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Old 25-03-2009, 18:56   #1
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Electrician’s Nightmare?

I am planning to modify the battery banks on my boat. The planned changes seem OK to me, but then I donít really know anything about batteries. Hereís what I have and what I am considering. Any suggestions would be greatly appreciated.

The main house bank consists of two SeaVolt group 31 105 Amp AGM batteries wired in parallel. They are served by a Newmar 40A charger.

The starting battery is an Optima Blue Top 34M 50 Amp AGM starting battery, which is wired to the starter/alternator. The main house bank and the starting battery are connected by a Blue Sea 7610 Automatic charging relay, so they are combined when there is a charging current to either battery, but separate otherwise.

There is a third AGM battery bank forward (two Optima 55A dual purpose AGM batteries) that is used to power the windlass and the electric toilet. It is served by a 10 Amp battery charger. It is only recharged when we are connected to shore power or the generator is running. What I am contemplating is to connect this battery bank to the starting battery with another Blue Sea 7610 Automatic Charging Relay, so that this battery is also charged when the motor is running.

After this change, all three batteries would presumably be integrated when there is charging source, but isolated otherwise. I have a number of questions:

1. Does any of this make any sense?
2. Is the 40 Amp charger connected to the main house battery big enough for this system?
3. Are there any problems from having the 40A charger on the main house bank and the 10A charger on the windlass battery charging at the same time when all the batteries are integrated?
4. How does voltage regulation on a charger or alternator work when you have three different sized batteries integrated and each may be at a different state of charge?

Any and all help would be greatly appreciated.
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Old 25-03-2009, 19:54   #2
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Instead of the Blue Sea 7610 Automatic Charging Relay, consider an Echo Charge: Xantrex Technology Inc. - Recreational Vehicles - echo~charge - Product Information
You can set it for battery type...

Here's to swimmin' with bowlegged women!
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Old 25-03-2009, 19:56   #3
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The charging pattern matters most based on type of battery and the ability of the charger to respond as the battery gets charged. Batteries accept charge based on the state of the battery.,If you send too much charge to a battery then your are damaging it to some extent. Once charging source can not know what the state of multiple batteries are. It can only tell what the state of the group is is.

If all batteries are connected to the same source and each is at a different state of charge it sees what the "group" is at. That means it is sending charge at a rate based on all of them and not at all what each needs. The batteries that are at near full are being over charged and the ones lower are not getting enough. It means you are beating both up and will shorten their lifespans.
Paul Blais
s/v Bright Eyes Gozzard 36
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Old 25-03-2009, 20:25   #4
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The only thing in your description that doesn't make sense, IMHO, is trying to charge the two forward batteries from the alternator. You don't say how far forward that bank is but the voltage drop may be significant without installing pretty hefty cables. Charging current could be quite large at the beginning of the charge cycle. Then you also need cct. protection at each end of those cables.

I think you would be better off running the forward charger from your inverter (I assume you have an inverter) or your generator if that is running. Running the charger from the inverter requires some management so you will have to switch the charger off when you are not running the engine. Could be a hassle.

It would be better if your alternator was connected directly to your house bank with starting battery connected through the automatic relay. Your house bank will need more charge current than the start battery.
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Old 03-04-2009, 21:14   #5
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Your idea is sound. I just designed the charging system for a 72' power catamaran with a 7610 supplying the navigation battery bank in the pilot house. Total wiring run was 80' (100 amp alternator). I used 1/0 for about 7% voltage drop. Size your cabling for < 5% voltage drop and put a proper fuse where you connect to the battery and you will be fine.

By the way, AGM batteries are sensitive to high voltage and charging voltage should be decreased with increasing temperature from standard 25 C (negative temperature coefficient) and the charging voltage should be increased as the ambient temperature decreases from 25 C. Most AGMs use a value of 4 to 6 millivolts/C/cell or 24 to 36 millivolts/C for a 12 volt battery. What all this means is that to ensure long life for your AGMs, use a charging source with the correct charging profile for your AGMs and ensure that the charging source uses temperature compensation to adjust the charging voltage in all three stages.

Now for your specific questions:
Q2> A good rule of thumb is to charge flooded batteries with a charging source of 15 to 20% of capacity. AGMs can easily handle a charging source of 40% with thin plate pure lead (TPPL) batteries able to absorb a charging source of 100% of capacity. In your system the charger is a bit undersized.
Q3> I don't recommend mixing charging sources. With different charging algorithms, they won't generally play nice with each other.
Q4> The engine starter may use 250 amp x 5 seconds x 1 hr/3600 seconds = 0.35 amp-hrs (assuming a well tuned, 80 hp engine). With your alternator connected to the starting battery, it will take only a minute or two to charge your starting battery and for the 7610 to close to start charging your house battery. So far so good. The starting battery will be ready to go into float but the combined bank will look like a battery that needs to be bulk charged. In fact, the starting battery will share some of its charge with the house bank. As the alternator charges the combined system from bulk to absorbtion and then into float, the starting battery will end up being subjected to bulk and absorbtion voltage for a longer period of time than necessary. No current will flow into the starting battery after it has reached equilibrium. You cannot pack more electrons into the electro-chemical process than the electro-chemical process will allow at a given voltage and temperature.

The system described is not laboratory perfect but it is robust and will work well. I have designed and installed many systems in sail and power boats from 36' to 72' just as described and these systems perform very well.

I would recommend installing a Blue Sea 6006 switch in parallel with each of your 7610's. This switch will allow you to bypass the 7610 should the need arise.
Charlie Johnson
ABYC Master Technician
JTB Marine Corporation
"The Devil is in the details and so is salvation."
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