 06-07-2010, 23:44   #1
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Two Engines and Charging

I am building a 38' cat which will have two 21HP engines each with a 70amp alternator. I will have 4x120amp AGM batteries as a house bank and each engine provided with a 75amp AGM start battery. I want to be able to utilise as much of the alternator output as possible to charge batteries, engine and house, in the shortest possible time. I had intended using a VSR on each alternator to control the charging but have heard that there can be troubles using this arrangement along with the alternator regulators fighting each other and reducing charging capacity.
Has anyone installed such a system and what is the best way to provide the best result?

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Peter
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Old 06-07-2010, 23:54   #2
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This is a very common issue on powerboats. The regulator manufactures will have solutions. You are correct: when not done properly the so-called "smart" regulators will fail to charge properly.
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Old 07-07-2010, 05:48   #3
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The Balmar Centerfielder is the solution for this common problem. This device electrically sits between the two regulators and makes them play nice. One caveat: the field conductors from the CF to each smart regulator must be the same length. Charlie
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Old 07-07-2010, 06:16   #4
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Charlie is correct - we have this set up on our cat with a 24V Balmar alternator on each Yanmar and the Centrefielder. Works well.
Paul
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Old 07-07-2010, 06:55   #5
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The Centerfielder will work, but so will the new Balmar MC-612 Dual. It will handle two alternators charging one house bank.

Two observations:

1. The two 70A alternators are gonna be WAY too small for fast charging of a 720AH AGM battery bank. That bank can easily take 700 plus amps. You might want to re-think the charging setup with the two engines. I believe that in your situation I'd think about fitting some larger alternators....even up to twice that size....on each engine and, possibly, de-rating them by using the percentage function of, e.g., the Balmar MC-612. This allows you to set the maximum output from the alternator as a percentage of it's full capacity. If you don't somehow control those 70A alternators and limit their output, the 720AH AGM house bank will likely burn them up by placing them under a large load for an extended period, especially when they're drawn down as would happen while cruising.

BTW, I understand that with two smallish engines you might be concerned about big alternators drawing too much power away from propulsion needs. The MC-612 regulator has a neat function which will allow you to fit a "half-power" switch....a simple toggle switch which will cut the alternator output in half when needed. Note: it also allows you to fit temp sensors for the alternator and the battery bank which, when they sense over-temps, will also cut the output in half.

2. The 70AH AGMs would be fine, and could be maintained by, e.g., an EchoCharge or DuoCharge device, drawing a bit of needed current off the house batteries while they're charging.

Bill
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Old 07-07-2010, 16:33   #6
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Thanks for the responses. Total amp hours is 630 made up of 4 x 120 aH house batteries and two 75aH start batteries. I do not have smart regulators but had hoped to use the regulators provided with the engine/ alternators. If I am only discharging the house bank to 50% then I will only need to charge 240aH.

To use the Balmar Centrefielder will require the purchase of two balmar regulators as well, starts getting expensive.

One option I have been told about is to split the house bank in two and have two independent systems each charged from its own alternator and incorporating a selection switch to determine which bank to use. On top of this the engine oil pressure switch could be used to activate a solenoid that combines the start and house bank for charging. When the engine is turned off. the solenoid disengages and separates the two batteries to ensure the start battery is fully charged for starting. When the engine has started and oil pressure built up, the solenoid engages to combine the batteries.

Any thoughts?

Peter
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Old 07-07-2010, 16:42   #7
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Yes, don't split the bank. Current practice is to have one large house bank. Your batteries will last longer because you wont draw each battery down as much and you don't have mess with/forget to change the switch over to the different bank.

In the previous post Bill mentions the Balmar MC-612 dual alternator controller instead of the Centerfielder and two regulators.

The built in chargers are really only meant for start batteries. You will be much, much happier with an external 3 stage voltage regulator.

Oh, by the way, you would only count the battery bank as 480 AH. The start batteries generally only need a small amount of charging to top them up.
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Old 07-07-2010, 20:55   #8
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The Centerfielder will work, but so will the new Balmar MC-612 Dual. It will handle two alternators charging one house bank.
Bill-The MC-612 Dual will only work when the alternators are on the same engine. Turns out minute differences in rpm can cause significant control problems.

Charlie
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Old 07-07-2010, 21:01   #9
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SpiritCat-Bilge the oil pressure switch/solenoid idea! Check out the Blue Sea Systems Automatic Charge Relays that combine battery banks when the charging voltage exceeds a ceiling for a set period of time and isolates the banks when the system voltage is reduced below a floor for a set period of time.
This one is a solid performer and can handle 120 amps: SI-Series Automatic Charging Relay - Blue Sea Systems
I prefer to use the latching relays with their integral manual over-ride switches. They are rated at 300 amps. ML-Series Heavy Duty Automatic Charging Relay - PN 7622 - Blue Sea Systems

Charlie
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Old 08-07-2010, 05:00   #10
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Bill-The MC-612 Dual will only work when the alternators are on the same engine. Turns out minute differences in rpm can cause significant control problems.

Charlie
Yes, Charlie....absolutely right. Thanks for catching that!

Memo to self: we're talking about TWO engines, here :-)

Bill
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Old 08-07-2010, 09:58   #11
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Originally Posted by spiritcat View Post
I am building a 38' cat which will have two 21HP engines each with a 70amp alternator. I will have 4x120amp AGM batteries as a house bank and each engine provided with a 75amp AGM start battery. I want to be able to utilise as much of the alternator output as possible to charge batteries, engine and house, in the shortest possible time. I had intended using a VSR on each alternator to control the charging but have heard that there can be troubles using this arrangement along with the alternator regulators fighting each other and reducing charging capacity.
Has anyone installed such a system and what is the best way to provide the best result?
Check with the battery manufacturer before planning how much current to give an AGM. You will read in places that AGMs have a very high acceptance charge rate. Just because the batteries may accept this current, doesn't mean it is good for them. Lifeline, for example, recommends charge current be no more than 20% of total capacity (200A for a 1000AH bank).

Also, consider that your alternators may not be able to put out full output for extended periods. I think your plan for 70A alternators is fine, and you probably want to limit their output via external regulator to prevent overheating the alternator and giving the batteries excessive charge current (per mfr specs).

Using external regulators such as Balmar MaxCharges and a Centerfielder is a necessity. The internal regulators will not do a good job and you will end up running your engines for a long time at a light load while the alternators trickle charge your batteries. As mentioned in another post, you can also use the external regulator to cut the alternator output in half or turn it off completely to give full engine power to propulsion.

Also, I think you want DuoCharge controllers for your start batteries. If you subject your start batteries to the house charge voltage by simply using a voltage sensitive relay you will cook them.

Your goal of charging the batteries as quickly as possible is understandable and common, but you should recognize that this will shorten their lives. Higher acceptance rates are achieved when the battery is at a lower depth of discharge (ie. 50%), however the deeper you discharge the fewer cycles the battery will yield. Life expectancy at 50% discharge in a cruising application (daily discharge/recharge cycle) for an AGM is only about 2-3 years. Batteries will yield more cycles at shallower discharges. Shallow discharge means lower acceptance rates during charging. Low current charging is not practical with alternators or diesel generators, only with wind and solar.

I learned this the hard way. I installed the external regulators and Centerfielder on my cat and saw the difference. I also killed a set of AGMs by cycling them between 50-80% and jamming in over 250A. I have put up solar and now try to cycle the new batteries between 70-100%. Charging takes a long time and the engines are underloaded for much of it. A little Honda generator fills the gap between when using diesel is silly and before the solar takes over to finish.
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Old 08-07-2010, 11:59   #12
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Jay H...

Much of what you say, and your experience, resonates well. However, the rate at which AGM batteries can take a charge...without damage...is one of the principal reasons for going with AGMs in the first place.

And, the 20% figure you quoted is WAY below those given by most manufacturers, including Lifeline. The following is from the Lifeline website:

Quote
AGM Batteries are not harmed by high amperage charge and discharge so long as the input voltage is carefully regulated.
Charge 12 volt batteries at 13.2 - 14.4 volts
Charge 6 volt batteries at 6.6 - 7.2 volts
Avoid over charging and remove from charger immediately (or divert to other load - PV, renewable). Over charged batteries will have a unnaturally short lifespan. AGM batteries are not affected by higher current charging as GEL and Flooded batteries can be.
Unquote

They say in several places that their AGM's can be charged at a rate of up to 100% of their AH capacity without damage. And, as for the depth-of-discharge parameter, they rate their AGMs at 1,100 cycles when discharged to an average depth of 50%.

That said, I believe it's better for extended battery life to charge at lower rather than higher rates. However, there are tradeoffs, particularly if engines or generators are to be primary sources of charging. Then, their acquisition, POL, and maintenance/amortization costs need to be taken account of, not to mention the noise pollution factor.

Using your AGMs at between 50 and 80% charge, as many cruisers do, may be OK so long as you get them to near 100% charge every week or two. Remember, all lead-acid batteries (flooded, AGM, gelled) are damaged by remaining at less than full charge for extended periods, allowing PbSO4 crystals to form on the plates and reduce ultimate capacity for storing and delivering energy.

An adequate-sized solar array thru a good MPPT controller may be one of the best things you can do for your batteries, "adequate" meaning enough to routinely approach 100% charge level.

Everything's a tradeoff: you gotta do what you gotta do in your own individual circumstances.

Bill
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Old 08-07-2010, 12:18   #13
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And, the 20% figure you quoted is WAY below those given by most manufacturers, including Lifeline. The following is from the Lifeline website:

Quote
AGM Batteries are not harmed by high amperage charge and discharge so long as the input voltage is carefully regulated.
Charge 12 volt batteries at 13.2 - 14.4 volts
Charge 6 volt batteries at 6.6 - 7.2 volts
Avoid over charging and remove from charger immediately (or divert to other load - PV, renewable). Over charged batteries will have a unnaturally short lifespan. AGM batteries are not affected by higher current charging as GEL and Flooded batteries can be.
Unquote

They say in several places that their AGM's can be charged at a rate of up to 100% of their AH capacity without damage. And, as for the depth-of-discharge parameter, they rate their AGMs at 1,100 cycles when discharged to an average depth of 50%.
You are correct, of course. My apologies for contributing bad information.

1,100 daily cycles is 3.0 years.
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Old 08-07-2010, 12:36   #14
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Also, I think you want DuoCharge controllers for your start batteries. If you subject your start batteries to the house charge voltage by simply using a voltage sensitive relay you will cook them.
Jay, this statement is not true. You cannot overcharge the start batteries by connecting them to the house during a charging cycle. A combined bank will have a combined voltage that the charger sees. The charger will bring this combined bank up to the preset voltage, after which each individual battery will only accept the charge it needs and the increased internal resistance of a charged battery (say the starting battery here) during the charging cycle will shunt the charge amperage to less charged cells (say the house battery). Ohms law.

Voltage sensitive relays are perfectly suited for combining multiple battery banks during a charge cycle and will not cause overcharging of smaller, more charged batteries, like a starting battery.

The only way to cook batteries is to follow wrong charging regimes. So if your start batteries are a different type than the house (gel vs. flooded, etc), then a combining relay is not a good solution unless the charging is targeted for the battery with the lower charge voltage. This would be fine if the house bank needed the lower charge voltage, but a waste of energy if the reverse.

Mark
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