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Old 04-04-2007, 10:04   #1
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House battery bank size

I have 2 4D AGM batteries (420 amp) that are around 5 years old (maybe a year or so older). They are probably going downhill and will need to be replaced when we return to the states next month. The boat is kinda energy hungry I admit, but I'm not willing to give up ice cubes or the deepfreeze. So I think I'll look into expanding the house bank. Is there such a thing as an average size? This will not be an easy job as I see the only place to put them will be under the bed in the bow. That will require running large cables back to tie them together with the ones in the stern locker. I can handle that. Ideas?
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Old 04-04-2007, 11:02   #2
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How Lead Acid Batteries Work

is a good place to start to begin sizing your battery bank and for dimensions on the various sizes of batteries. I would suggest that most cruisers in 40'+ boats seem to use 150 amps or so per day so a bank of 600+ A/H's igves you a couple of days between charges at that rate. 3 8d's or 4x 4D's would put you in that range but you can get there with other arrangements such as golf cart batteries as well.
Check out the linked site for some of the options and theory including wiring sizing and then if you have further questions, there are plenty of folks here that can anwswer more specific questions.
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Old 04-04-2007, 11:35   #3
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Another consideration is alternator size. Batteries like to charge at specific rates depending on type,
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Old 04-04-2007, 12:31   #4
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You might want to look at "taller" 6 volt batteries. You may be able to get the required amps into a smaller horizontal space rather than spreading them out into opposite ends of the boat. As for bank size, how much power do you use now? Is that enough, almost enough or not near enough? I think bank size really depends on personal useage and whether or not you plan to add new power hungry equipment.

Other things you can look at to decrease the need for a larger battery bank are replacing the anchor, running and cabin lights with LED units, adding solar panels, etc. Before increasing battery bank size take a look at your whole system including alternator size, shore power charger size, etc to see if you would need to increase the size (in amps) of those units. A systems approach will give you better results with fewer surprises.

Hope this helps...
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Old 04-04-2007, 12:44   #5
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I'd look at it this way.

First, deep cycle/equalize the batteries, check the electrolyte level, check the wiring (clean and tight) to make sure of what capacity you really do have. Depending on quality and usage, you might very well have 5-8 years left, although they might also be shot.

Now, before you come home, take some notes on your energy usage. How many amp hours do you use in the average day? 100? 200?

And, how do you plan to recharge, and how often? If you are recharging by using the main engine, what's acceptable to you? An hour a day? Two hours? Every second day?

Based on how much energy you use, and how you plan to recharge, you basically want to buy enough battery so that you can use that energy and follow that charging schedule, while never using up more than half the capacity of the batteries. Batteries get the best life if you only cycle them some 30-50% of the rated capacity, depending on whose numbers you believe.

But that's just a starting point. Then you need to figure out, if there's room enough for that much battery. Budget, weight. And if you can or want to upgrade your charging system instead. A good external regulator to optimize charging, or wind or solar panels to provide charging without using the main engine.

One other factor is "charge acceptance rate". Rule of thumb is that your batteries cannot be charged any faster than 1/5th of their rated capacity, i.e. a 100Amp alternator is all that you can actually USE to feed a 500AH battery bank. (Rough numbers) More power will be wasted. AGM batteries, which are more expensive, can recharge 25% faster, but either way it is very easy to provide more alternator power than batteries can actually use. Some reserve margin is good, just be aware there are limits.

I think you'll find lots of battery and charging threads if you look at the older ones here.
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Old 04-04-2007, 12:45   #6
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Eliminate the guess-work

Apparently you do not have a battery monitor else you would KNOW just how much energy you use cyclically between your normal daily discharge usage and charging times. The first thing to do is install a "real" battery monitor and measure your energy usage. In addition, you can use the monitor to determine your bank internal cell resistance which is one prime indicator of capacity degradation (not the only one) along with current charge acceptance (another good indicator).

Multiply the measured daily Amp-hour consumption, multiply by 2.5 and that is the rating of your battery bank for AGM batteries that are cycled between a 50% state-of-capacity and a 90% state-of-capacity. Notice that the wording is not, "state-of-charge" here because it is possible to apparently cycle between two states of charge on a cyclical basis and not reach the same state of capacity upon recharge if sufficient absorption voltage is not reached for a sufficient period of time, thereby gradually losing capacity with each charge/discharge cycle.

Obviously when you have longer recharge times available you will reach more than 90% state-of-capacity if a proper charge regimen is applied. If you have a charge source with a current rating high enough you can recharge between the 50% and 90% "dynamic range" in one hour while maintaining a low internal battery resistance at the end of each charge cycle.

Read some of the related threads in the history of this section related to such topics.
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Old 04-04-2007, 15:18   #7
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Quote:
The boat is kinda energy hungry I admit, but I'm not willing to give up ice cubes or the deepfreeze. So I think I'll look into expanding the house bank.
Sorry there isn't an average size but 2 - 4D's is not an uncommon sized house bank. Adding more batteries means you have more charging to do with maybe little benefit. You also will charge better with same age same brand batteries. If well cared for your 5 year batteries could have a few more years left and maybe more than you think.

You only want all you need. Extra has little use if you can cycle as Rick notes above. When you take the whole bank down to 50% then you need even more charging to get it back up. Were you deep cycling too deep often then more batteries or less use would be indicated. If you could compute it you would know.

I would agree about the monitor. Just about the best electrical gadget you can have. Knowing what you use and where you are at sets up everything you'll ever need to know about power. I would say for sure you are not average so it sure would be nice to find out what you really use and how and how well do you charge back.

You may get more benefit with a monitor and switching out a few power hungry items for less hungry ones. You may be perfectly set up with what you have now. Right now lots of lighting options that eat a lot of power using incandescent and halogen. LED's and Cold Cathode Fluorescent are pretty nice when used in the right places. Adding a bit of insulation to the fridge is never a bad idea if you can. Use DC directly where ever you can and avoid Inverters as much as possible also helps. Adding a solar panel isn't ever a bad idea either.

I would do what ever is practical to see if you can live just as well without adding more batteries.
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Old 05-04-2007, 05:34   #8
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Some additional info.

I have two 130 amp solar panels, AirX wind generator, Heart 2000 inverter with 100 amp charger and a 100 amp Balmar alternator w/ remote 3 stage regulator.

I now have to run the generator twice daily to recharge the batteries. I charge last thing at night and by 7AM voltage is around 12 to 12.2V

The frig and freezer draw about 10-12amps combined and the anchor light maybe 2. That's why I think the batteries are going south. I'm hesitant to equalize them as I don't have a way to monitor battery temp as they are AGM (non vented).
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Old 05-04-2007, 06:05   #9
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Also a Link 2000 monitor (which I haven't completely figured out yet).
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Old 05-04-2007, 09:00   #10
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"I see the only place to put them will be under the bed in the bow. That will require running large cables back to tie them together with the ones in the stern locker."

If I understand correctly you plan to have 1 battery bank, with part in the bow and part in the stern. I strong suggest you don't do that, best to keep all the batteries in same location.
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Old 05-04-2007, 10:26   #11
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I may not have a choice, but I'd like to know why that's not adviseable.
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Old 05-04-2007, 11:02   #12
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Bottle-
"I charge last thing at night and by 7AM voltage is around 12 to 12.2V "
that means either something is wrong, or your batteries are half dead. Typical voltage for a fully charged lead acid battery is 12.6V fully charged, 11.6V fully discharged (under loads).

A battery bank up front isn't a good idea, you'll add a lot of pitching motion to the boat from that weight at the end. And, you may need large and expensive cabling to charge them properly.

If you want the regulator to work properly, it needs to sense the battery voltage without any loss or drop in the sensing lead. And if you want to charge quickly and with less load on the alternator, you want near-zero drop in the charging cables. Look at the round-trip cable length (you count both legs) from the charger to the battery, consider the amperage load you want to charge at (40A? 60A?) and look at the size of fully tinned marine grade cable you'll need to do that. I'm guessing something like an SAE 0 (zero) cable size. Ain't gonna be cheap.
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Old 05-04-2007, 13:52   #13
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Some considerations

I assume that you meant two 130 Watt pv panels, not 130A, is that true?
The Link 2000 has a pv mode that allows you to monitor your pv output and accumulated energy contribution on one shunt element, and the other channel is dedicated to your house bank, should you desire to set it up that way.

In my opinion your alternator is undersized. A more optimum would be a 165A hot rated large frame for shorter charging times or two small frame alternators unless you generally get a good contribution from your pv panels and wind generator.

If you review the concept behind Amp-hour law charging (in this section history) you can safely charge your AGM batteries without a temperature sensor (assuming that the ambient temperature is not over 100 deg F) if you keep within the strictures of the "law". At the least, with a monitor it is well worth your while to apply several cycles of high acceptance voltage (when your charge source will deliver the current) and log down your incremental internal battery resistance as an indicator to note any improvement.

One key element is to make the parasitic cable and connector resistance identical for each battery in the bank (assuming that the batteries are equivalent in energy storage and internal resistance rating). The use of a single positive and a single negative distribution post facilitates this concept so that all loads and all charge sources "see" the same resistance to each battery. That way, it will not matter where each battery resides relative to the other as long as their ambient temperatures are within about 5 deg F or less if possible. Regardless, I agree with others in that keeping the batteries away from either end of the boat keeps the pitching moment down. Have you examined the possibility of vertically stacking your batteries (build a rack) if you need to shoehorn one more in? With gel-cell and AGM batteries you can do this making sure that you can inspect and tighten the connectors without undue disassembly of the rack.
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Old 05-04-2007, 14:03   #14
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bottleinamessage,

you said,"I'm hesitant to equalize them as I don't have a way to monitor battery temp as they are AGM (non vented)."

AGM batteries should never be equalized. They could be damaged and due to the construction, and they don't need it in the first place.
As long as you don't exceed the maximum recommended voltage while charging, you should be fine. Check with the mfr. for your specific batteries for the max voltage. I have flooded batteries in the boat and AGM's in our house (a real house not a boat) for grid failure backup system. The Northstar battery site says it's ok to 14.7v. but they prefer 13.64 v for constant voltage charging.

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Old 05-04-2007, 14:28   #15
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Equalizing AGM batteries

All lead-acid batteries can be equalized. They ALL follow the same general electrochemistry (AGM, gel-cell, flooded-cell). The problem is that almost no one has the proper equipment to perform a real equalization cycle. Virtually no inverter/charger or other charger sold into the marine market is set up to do that properly. They are approximations to the regimen and, therefore, CAN ruin an AGM.

I have brought many AGM batteries back from the "dead" by properly charging and equalizing them. Equalizing takes a variable constant-current source having a compliance voltage near 20Volts for "dead" AGM batteries.

If you refer to the Amp-hour law you can visualize that it is a myth that there is any single "set-point" voltage that is an upper limit for any lead-acid battery charging regimen. Such single set point voltages are established when using 3-step or less types of chargers so that the Amp-hour law is not radically exceeded. Keep in mind that even the Amp-hour law current limits (and whatever corresponding compliance voltage that results in order to get the Amp-hour law current to flow) are not sacrosanct...they can be exceeded for short periods of time. Such a "law" was determined over 70 years ago for lead-acid batteries as has the constant current equalization regimen.
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