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Old 17-10-2008, 08:51   #1
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Let's revisit this battery question...

OK, i have seen where this was touched upon several times, but I am not sure the opinions are as valid today as when these issues seem to have been addressed.

I see more and more auto battery chargers -- at Walmart, for instance- that are three-stage. Why can't these be used to charge marine deep cycle batteries?

The Schumacher brand, one model is sold at Walmart as "marine", seems to fit the bill at 15 Amp, then down to a trickle charge once near the batteries top end.

My questionis, I am replacing my current house bank with 3-4 8D batteries. Can I use one of these chargers? What's the down side? they say they are multi-stage, etc.

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Old 17-10-2008, 09:43   #2
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They might not last quite as long in a salt-water environment. For the price difference, though, maybe worth it.

The other issue is capacity. The ones sold at Wal-Mart are rarely intended for charging anything more than a 100-150 ah battery bank. If you're trying to charge a 1000 ah battery bank with that it's going to take a LOOOOONG time!
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Old 17-10-2008, 10:15   #3
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Yes, I can see your point about how long they might take! I am looking at having about 800 ah on board. What's a good estimate on a reasonable charger? Say, 30 amps total???
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Old 17-10-2008, 10:51   #4
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Also something about an auto-motive battery charger not being grounded the correct way so it could cause damage.
(Forgot the details, but remember it being a big no-no)

As for capacity, I think 10% is recommended.

(400 amp/hour bank. 40 amp charger)
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Old 17-10-2008, 11:23   #5
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I heard the point about grounding before, but I don't get it. I mean, if the shore power cord is grounded to the deck connection point, why would it not be?? Otherwise, would anything be grounded right?

The percentage seems to make sense to me. That would allow a full recharge in about 24 hours, assuming 50% discharge on the battery and the different charge rates in the three stages...

Or am I missing something again???
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Old 17-10-2008, 11:59   #6
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"Or am I missing something again???"

'Fraid you are.

First, a 40A battery charger is much too small for a house battery bank consisting of 3 or 4 8D's. If you have a battery bank that size (say, 750-1,000AH nominal), then one would assume that you have sized your bank properly, and that your daily usage is on the order of 250 to 350AH. That's a lot, BTW.

To recharge a bank of this size with a 40A charger would take on the order of 7-12 hours daily. Do you really want to run your generator that long???

If you're not intending to cruise, but just sit at the dock, then 40A is fine. But if you're going to be using the boat and need to recharge the batteries using a battery charger powered by, e.g., an onboard generator, then 40A is far too small.

The recommended size, BTW, is NLT 20% of total battery capacity. For a 750A bank, this would be a 150A charger.

If you're looking for a really good marine charger which will do the job very well and which won't break your budget, look at the Iota series. They come in a variety of sizes up to 90A. And, two identical ones can be combined to double capacity if needed, e.g., two 75A chargers can provide up to 150A charging. BTW, the Iota chargers are hi-tech, very reliable, and provide more amps per hour than other chargers their size. Highly recommended and, for what they do, very inexpensive.

Forget the Walmart, Sears, etc. chargers....a very poor choice for a marine application, IMHO.

Bill
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Old 17-10-2008, 12:13   #7
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I will be honest and say that the reason for 4 8D batteries is that I was able to get them at a good price, and I am thinking that rarely do people regret having too much capacity. I don't want to re-configure the engine room area under the center-cockpit of my O'Day 32. I know it's a lot of capacity, but with technology always increasing and the battery draw always rising, can "too much" be bad? lol
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Old 17-10-2008, 15:30   #8
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multi steep chargers

One point often overlooked is some chargers default after say 6hrs to the trickle charge rate so if the wrong match is selected you have lost all the advantage of a multi stage charger!!

Regards Bill Goodard
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Old 21-10-2008, 14:45   #9
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I can see the points much more clearly now!!
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Old 21-10-2008, 15:03   #10
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4 8D's seems like overkill. That's a lot of weight, where do you put it all? Even if it's a great price 2 or 3 8D's might do you.
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Old 21-10-2008, 18:11   #11
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Well, I was SOOO tempted by the fact that I have a center cockpit and thus an "engine room" below, and I guess I was just going nuts over how I would use all that space. lol
Seriously, they came as a set of four. I originally thought I would sell one or two. Then I started wondering if maybe I should keep three, sell one. But, since they are all the same model and and age, etc., I thought, well, when would I luck into 4 like these again? And who has ever complained of too much juice??

I don't know, I just have this feeling I am damned if I do and damned if I don't!
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Old 21-10-2008, 18:52   #12
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If you're on the hook or cruising, I think you made a terrific deal. If your boat is at the dock, I have to ask why you need this capacity. Here's the chart that Gord posted showing maximum life of batteries, versus % of capacity discharge.

10% DoD (100 A-H x 0.10) x 6,200 cycles = 62,000 Amp-Hours (lifetime)
20% x 5,200 = 104,000
30% x 4,400 = 132,000
40% x 3,700 = 148,000
50% x 2,900 = 145,000
60% x 2,400 = 144,000
70% x 2,000 = 140,000
80% x 1,700 = 136,000

4 8D's are roughly a 1000 amp hours. 40% discharge is 400 amps. At a 5 hour recharge cycle you'll need an 80 amp - 100 amp battery charger. That's monsterous.

Did you install 12 volt air conditioning in the boat? Also, does your slip have an AC breaker? Will it handle this size charger?
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Old 21-10-2008, 19:08   #13
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With a 30 amp shorepower outlet, you should have no problems with a 100A to 150A 12 volt charger.

30 amps times 120 volts is roughly around 3600 watts. Figure for some losses so 3200 watts would be a conservative estimate. A 150 amp 12 volt battery charger would need to produce 14 volts max times 150 which would be 2100 watts...about. So you have a difference, 3600-2100 that equals 1500 watts. A 1500 watt difference is more than enough to cover inefficiencies. I would not though consider a battery charger of that size if your at a marina with a 15 amp outlet. A 20 amp outlet is iffy.

With a lot of inverter/chargers you can control the maximum charge. I have three 8-D's onboard and have my charger set at a maximum charge of 100 amps.

That is a lot of amp-hours for a boat of your size but then if it works for you then great....you could run the margarita blender all night if you want! Be sure to tell your wife its fine to run the blow dryer as well.
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Old 21-10-2008, 21:44   #14
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A couple of years ago, before I got there. One of the boat owners bought an automotive charger, clipped it on and went to lunch. One of the clamps popped off, sparked and burnt the whole marina down. I would hard wire it in. I would never us a clip on type unless I was there to watch it.
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Old 22-10-2008, 03:05   #15
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Quote:
Originally Posted by Tropic Cat View Post
... Here's the chart that Gord posted showing maximum life of batteries, versus % of capacity discharge...
Caution: Iíve never independently confirmed that Trojan (nor anyone) endorses the chart, and itís implication that the most efficient usage discharges a flooded battery to 40% DoD.
Remember, the "conventional" wisdom recommends that we discharge to only about 60% (or 50% at most) DoD.

DoD = Depth of Discharge (energy used), not itís inverse, State of Charge (SoC) (energy remaining).
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