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post #11 of 17 Old 10-27-2007
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Originally Posted by dohenyboy View Post
Okay here is illustrated a technical fact where, I believe, cruisers err.
If you have say 6 golf cart batteries (6v) with 100 amp hour ratings each and have them paired to produce 12v, at 12v usage the total amp hour capacity is only 300 amps. If you have a 6 volt system yes you would have 300 amp hours. Amps time volts equal wattage. Am I not correct?
If you have six 6VDC golf cart batteries with a capacity of 100 amp-hours each, and pair them to get the equivalent of three 12 VDC batteries, with a total bank capacity of 300 Amp-hours... however, if your system is 6VDC, you would get 600 Amp-hours out of the same batteries... If you had both 12 VDC and 24 VDC systems aboard, you could get one 12 VDC bank (two batteries) with a capacity of 100 Amp-hours, and one 24VDC bank (the remaining four batteries in series) with a capacity of 100 Amp-hours as well.

Don't confuse amp-hours with wattage.

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Also. AGMs can support 30 to 40 percent charge rate, so if you want to minimize charging time you will go with the biggest charger you can with AGMs.
Yes, AGMs can support more than 30-40 percent charge rates, but getting too large a charger is also not very cost-effective, unless you're battery management skills are pretty weak. All batteries, whether they're AGM or wet-cell, will accept a fair amount of current up until they reach about the 80% charge state, and then the charge acceptance level will drop like a rock...and you'll be float charging them until they are topped off. The current acceptance rate during the float charge portion of the charging cycle is relatively low.

Let's go with the original poster's 600 Amp-hour bank. Say you get a 90 Amp charger for it... You can use about 300 Amp-hours before needing to recharge it. That means you will need to replace 180 amp hours to get to the 80% charge state. This will probably take about 2–3 hours with this size charger—a larger charger might reduce this slightly. However, getting from 80-100% will take much longer—regardless of what size charger you have.

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post #12 of 17 Old 10-27-2007
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"The 10% rule is helpful, for then I can size the charger appropriately."
Bear in mind, different batteries can accept a charge at 20-25% of their rating, so a 750AH battery bank can make use of a 180A battery charger, if you ever want to "get them charged and get out of here".
There is also the matter of how smart a battery charger is, the "3 stage" is still probably considered the norm for "good enough" and some makers offer a 4-stage while others claim a 5-stage (which really appears to be a 4-stage plus the "off" stage[g]). And the charging logic they use (like MPPT controllers for solar arrays) can also affect charging time and efficiency.
Odds are you'll find a brand name with a good warranty, an affordable price, and stop there before Too Much Information sets in.[g]
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post #13 of 17 Old 10-27-2007 Thread Starter
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The 6 batteries installed are wet cell Exide GC-5's. According to the Exide website, the reserve capacity minutes @ 25 amps are 480, and minutes @ 75 amps to 5.25v are 135. How does this work out to get real capacity when the batteries are connected in pairs to make 12v? Appreciate all the information and advice.
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post #14 of 17 Old 10-27-2007
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kavakava-

The Exide GC-5's are about equivalent to the Trojan T105's I have on my boat. A pair wired in series is about 200 amp-hours @ 12VDC. According to their specifications, they are rated at 186–226 amp-hours at the 20 hour discharge rate... 200 amp-hours is a good number to use for your calculations.

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"The 6 batteries installed are wet cell Exide GC-5's. According to the Exide website, the reserve capacity minutes @ 25 amps are 480, and minutes @ 75 amps to 5.25v are 135. How does this work out to get real capacity when the batteries are connected in pairs to make 12v?"

"Reserve capacity" is a term used for automotive batteries in automotive use. Forget about it for deep cycle boat battery banks. All we care about are amp-hours at discharge rates, i.e. 100 AH at a standard 20-hour discharge rate, or 80AH at a 5-hour discharge rate--which can be the same battery measured at two different rates.

I didn't find the Exide specs (you can call them Monday) but found a similar note:
NAPA 8146 (mfg. by Exide - equivilalent to Exide GC-5)
480 Min @25Amps, 135 Min @75Amps, 226Ah @20 Amps, 65 Lbs

In other words, IF they are the same battery, the capacity is 226Amphours at the satndard 20-hour discharge rate. About 11 amps per hour, for twenty hours.
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post #16 of 17 Old 10-27-2007
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Dohenboy...actually good AGM's can be charged at 100% of their rated amp hour capacity if you can find a charger that big! In fact LIFELINE AGM's have no restriction on charging current.
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The one problem with what cam suggests is the cables connecting the charger to the battery banks have to be large enough to handle the maximum current load... and getting a charger that big requires using really large cables...

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—Cpt. Mal Reynolds, Serenity (edited)

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