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  #1  
Old 10-16-2006
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Question AGM Batteries

OK gang, I have 3 AGM batteries and need to replace them next spring. The simple question since I really don't NEED 600 amp hrs is: should I just replace the 3 4D's with 2 (cutting my hours to 400) or keep the 3? The real question is probably: will the lifespan of 3 AGM's outlast that of 2 AGM's if I usually just burn them down to about -150 hrs before recharging them?

Thanks for the suggestions
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Energy Budget

What is your energy budget and how do you charge your batteries. AGMs suffer if you undercharge them or overcharge them. If you charge them with solar or wind you may have to have more capacity to prevent deep discharge, also a way to shorten there life. We need more info to give you any kind of coherent answer.
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Should have been a little more specific in my question! Well, here goes: I'm on a 46' sailboat, 3 4D AGM's now, FisherPanda generator, no wind or solar charging, a Heart charger (allows reading amp hr used), and am based on a dock (so have full electricity when at home) to fully recharge whenever home.
My usual situation consists of 3-4 day trips with various loads (sometimes autopilot, refrig, lighting, coffee pot in the AM, etc) which usually burns around 125-150 amp hrs in a 24 hr period. I can recharge while on anchor with the FisherPanda if needed. I usually let the currect 3 batteries (600 amp hrs) burn down to a point of about -200 amp hrs before starting to recharge them (about 1/3 of the 600 hrs available). I could just as easily recharge them at a point of -150-175 if I was to change to the 2 battery setup. So the question is am I better off with -200 on a 3 battery system or -175 on a 2 battery system. When I recharge them on anchor, I can get them to "0" amp hrs used and when I get back to the dock, the boat is connected to shorepower for several days, so things go back to "full" and stay there for a while.
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Old 10-16-2006
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Plunderer... you should never let your batteries get down 12.25 volts (measured at rest after an hour with nothing on) or you will kill them much faster than normal. Since a 4D delivers about 155 A/hrs I am wondering how you are coming up with 600 for 3 of them...more like 450. So...the short answer to your question is you have 450 a/hrs now and you should never go below -225 but since you can charge daily you can reduce your batteries to 2 and that will leave you with 150 amp/hoursusable per day but you will need to charge daily and have NO extra capacity. I assume you have a separate starting battery to these or my answer would be to keep 3.
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According to the latest West Marine catalogue, their 4D has a rating of 200 amp hrs and according to Lifeline's paperwork, theirs is rated at 210 amp hrs.
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Old 10-16-2006
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Though you could get by on two, I would keep three. Better to have the extra in case the genset goes down, or you are out longer than antcipated. I assume you mean you draw them down the amounts you state, not draw them down to that much capacity left. Batteries should generally stay within 50-85% of available capacity to last. The last fifteen percent would require extended engine or genset running to get them up to 100%.
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Plunderer..I had a whole thing written with internet links and then thesailnet site went down! So...rather than go through all that again, let me just say that I believe battery mfrs. exaggerate a bit in their amp hour claims...not that some are not a bit better...but most independent sources say avg. 4D is 150-160Amps. Check your Heart Inverter site for this or the manual.
In any event...let's assume you DO have 200x3 which gives you 600 total or 300 "working" amps...that's enough for 2 days o the hook without the generator.
If you go to 2 batteries and a 200A/H working total...that will give you 1 day + 50 amps. Which means you have to run the genset every day AND which means the batteries will be closer to 80% full QUICKER as you are running the genset which will make you run it LONGER to get those amps back in as the regulator will clamp down on the amperage. In other words, if you are out cruising for 4 days you will run your genset every day longer than if you had a bigger bank and waited to run it every two days. My guess is the diesel fuel will more than pay for the extra battery over time!
Even though you can fully charge at the dock...you will use 600 A/H's as you anchor for 4 days so you must run the genny daily with 2 batteries or you'll fall below the magical 50% full "mark". I'd stick with the 3 even if you do have 200A/H 4D's!
Hope this helps.
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Old 10-17-2006
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AGMs

The shallower you discharge your AGMs the longer (more cycles) they will last. The better you charge your AGMs the longer they will last. The cooler you keep your AGMs the longer they will last. Now charging AGMs takes 24 to 36 hours to get a full charge so when your on the hook I doubt that you fully charge them, which will also lower there life.

The best you can do for your batteries is to discharge them to 15 to 25% and charge them immediately after discharge. They should last up to 4 times longer then if you discharge them to 50%. However, since you don’t fully charge them each time unless your connected to shore power they will loose some life.

You can operate on the configuration you described, just be prepared to replace your batteries at least twice as often.
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Old 10-17-2006
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Plunderer, each manufacturer has different claims about the number of charge cycles you can get versus the depth of discharge. To some extent, you pick the ones you might want to buy from (or trust) and then compare what they are claiming in print.

If they say their AGM 4D has a capacity of 200AH, they may also give you curves or charts saying:
200AH @ full discharge @ 20 (50, etc) hour rate, 50 cycles total life
150AH @ 75% discharge...220 cycles total life
100AH @ 50% discharge...600 cycles total life
65AH @ 30% discharge...1200 cycles total life
20AH@ 10% discharge...3500 cycles total life

And you can easily do the math, multiply the number of cycles they claim, by the capacity at that cycle depth, to get the total number of amp-hours that you can put in and out of that battery--at that discharge depth. This is a curve, not a flat-line relationship.

If you buy 2 batteries instead of 3, and you can still manage to run them at 30% or 50% discharge cycling, you may save money. But if buying one less battery means that you'll be cycling the two new ones much further, you may actually be paying far more "per amp" since the batteries won't last as long.

This applies to pretty much all rechargeables, not just AGM. Industrial wet leaed cells will last longest--but of course that's wet lead, and industrial cells tend to cost more and be heavier because of construction differences. Same thing applies to industrial AGM batteries, which your local chandlery won't even carry.

Rule of thumb? Buy as much battery as you can afford (money, size, weight) and live with whatever other compromises that leaves you.
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Old 10-21-2006
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I'd stick with the three batteries...as excess capacity is rarely a problem. The larger the bank, the less discharge depth you'll generally need, and the more safety factor you have if you have a problem with the charging system on your boat. Both of these are good reasons to keep with a three battery setup. As others have pointed out...the fuel savings alone will probably pay for the extra battery in fairly short order. Might also be worth investing in some renewable energy systems for your boat. Solar or wind would be ideal for topping off the batteries while not using any fuel.
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