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Old 20-11-2009, 09:46   #1
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Paging Forum Electrical Geniuses - (You Know Who You Are)

I got some great advice from some of you about the electrical system on my boat, which has now raised some new questions.

Our boat has two starting batteries and two banks of service batteries (220ah x 24 volts each) for a total of four separate battery banks, two twelve volt (engine and generator start respectively) and two 24 volt.

The two service banks are separated into one house bank, which services lighting, water system, electronics, refrigeration, and domestic consumers, and one service bank which services four electric winches, bowthruster, and windlass.


The problem (one of a number of problems, but the problem this post is about) is that the house bank is too small to maintain the domestic systems for very long at anchor. The two service banks are not used efficiently -- the house bank is drawn down at anchor; the service bank is only used momentarily when sailing, docking, or anchoring. So I end up with a flat house bank even while I still have a fully charged service bank, and I have to run the generator to charge even though half of my service batteries are still up.

Someone here suggested combining the two banks. I'm going to try that -- first by connecting with jumper cables.

Someone else told me to be careful -- if the banks are connected when I run the engine, I can blow either the alternator or the diode splitter.

Question #1: How is that, that having the two banks connected will blow an alternator, which is connected to both of them anyway, and/or the diode splitter? Is it true?

Question #2: What do you guys think of the idea of putting in a combiner switch, so that the two banks are combined only at anchor. When I get ready to weigh anchor, I will disconnect them. That would eliminate any risk of voltage shocks from heavy consumer like the thruster affecting the electronics.

I think that the reason the banks are separate in the first place, is those voltage shocks. If so, then it's an awfully inefficient way to do it -- tail wagging the dog.

I know another way to do it would be to combine the two banks permanently, then install yet another bank-- say, two 110ah x 12v batteries to get 110ah x 24v -- in the bow to deal with the thruster and windlass. Leaving the winches on the main, now combined, 440ah x 24v bank.

There is no reason in principle not to do that -- there is space in the bow and so forth, and the boat is heavy enough (44,000 pounds displacement) that a couple extra batteries in the bow is not going to be a big deal in terms of trim. But if I can solve the problem with something like the above, I think it would be simpler, cheaper, and more elegant, than just throwing more batteries at the problem.



What do you guys think?
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Old 20-11-2009, 10:08   #2
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I think the best thing to do would be to sit down and make a list of your loads in amps and time used. you really need to understand what your needs are before you can figure out what is the best solution. You really do not give enough information for anyone here to really help you. It boils down to basic math, figure out how much you plan to take out and how fast you can replace that. It sounds like you have a lot of both short use and long use loads there is a lot to consider here. I would start by making a list of short use loads IE winches starter etc. and then a list of long term loads like lights radio inverter etc. Then you can start to figure out how much battery you will need, then you can figure out how best to charge those batteries.

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Old 20-11-2009, 10:51   #3
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I'll do that -- thanks.

But if there is no harm in putting a combiner switch in the circuit, then in parallel with doing that (will take some time since I am changing various things in the system, experimenting with LED lighting, choosing a new charger/invertor, etc.) I can just go ahead and put in the combiner and live with it a little (life being a lot more informative than any calculations).

So what do you think -- harmful to the battery or regulator, to combine the banks with a combiner, when the two banks are charged by the same (24 volt x 110 amp) alternator, through a diode splitter (more precisely, X-splitter)? That's kind of a threshold question.

Cheers, C.
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Old 20-11-2009, 11:06   #4
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I have used a diode based battery combiner for several years now. I have encountered no issues to date. I also have an AB switch that allows me to use the starting battery as the house bank or the reverse, to use the house bank as a starting battery. My default is the alternators charge the starter battery. My solar, wind and generator charge the house bank. My diode combiner will allow the alternator to recharge the house bank, but I am not certain that is essential.

So fundementally, if you wire it and understand your loads you will be okay. The second issue is one that you have started to become versed in. Why having one house bank will provide better overall service than separate banks. The only benefit to having separate house banks that I can see is the ability to use different types of batteries in the different banks, but that's a far more involved discussion that has been well articulated in other threads.
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Old 20-11-2009, 11:07   #5
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I have been on two boats configured like yours, but with a crossover switch which combines the two banks. Both boats ended up with the crossover switch closed on a permanent basis when away from dockpower, for the reason you have come up with--the service battery sees limited use. Combining the banks will not hurt the alternator or the splitter.
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Old 20-11-2009, 11:32   #6
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The down side of combining these batteries is you now have two differently aged banks. If they were near new at the same time the age of the batteries combined can be an issue. You would not normally combine batteries of different makes, brands or age. The weak link drives the charging and so the better batteries get over charged. One bad cell in a flood battery bank can overcharge all the rest of the cells. Being able to switch out batteries is a good thing. You just may have a hard time telling unless you notice a radical water loss in one cell or more.

The second issue is if the batteries were all identical and combined you'll find the rate of acceptance for the whole group will be lower overall. With a 50% discharged bank the charging rate is high in the beginning but the last 10% goes pretty slow. Combined with more batteries that 10% is a lot more amp hours that need to be replaced at a much slower rate. You run the charger longer as you dribble back the last 10%.

Doing all the math about usage is where the payoff comes from. If you would normally be drawing the one bank down to 50% then charging the idea of combining it is not to your advantage. If you are drawing it to a very deep discharge then combining may be better overall yet it will take longer to recharge the exact same number of amp hours. Reduction of the power used always has the best pay off. Once you go that far, you want to size the bank correctly. Too much bank means less efficient charging. The charger runs longer for the super sized bank drawn down the same number of amp hours.

You want to try and stick with no more than 50% discharge then recharge to a full 100%. It's the sweet spot on the wallet. It may be best to add the ability to transfer to the other bank when required then charge both banks 100% only when needed. Treating them as separate allow for maximum flexibility without the penalty for getting them both back to 100%.

I agree with others there is no danger combining them but it might not be the better plan. Your usage dictates the most economical plan. Wayne is also correct about that too.
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Old 20-11-2009, 12:15   #7
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The main rational I have used for allocating one bank is the time factor associated with bringing the bank back up to 100% As you charge the batteries, the charge acceptance rate of that battery goes down. In essence, even though you have available current, the batteries can only accept so much before they start generating excess heat and possibly permanent damage to the batteries. After you get past about 85% fully charged, the charge acceptance rate is not linear, it can decrease significantly. Most of the modern chargers have three phase charging to help mitigate this issue (Bulk, acceptance and float). So, for example, to get the last 10% of the charge into the battery may take 25% of the overall time, even though more charging current is available. So, there is this "penalty" for getting the battery up to 100%. If I have two banks, and I drain 1 to 50% (Don't go bellow 50%! The number of times a battery can be fully cycle down to that level is drastically worse if you stay above the 50% level. Each manufacture typically provides a curve of the life of the battery in cycles based on the level of discharge) and the other to 50%, then charge them both back up to 100%, the overall time I necessary to charge them both back up is significantly more than if I only had to do it once. This, assumes I can present close to maximum charge acceptance levels in both cases.

The other issue is one of available current. The slower I discharge a bank, the more current there is available (Peukert's constant). You'll find that while you may be able to get 500 amp/hours from a 1000 amp/hour bank at a 20 amp/hour discharge rate. You might only get 450 amp/hours from two 500 amp/hour banks at the same 20 amp/hour discharge rate. Combine this with the having to incur the recharge penalty twice, and it makes for a more compelling reason to have the one bank.

Having said that. I agree with Paul on the issue of having batteries of the same type, capacity, age. If they are not, there will be charging issues. Not generally the somethings going to blow-up or burn-out, but definitely not as efficient as it should be. For instance. As I was saying before, if you trying to provide too much current, the batteries start heating up. A certain amount of this can be okay. The good battery chargers have a temperature monitor that you can put on one of the batteries and they will vary the current to keep battery below its detrimental gassing level (i.e. temperature). There is only one temperature sensor so the charger will present the current based upon the temp of the one battery. If all are the same. Great! But if the others are different, or defective, the charger is not going to make the right decision for the other batteries. But, I don't think this is typically to high a price to pay for better overall life, and shorter recharge cycles.
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Old 20-11-2009, 12:29   #8
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The problem i see with just throwing a combiner in and carrying on for now is that these temp fixes become permanent. Then you start having problems and throw in another temp fix, and so on till you have a mess and a system that just pisses you off. It should not be that hard to figure your current use and predict what you future needs will be. If in doubt plan for more than you think you will need. Odds are it will still not be enough in a couple of years. What the others have said stands true as well. Combining banks of different age and use batteries is not a good idea. There is a school of thought on this forum that you should have just one bank and use that for house and start loads. I do not agree with this personally. I want my engine wiring completely separate so that if something goes wrong on the house side i can still use my engine. And be able to still charge the house side. A combiner will keep the banks separate except during charge. So I like it for that reason. I also separate my house and inverter batteries once again so that i have a backup if one system goes down. You have loads that are high amp and fast draw ie starter, winches, etc. These will want a different battery type than that of say your inverter as that would be a deep cycle battery. Spend the time to figure it out now and set it up right the first time or keep screwing with it putting band aids on and having problems. The one thing i see more than any other problem is crappy electrical systems because they are patched together over and over. Sure they work but not very well. Do your home work or call a professional to consult with you. You ask advice here and you are going to get 20 different opinions, 18 of which are likely to be right. You have to know what is right for you and the only way to do that is analyze your system and needs. Do it wrong a bunch of times or right once... your call.

Good Luck

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Old 20-11-2009, 13:47   #9
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If you want to do some planning and system reconfiguration the best overall site I have seen starts here Comparing marine batteries (Gel, Absorbed Glass Mat (AGM), flooded lead acid) Von Wentzel has an excellent treatise on configuring a system. I LOVE the spreadsheet he provides it is the best tool I have seen to date.

I had the opportunity to reconfigure my systems. I still have my original spreadsheets, but, one caveat. They WILL be off! My wind generator put out a lot less at anchor, my solar panel put out a lot more than planned, my freezer used a lot more than planned, my refrigerator used less than planned. I used my computer more than planned sometimes, less than planned other time. I didn't use my radar at all. Never did measure my actuals on my autopilot.

This is not to say don't do the planning! I am really happy I did it. But after doing it and tailoring my chargers and optimizing my consumers (haven't finished all the LEDs, but then again, I don't use those lights enough to justify amp/hour saving, and they haven't gone out yet!), my conclusion for meis to tailor my producers to be higher than my consumers, by about 20% or better. Then put in batteries that are as large as will conveniently fit into the battery boxes that are installed on my boat. Currently, this is 750 amp/hours (20 hour discharge rate). I'll be replacing them with 800 amp/hour AGM batteries during this winter (got 8 years out of the flooded cell batteries).

The other factor that comes into play here is, I found I was changing my usage patterns to sync with my production. When I ran my generator, I ran my water maker. When I had excess power, I'd run the stereo/computer more. My freezer was consuming too much, I cut down on the interior volume and raised the temperature from 15 to 22 degrees. I'll add two more solar panels, re-insulate my freezer and drop the temp back down to 15. When I used more energy than my passive suppliers would make, I bought and ran my portable generator for longer.

It seems to me, many of the notions on sizing and planning were/are predicated on the notion that you have one large (or two), engine driven alternator(s) and that one is trying to minimize the amount of time the engine is run and maximize the interval between running the engines. We hung engine driven water makers, and holding plate compressor to take advantage of the notion that we had to run our engines for power anyway. It is my experience that these notions no longer hold true. We have multiple, practical, energy producers (Solar, wind, water, gen sets, and generators). We have more efficient consumers (DC driven water makers, evaporator plate refrigeration, LED based lighting computer optimized systems) we have better storage technology (AGM, batteries to name just one of many) Manipulating one of these variables is complex enough! Trying to manipulate them all is an interesting application for differential equations! I don't use my engines to generate power, I use them to move from one location to another. I found I sailed or anchored for FAR more time than I used my engines to move. After this discovery, I decided to re-orient my systems to support that paradigm. My experience was I didn't anchor in windy spots as often as I was originally supposing so, spending more to get a better wind generator is not serving my usage patterns well. The sun shines a whole lot more in the areas I like to sail so increasing the amount of energy that I can produce by sun serves my usage pattern well. My catamaran sails a lot better when less loaded, so a portable gas generator serves me better than a diesel genset. I don't do much extended ocean crossing so my autopilot works better for me than a wind vane based steering system. I guess what I am suggesting is that it helps to know what you're going to do on your boat, how you'll use it, understand what you like, THEN start optimizing for that usage. Find out what is important for you and the way you use your boat, THEN spend all the money on it you can't afford

This was a little far afield from the original questions, but I hope that was answered in the prior post. Yes, you can use a combiner, no, it won't destroy your alternator (not if you have the appropriate regulator anyway!). There is some current loss and degradation of efficiency from the diode based combiner but given the size of your system it is such a small amount. But, this is the reason I used a switch as the primary diverter in my system.
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Old 21-11-2009, 02:09   #10
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I think some of the experts are over-analyzing the problem. You already have separate starting batteries, so you have the backup ability to start the engine or genset. Combining different age batteries in parallel does not do evil things like combining them in series does, and one big bank is more efficient (both discharging and charging) and will last longer than two smaller banks. The combining switch is cheap to install, and you can always leave it open if you don't like the results.
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