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Old 09-22-2012, 10:00 PM   #1
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Size of Battery Bank

Size matters, but it may be more of what you do with it that counts. I, being a little slower to pick up on things, have finally come to an epiphany. While contemplating adding another 8D AGM battery to my house bank, it dawned on me that there could be no real advantage. As long as I can run all items on the inverter circuits for 24 hours before the bank is at 12.2 volts why enlarge the capacity? I would not have to run the generator as frequently, but it would still have to run the same amount of time for charging. Or would it?

With that stated it seems that more battery capacity is not needed, and would not really save any generator hours. By turning off my biggest power drain, the ice maker, all else can run 48 hours. So, is the 510 amp hr house bank currently installed large enough?
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Old 09-22-2012, 10:47 PM   #2
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Originally Posted by Moonstruck View Post
Size matters, but it may be more of what you do with it that counts. I, being a little slower to pick up on things, have finally come to an epiphany. While contemplating adding another 8D AGM battery to my house bank, it dawned on me that there could be no real advantage. As long as I can run all items on the inverter circuits for 24 hours before the bank is at 12.2 volts why enlarge the capacity? I would not have to run the generator as frequently, but it would still have to run the same amount of time for charging. Or would it?

With that stated it seems that more battery capacity is not needed, and would not really save any generator hours. By turning off my biggest power drain, the ice maker, all else can run 48 hours. So, is the 510 amp hr house bank currently installed large enough?
Don, how long the genset would have to run is a function of its output, and the absorption rate of the batteries. The greater the capacity of the bank, the longer the interval is between re-charging, but once you start charging, you are correct that the charging time is going to be about the same, unless the absorption rate exceeds the output of the charger. My personal opinion is that if you can sit at anchor for a minimum of 24 hrs before having to start the genset, you probably aren't going to add much unless you double the bank size to add another day. Adding one more battery isn't going to accomplish that. Just one man's opinion....
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Old 09-22-2012, 11:42 PM   #3
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Sounds like it's working well for you now, Don. With another 8D, maybe you'd find you can go longer between genset runs, but if you consume x amps per day, theoretically you need to replace x amps unless you don't mind returning to shore with something less than a full charge.

In reality, since the charge rate accepted by the batteries for the last 10% or so tapers off significantly, few of us recharge to a full 100% via genset. After a few days on the hook, I seem to live between 50 and 90% to spare the extra generator hours required to get significantly above 90%.

So let's say you live on 40% of your bank AH. 40% of 510 AH bank is 204 AH. Assuming a 100A charger, it'll take 2 hrs to sufficiently charge the bank. Add a third 8D for a bank of 765 AH and your 40% range represents 306 AH/3 hrs genset run. Unless you need the extra capacity, you're only diminishing the depth of discharge of the bank each day (longer life) and carrying around the extra weight. And that cost you 50% more in batteries!

When it comes time to replace those two 8Ds, you might find that switching to 6V golf cart batts will give you the same 50% increased AH capacity in the same footprint as the 8Ds. Due to their lighter weight and smaller size, they're also MUCH easier to move in and out of the engine room.
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Old 09-23-2012, 01:41 AM   #4
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Originally Posted by Moonstruck View Post
Size matters, but it may be more of what you do with it that counts. I, being a little slower to pick up on things, have finally come to an epiphany. While contemplating adding another 8D AGM battery to my house bank, it dawned on me that there could be no real advantage. As long as I can run all items on the inverter circuits for 24 hours before the bank is at 12.2 volts why enlarge the capacity? I would not have to run the generator as frequently, but it would still have to run the same amount of time for charging. Or would it?

With that stated it seems that more battery capacity is not needed, and would not really save any generator hours. By turning off my biggest power drain, the ice maker, all else can run 48 hours. So, is the 510 amp hr house bank currently installed large enough?

You are correct.

Unless you increase the size of your charger and assuming the larger battery bank can accept more amperage.

No matter how large you make the battery bank you'll still need the same amount of hours of generator time per day.
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Old 09-23-2012, 04:05 AM   #5
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Apart from the obvious charging time differential, is there any pros or cons in using a smaller 40amp charger as against a 100amp model.

Also are there life expectancy issues with golf cart type batteries as against the standard 8D's? Our local marine electricians don't seem to keen on them.
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Old 09-23-2012, 07:16 AM   #6
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"Also are there life expectancy issues with golf cart type batteries as against the standard 8D's?"

8D is a size like 10D in shoes.

It is not an explanation of the type battery,deep cycle , start , "combination".

Assuming both are true deep cycle batts the configuration is not the concern , its the pounds of LEAD that does the work and needs the charge.

To live on the hook most will run 85 % SOC to 50% SOC with wet batts.

This constant lack of full charge will reduce the batt capacity over time , plan on it.

AGM are better if you are willing too pay double to run 40%-85%

For the silence minded the more rapid absorbtion rate of the AGM MIGHT be worth it IF the charge system is upgraded to provide the higher amp charge they can accept.

Life of most styles of batts is a matter of the depth of discharge .

On most web sites , but 400 cycles is fairly common for wet.Look up Trojan or Surette.

As always a SOC meter should be the first purchase , before contemplating any cruising on batts.
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Old 09-23-2012, 08:55 AM   #7
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Andy:

There are very few true deep cycle 8Ds. Rolls, Lifeline and Surrette are a few but they are very exensive. Look at the manufacturers rating. If it is rated in MCA or CCA and not amphours, then it almost certainly is not a deep cycle battery.

So if your electrician can find you truie 8D deep cycle batteries, you can afford them and he has the brawn to move them, ok.

But note that there are probably no golf cart batteries that aren't true deep cycle.

David
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Old 09-23-2012, 09:48 AM   #8
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Originally Posted by Moonstruck View Post
Size matters, but it may be more of what you do with it that counts. I, being a little slower to pick up on things, have finally come to an epiphany. While contemplating adding another 8D AGM battery to my house bank, it dawned on me that there could be no real advantage. As long as I can run all items on the inverter circuits for 24 hours before the bank is at 12.2 volts why enlarge the capacity? I would not have to run the generator as frequently, but it would still have to run the same amount of time for charging. Or would it?

With that stated it seems that more battery capacity is not needed, and would not really save any generator hours. By turning off my biggest power drain, the ice maker, all else can run 48 hours. So, is the 510 amp hr house bank currently installed large enough?
Energy is energy so you would need to run the genset to make up for energy used, regardless. Unless - you are underway and recharging your batteries with the engines.

The answer lies with how you use your boat and your willingnees to manage energy vs. just using it and then worrying about replenishing it.

Two factors to consider are the original and eventual replacement cost of additional batteries and the additional weight of additional batteries.

My thought - I suspect you are fine with what you have.
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Old 09-23-2012, 10:19 AM   #9
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Thanks for the replies and advice. We usually don't run for as long a period of time as a trawler to get to destinations. Therefore, we don't depend on the engines to fully charge the batteries while travelling. When we have a long run such as relocating north or south that works. We like to anchor for 2 to 4 nights at a spot. That means generator time for charging. We have plenty of capacity for that. 2 hours in the morning and 2 hours in the evening will take care of the charging we need. Ron, because we cruise a lot with guests on board, I try to set it up without worrying too much about load shedding. The heavy loads are not on our inverter, so only work on the generator. Hot water is usually sufficient throughout the day. We run the generator when doing our morning and evening meals.

I think that the 3 stage voltage regulator is the only upgrade to the system now. Maybe more later, as the inverter has more capacity.
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Old 09-23-2012, 12:04 PM   #10
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In thinking about whether a larger House battery bank is needed, two things need to be balanced out: the maximum depth of discharge you will tolerate (not greater than 40% would be a good target for flooded batteries....ie: 60% remaining) & the generator use frequency you are prepared to tolerate. For example, if you only want to use your genset once in 24 hours but at that point, the battery bank is down to, say, only 50% capacity remaining rather than the target 60%, you could consider adding capacity to the bank. Alternatively, you could take steps to reduce energy consumption over the 24 hour period (LED lighting, better insulated refrigerators, turning off un-needed equipment, etc). As others have pointed out, you don't need to run the genset to the point where the battery bank is 100% charged. It is perfectly ok to operate the bank between 60% & 80% almost indefinately when cruising and top it back up to 100% only when shorepower is available again.

Minimizing generator run-time to stay within the 60-80% charged range is a different question again. Getting the most out of your engine alternators is a good start, involving a 3-stage external regulator for the alternator charging the house bank; combining the output of both alternators and directing the total charge to the house bank in twin engine vessels is a good 2nd step; upgrading one or both alts is another. Of course, this assumes the battery charger being fed by your genset is suitably sized to make full use of your genset's output without exceeding 20% of the battery bank's capacity (flooded batteries).
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Old 09-23-2012, 08:13 PM   #11
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Two significant advantages of AGMs are the charge acceptance rates and the ability to take deeper discharges over a high number of cycles. Only discharging to 60% is pretty conservative for AGMs, and your batteries will probably accept the capacity of your charger.
It sounds like you are consuming between 100 and 150 AH per day if you are discharging to 12.2 (60%) and recharging to either 80% or 90%. If you discharge to 40% that is another 100 AH, or another day on batteries, and all you are doing is utilizing the capability of the AGMs that you paid for.
Since the batteries will probably accept the charger's full output your generator run-time will go up accordingly.
Another alternative for the ice-maker is a small Engel freezer for ice, probably quite a bit lower power consumption.
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Old 09-23-2012, 08:45 PM   #12
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I agree wth all av8r says. I'd add that AGM's have 2 significant disadvantages: they are much more expensive up-front and they are highly sensitive to charge voltage....they are easily killed by a too-high charge voltage, so a quality regulator is essential. Also, recent testing suggests that AGM manufacturers' claims of being able to re-charge at the full Ah capacity of the battery are somewhat overblown. Charge rates at more than 80% of capacity are apparently likely to shorten the life of the even quality AGMs and cheaper AGM's may not even take this much. Still better than flooded, but not quite as good as typically claimed.
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Old 09-23-2012, 08:56 PM   #13
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Apart from the obvious charging time differential, is there any pros or cons in using a smaller 40amp charger as against a 100amp model.

Also are there life expectancy issues with golf cart type batteries as against the standard 8D's? Our local marine electricians don't seem to keen on them.
Andy,I suspect IGs of our vintage are unusual,with no dedicated house batteries.For that reason I use Century dual purpose (ie start & house capable) 200amp/hr wet cell batteries, cost is reasonable and so far they start the engines and run the house without problem. They are never undercharged when I set off, each has 90 watts of solar via a multi stage regulator.
An electrician told me a charger should put out at least 10% of the capacity of the battery it is charging. If that`s right, 40 amps may be enough. Before I renewed batteries, if I turned my ancient original charger to a high level it sometimes tripped out, but that might be from trying to fast charge tired old batteries which I`ve not needed to do with the present batteries.Even so, it may be a charger can put out more than a battery can accept. BruceK
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Old 09-23-2012, 09:10 PM   #14
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Commenting on Bruce's Reply...while 'dual purpose' batteries will do the job, in the medium-term they will cost more than batteries that are purpose-specific: deep cycle for House purposes, thin-plate flooded or spiral-wound AGMs for Start. And yes, 10% of capacity is generally accepted as the minimum re-charge capacity (and is in fact about the optimal rate for flooded), with an upper limit of 20% (some would say 25%) for flooded. If you want to re-charge at 20-25% of bank capacity, a charger with battery temperature sensing is a good idea.
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Old 09-23-2012, 09:35 PM   #15
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AGMs accept the same charging voltages as flooded-cell and are not particularly sensitive. Gel-cells supposedly are quite sensitive to charging voltages.
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Old 09-24-2012, 05:13 AM   #16
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Bruce your right, the antiquated battery set up our IG's have, is at best a juggling game. That is the reason I am persevering with getting my troublesome Eutectic refrigeration system overhauled.

There is simply no room in the engine bay to install more batteries that I think I would need if I switched to 12volt fridge and freezer set up, not to mention the cost of of it all.

The present system with a good 7KW gen back up works pretty well for me.The 40amp charger seems to be about right, from what everyone is saying.

Feeling a bit guilty as I realise sitting here in the boat I am running 7x 20watt lights in the saloon alone. Just did a count I have 20 x 15/20 watt internal lights in the boat not counting the engine room.This was all put in by the PO. Wondering, is this a little excessive or are we all running similar set ups?
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Old 09-24-2012, 05:53 AM   #17
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It is always cheaper to save (not use) power than to continue to make it , store it then convert and use it.

Better refrigeration is costly , but with large batches of lights that may be on long hours , different bulbs would surely pay for themselves.

Nothing is as cost effective as a SOC meter , to let you know what is happening.

For first time boaters Sams Club or Cosco golf cart deep cycles are preferred as their warentee is great , till the computer figures out how many sets have been returned ,ruined.

Starts are the worst , but any batt even a deep cycle is ruined if discharged to ZERO!
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Old 09-24-2012, 02:11 PM   #18
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Bruce your right, the antiquated battery set up our IG's have, is at best a juggling game. That is the reason I am persevering with getting my troublesome Eutectic refrigeration system overhauled.

There is simply no room in the engine bay to install more batteries that I think I would need if I switched to 12volt fridge and freezer set up, not to mention the cost of of it all.

The present system with a good 7KW gen back up works pretty well for me.The 40amp charger seems to be about right, from what everyone is saying.

Feeling a bit guilty as I realise sitting here in the boat I am running 7x 20watt lights in the saloon alone. Just did a count I have 20 x 15/20 watt internal lights in the boat not counting the engine room.This was all put in by the PO. Wondering, is this a little excessive or are we all running similar set ups?

Don't compromise your comfort by pulling lights out....just swap out the incandescent bulbs for LEDs and save 70% of current lighting power draw right there (even more if for lights you use a lot, like in the saloon, you replace not just the bulbs but the whole fitting with engineered LED light fixtures)
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Old 09-24-2012, 08:43 PM   #19
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Don't compromise your comfort by pulling lights out....just swap out the incandescent bulbs for LEDs and save 70% of current lighting power draw right there (even more if for lights you use a lot, like in the saloon, you replace not just the bulbs but the whole fitting with engineered LED light fixtures)
A while back I bought on Ebay, from a Hong Kong trader, for about $120 posted, 10 Edison screw "wide spot" LED "globes" drawing about 3 watts, to replace the 25 watt 12v incandescents in the saloon/cabin light fittings. While not as bright, the recesses largely diffuse the light, and all 5 draw less than one of the 5 they replace.There is an article in September`s Afloat on changing fittings,but I`m ok with LED bulbs, which I`ve also fitted selectively elsewhere.
Andy,if your Nav. Light main switch turns on the instrument lights like mine did,put in a switch so they are not pointlessly on all night, up and down, with the anchor light.
Despite the odd IG "no house battery" set up,it seems to work,thus my "dual purpose"compromise batteries. Genset is back up if you overdraw. I think persisting with the eutectics is right, it gets the fridge off the batteries. The original built in 12v under settee fridge is good for day use; my batteries are solar fed, which helps run it and maintain batteries.A 35L Waeco I occasionally use,and carry on multi day trips just in case,would draw a lot less. BruceK
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Old 09-24-2012, 10:04 PM   #20
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Andy,

I will second what BruceK said. If your bulbs are the g4 type they can be bought very cheaply on eBay. I paid about $3.00 each for them three years ago. Do a little research first to be sure the led's you buy have the same output in lumens as what you have now. You will also want to look at the color temperature. I think warm is between 2700 and 4000k. I'm sure someone else can confirm this.

We saw a significant drop in our daily amp usage after making the switch.

Rob
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