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Old 03-06-2010, 05:20   #1
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Complicated Electrical Question - Good Puzzle for You Electrical Geniuses

Our boat has two banks of 4 x 110ah x 12v batteries each, wired to provide 220ah x 24v from each bank. One bank is the "service" bank, powering the winches, thruster, and windlass. The other bank is the "domestic" bank, powering everything else.

The "domestic" bank does not have enough capacity to run my large refrigeration plant and other loads at anchor for reasonable periods of time. So after a lot of thought and consultation here and with various electricians, I have wired the two banks together so that I can draw on both "domestic" and "service" banks while at anchor.

The way I wired them together is as follows. Since the banks are quite far apart from each other (about 20 feet at least, I would say), and to avoid extensive reconnecting of the various loads or reconfiguring the chargers and splitters, I simply ran a heavy cable between them with a heavy duty mechanical "on-off" switch. When I am at anchor, I join the banks by flipping the switch. When not at anchor, particularly when charging or motoring, the banks are disconnected and are charged and work the way they were originally designed.

This seems to work fine, although the act of connecting and disconnecting the banks may be a needless hassle, compared to if I had truly integrated the two banks. Note that this method of joining the banks has one possible advantage over fully integrated banks -- it preserves the originally designed function of isolating the electronics from voltage shocks from operating heavy equipment like windlass, winches, thruster. In all cases when that equipment is being used, the banks are disconnected from each other and the system works as the factory designed it.

Here's my question, or rather questions:

Even before I started reconfiguring the banks, I suspected my battery charger of malfunction. It seems to be cooking the batteries. I have 29.25 volts or so when the batteries are fully charged and the charger is connected, corresponding to 14.675 volts in a 12 volt system. That does not seem to be float charge voltage to me. And indeed the batteries have started consuming large amounts of water, which they did no previously do.

The charger is a Newmar multistage unit, 10 years old and original to the boat, of apparently high quality, 40 amps x 24v, a little on the small side for 440ah x 24 volts of batteries. I did not have any problems with it until fairly recently, although the problems started before I started messing with the battery banks.

Question 1: WTF? Does 29.25 volts at full charge mean the charger is on the fritz? If so, is this type of thing fixable, or do I toss it and buy a new one?

Question 2: If I have to replace the charger (I had not been planning on doing it this year), should I leave the batteries semi-integrated as they are now, leave in place the low-loss (Driftgate) splitter (which services only the alternator; the charger has separate outputs for the two banks)?

Or should I permanently connect the two banks? In that case, do I just remove the splitter, and remove the charger and alternator connections to the former "service" bank, and let that bank be charged along with the "domestic" bank through the connecting cable between the banks? Or will the distance between them impair the charging process?

A bonus question (Question 3): If I were to forget to disconnect the battery banks before cranking up the engine, and if charge current were to flow into the two banks through the Driftgate splitter, even though the two banks are connected to each other, will anything explode or melt down?

Boy, I know this is a tough one, but I also know we have some really remarkably knowledgeable people here, so maybe someone has some insights.
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Old 03-06-2010, 07:50   #2
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41 page views -- 41! And not one answer! Sigh. I was afraid everyone would be as stumped as I am.
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Old 03-06-2010, 08:06   #3
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I'll attack just one of your issues. I agree that 29.xx volts is not a 'float' voltage, I would guess float should be around 26.8? volts. IMO, 29+ volts would be the bulk/absortion voltage. Since I just went through replacing batteries, I wonder if you have a battery problem vs. a charger problem. My reason is that my batteries started using water and I attributed it to a weak cell (or two) that wouldn't come up to voltage, hence the charger just kept thrashing the batteries trying to get them up to voltage and boiling the water in all the cells. My new batteries (Trojan T105s) haven't used any water in 4+ months using the same charger.
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Old 03-06-2010, 08:07   #4
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I believe (although I may be wrong) that 14ish v is about right for charging voltage.

If you want to check the batteries are working properly, do you have a way of measuring Ah in and Ah out?

I'm sure that separating the 2 banks does have the advantage of protecting the domestic side from shock loads, but I was always of the opinion that the separation was to make sure that you didn't flatten the engine battery with the freezer, as once you've got 4 flat batteries, you have no way to start the engine to charge the batteries.
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Old 03-06-2010, 08:09   #5
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Ok I will attempt to help a bit. Quest. 1. Your charger could be fried but not necessarily. First off when you get into multiple battery configurations the chance of connection problems increases. A bad connection may prevent your battery charger from going to float. So clean and recheck all connections inbetween the batteries and to the charger.

Q2: Shouldn't hurt.
Q3: No
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Old 03-06-2010, 08:18   #6
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Are you leaving the batteries separate while you're charging? Just double checking if your problem started after you changed things even though you said it started before.

Did you change anything else, like leaving something on that you didn't before? My charger hits 14.7 volts while bulk charging. It is undersized for the house bank. If I leave anything on, the current draw is high enough that it fools the charger and it never trips to float stage.
Is your boat sinking? Are your pumps operating more than normal? Do you have an hour meter on your pumps?

Have you checked the splitter with a DVM?

If you leave the system in its original state and have changed nothing else while charging, I'd say the charger has failed.

John
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Old 03-06-2010, 08:24   #7
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Originally Posted by Lancerbye View Post
Ok I will attempt to help a bit. Quest. 1. Your charger could be fried but not necessarily. First off when you get into multiple battery configurations the chance of connection problems increases. A bad connection may prevent your battery charger from going to float. So clean and recheck all connections inbetween the batteries and to the charger.

Q2: Shouldn't hurt.
Q3: No
Thanks! That's a great tip on the battery connections. I'll do that first.

On Question 2 -- did you mean shouldn't hurt leaving it as it is (with switch between banks and the splitter left in place)?

Thanks again.
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Old 03-06-2010, 08:27   #8
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Originally Posted by cal40john View Post
Are you leaving the batteries separate while you're charging? Just double checking if your problem started after you changed things even though you said it started before.

Did you change anything else, like leaving something on that you didn't before? My charger hits 14.7 volts while bulk charging. It is undersized for the house bank. If I leave anything on, the current draw is high enough that it fools the charger and it never trips to float stage.
Is your boat sinking? Are your pumps operating more than normal? Do you have an hour meter on your pumps?

Have you checked the splitter with a DVM?

If you leave the system in its original state and have changed nothing else while charging, I'd say the charger has failed.

John
Thanks, that's also very useful.

Yes, the problem started before I started messing with combining the banks. No, I didn't change anything other than -- well, I did install an isolation transformer about the same time the problem started, but I can't quite imagine how that could have any effect.

No, there is no bilge water problem and no new loads.

I haven't checked the splitter but don't suspect it -- should I? The main battery charger, which has its own dual outputs, does not use the splitter. The splitter is only for the engine-driven alternator.

Is there some way to check the charger for correct function?
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Old 03-06-2010, 08:34   #9
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Originally Posted by YourOldNemesis View Post
I believe (although I may be wrong) that 14ish v is about right for charging voltage.

If you want to check the batteries are working properly, do you have a way of measuring Ah in and Ah out?

I'm sure that separating the 2 banks does have the advantage of protecting the domestic side from shock loads, but I was always of the opinion that the separation was to make sure that you didn't flatten the engine battery with the freezer, as once you've got 4 flat batteries, you have no way to start the engine to charge the batteries.
That's not applicable in this case because neither 24v bank is used for starting. The engine and generator each have their own completely separate 12v starting batteries each with its own separate 12v alternator.

I hesitate to conclude that there is anything stupidly designed on my boat, because it would be the only stupid thing I have found so far. It would not be like the designers who did such a good job with everything else. But for the life of me I can't come up with a good reason for these dual banks. The electronics shouldn't be so shocked anyway, because 440ah x 24v is a whole lot of capacity to absorb those shocks, plus the windlass and thruster can't even be used unless the engine is running, feeding and damping those loads with the massive 110amp * 24v alternator (can supply about 2.5kW).

It's poor use of battery capacity, because the "service" bank doesn't do anything at all to draw down capacity long term. Windlass and thruster can't draw down the batteries because those loads will be supplied (or quickly made up) by the alternator. Electric winches are just not used for long enough periods to pull the batteries down. So half of the boat's battery capacity basically sits there doing nothing.

So I finally decided to deviate from the original design and combine them.
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Old 03-06-2010, 08:50   #10
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Originally Posted by DotDun View Post
I'll attack just one of your issues. I agree that 29.xx volts is not a 'float' voltage, I would guess float should be around 26.8? volts. IMO, 29+ volts would be the bulk/absortion voltage. Since I just went through replacing batteries, I wonder if you have a battery problem vs. a charger problem. My reason is that my batteries started using water and I attributed it to a weak cell (or two) that wouldn't come up to voltage, hence the charger just kept thrashing the batteries trying to get them up to voltage and boiling the water in all the cells. My new batteries (Trojan T105s) haven't used any water in 4+ months using the same charger.
Oh, dear. That would be bad if true. My batteries are almost new, were new last season. I guess I could test them.
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Old 03-06-2010, 09:00   #11
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By all means, test them. Also, check that all connections are clean and tight.

Seems to me your boat is well set up and that you've solved the issue of the two banks (by combining them at anchor and/or dockside), and separating them when underway.

While you're right that the big battery bank provides a pretty good capacitor, the issue is sometimes one of response time, i.e., the batteries can't clamp high transient voltages fast enough. Motors like winches and windlasses and bow thrusters can throw a very high voltage on the cables when they stop. Of course, you could combine the battery banks full time, and get a good DC-DC converter/regulator to run the sensitive equipment.

Another thought: be sure your metering is accurate. Check against a calibrated meter. A few tenths of a volt error can make a big difference.

Bill
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Old 03-06-2010, 09:26   #12
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By all means, test them. Also, check that all connections are clean and tight.

Seems to me your boat is well set up and that you've solved the issue of the two banks (by combining them at anchor and/or dockside), and separating them when underway.

While you're right that the big battery bank provides a pretty good capacitor, the issue is sometimes one of response time, i.e., the batteries can't clamp high transient voltages fast enough. Motors like winches and windlasses and bow thrusters can throw a very high voltage on the cables when they stop. Of course, you could combine the battery banks full time, and get a good DC-DC converter/regulator to run the sensitive equipment.

Another thought: be sure your metering is accurate. Check against a calibrated meter. A few tenths of a volt error can make a big difference.

Bill
Thanks for all that! One purpose of combining the banks using a switch for the time being was to be able to experiment to see whether the instruments are really bothered by the heavy gear or not. In case they are bothered, then I could just keep disconnecting them when I'm underway.

I can't do that until I'm sure that I won't be hurting anything, by charging the two banks through a splitter as if they were separate, despite the fact that the switch is on and they are combined. Is there any feedback through the splitter or any other kind of problem to expect from that, or can I leave the switch on and experiment away? Can you comment on that?

As to accuracy of voltage -- I've got the original analogue volt meter plus a recently installed Victron battery manager. They agree on the 29.25 volts so I'm inclined to believe that, unfortunately, they are right.
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Old 03-06-2010, 09:53   #13
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I agree with above possibility of one or more bad cells. If shorted, they would fool the charger into thinking the bank is still partially discharged.

My house (an actual house) has a smallish charger to float 8 trojan 220ah 6volt batteries in a 12 volt configuration, and a wind generator to charge when the power goes out.
The charger has no problem keeping them full.

Try disconnecting all batteries from each other when they're supposedly full, let them stand for a few hours and I'll bet you will find at least one which measures 11 volts or less.
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Old 03-06-2010, 09:57   #14
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Even before I started reconfiguring the banks, I suspected my battery charger of malfunction. It seems to be cooking the batteries. I have 29.25 volts or so when the batteries are fully charged and the charger is connected, corresponding to 14.675 volts in a 12 volt system. That does not seem to be float charge voltage to me. And indeed the batteries have started consuming large amounts of water, which they did no previously do.

The charger is a Newmar multistage unit, 10 years old and original to the boat, of apparently high quality, 40 amps x 24v, a little on the small side for 440ah x 24 volts of batteries. I did not have any problems with it until fairly recently, although the problems started before I started messing with the battery banks.

Question 1: WTF? Does 29.25 volts at full charge mean the charger is on the fritz? If so, is this type of thing fixable, or do I toss it and buy a new one?
I hope what i write below helps even though it does not directly answer your questions.
If its working properly it will maybe do something like this...
The charger first bulk charges at 40a until the voltage is 29.25, it then enters absorbtion mode where it maintains 29.25v and gradually reduces the amps for a few hrs or until about 10a all that can be absorbed. It then enters float mode at 26.x volts.
The battery monitor should show this behaviour between volts and amps.
Does the charger have dip switch settings?

At anchor you will find the following problem...
40a charger is not enough. You need at least 100a.
When you run your generator at anchor you want to do it for the shortest period possible so you need the charger to put as many amps as possible into the battery. Imagine at anchor you start your generator and the 24v water heater is on, then this will draw 40a and so you will only have 60 left for charging the 440amphr bank. Thats why 100a is the minimum.

I have a 700a 24v battery bank, 7kw generator (cannot be heard from other boats) and 150a of 220v charging. The bank in the center of my 15m boat runs windlass and bowthruster and I have no spike type problems. Charging takes 40 mins a day at anchor if I stayed for a week. I can anchor 2-3 days without charging.
I used to have 80a of 220v charging and it meant the generator ran too long. I am now adding solar as well to make it even shorter.

I have no experience of what happens when you join 2 banks with 20 feet of cable... I think I would be inclined to leave them combined all the time but if I was leaving the boat for a few days or more I would probably seperate them. The only reason I say this is that paralleled banks drag each other down so I heard.
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Old 03-06-2010, 10:23   #15
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At anchor you will find the following problem...
40a charger is not enough. You need at least 100a.
When you run your generator at anchor you want to do it for the shortest period possible so you need the charger to put as many amps as possible into the battery. Imagine at anchor you start your generator and the 24v water heater is on, then this will draw 40a and so you will only have 60 left for charging the 440amphr bank. Thats why 100a is the minimum.

I have a 700a 24v battery bank, 7kw generator (cannot be heard from other boats) and 150a of 220v charging. The bank in the center of my 15m boat runs windlass and bowthruster and I have no spike type problems. Charging takes 40 mins a day at anchor if I stayed for a week. I can anchor 2-3 days without charging.
I used to have 80a of 220v charging and it meant the generator ran too long. I am now adding solar as well to make it even shorter.
FWIW, Trojan recommends charging their deep cycle flooded batteries at a rate of 10-13% of capacity and 20% of capacity on their gel and agm batteries.

see: http://www.trojanbattery.com/pdf/TRJ...uide0210LR.pdf
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