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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance > Electrical Systems
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Old 06-01-2009
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Another Battery question.

I'm sure my batteries are at the end of their life and I'm going to replace them but I have a couple of questions. Voltage and how it relates to stated Amp hrs.

I have a 12V house bank with 6 - 6 volt deep cycle batteries. Each Battery (Trojan T-105) is rated at 225 AH (20 hr Rate).
When I'm not at the dock I charge using a generator until the (Outback Inverter) meter reads 14.1 volts.
I shut off the gen and let it set and it drops to 12.3
I have a draw (again from the Outback panel) of one amp constant (Inverter is supplying a ice maker).
I also have a DC draw on my DC meter of < 5 amps.

After 6 hrs my voltage is down to 12.2 At 10 hrs it's down to 11.9

So, if I HAD 225 AH * 6 batteries I should have had 1350 AH (Assuming the 20 hr table) correct?

If I'm using 6 Amps (metered) and drew my batteries down to 50% would that mean that I should have been able to run the 6 amps for: 1350 AH total / 6 amps draw * 50% capacity level = 112 hrs?

If I replaced all the batteries with new, how would I calculate the total time it would take to bring the battery bank down to 12.1 volts assuming a 6 amp draw?
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Old 06-01-2009
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Six 6-volt T-105s in series/parallel configuration yield a total of 675AH @ 12VDC at the 20-hour rate. It takes two T-105s to make 12V, and you don't add the AH, i.e., two T-105s in series = 225AH @ 12vdc.

Figure only half of the total capacity as being available, so only 337AH are theoretically available from your battery bank before you draw it down to 50% charge level.

Note that 14.1VDC is insufficient to fully charge the T-105s. They need a minimum of 14.4v; more is better. Just gotta be sure you don't run them dry.

If you don't regularly exercise them and periodically equalize them (15.5 volts or more for a few hours), this process could help to rejeuvenate them if they're not totally gone. You didn't say how old they are. In EXCELLENT charging situations, you can expect only 4-5 years from these. Inferior treatment yields less service time.

RE: your last question, 12.1 volts is about 60% discharged. So, 60% x 675AH total capacity = 405AH. You'd have to draw 405AH to bring the (new) batteries down to 12.1VDC; that would be something like 67 hours time @ 6 volt draw. Note also that the 225AH rating of the T-105s is at the 20-hour rate. At lesser amperage draws (like your 6A), the rating would be considerably higher, and thus it would take more than 67 hours to reach 12.1vdc. Note also that these voltages are RESTING, i.e., NO LOAD and NO CHARGE for several hours.

Bill

Last edited by btrayfors; 06-01-2009 at 05:05 PM.
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Old 06-01-2009
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First, as pointed out, T105 batteries are added in pairs to make a 12 V bank, and as such the voltage doubles but the amp-hours don't... so you're bank is considerably smaller than you're thinking.

Second, the voltage of the batteries is a surface charge that is not representative of their true state of charge. Read Maine Sail's post on voltage as a way to check state of charge—it doesn't work.

Third, what you really need, and it sounds like you don't have, is a battery monitor. This is one device that will pay for itself reltatively quickly, since it will show you whether you're actually undercharging your batteries, over-discharging them and prolong their life by allowing you to maintain them properly.

Also, you're making a really basic mistake in that the amp-hour capacity of the batteries isn't linear. It changes based the amp draw. The larger the amperage draw in relation to bank size, the lower the amp-hour capacity of the bank will be—this is due to the Peukert factor for the lead-acid batteries, and is the reason the FIVE-HOUR rate is so much lower than the TWENTY-HOUR rate when you look at battery specifications.

BTW, is the outback panel showing the AC amperage or the DC amperage. ONE AMP at 120 VAC is basically TEN AMPS at 12 VDC. I can't think of a 120 VAC ice-maker that would work at only .1 amp (12 watts)—freezing ice takes a lot of energy.

Quote:
Originally Posted by carvendive View Post
I'm sure my batteries are at the end of their life and I'm going to replace them but I have a couple of questions. Voltage and how it relates to stated Amp hrs.

I have a 12V house bank with 6 - 6 volt deep cycle batteries. Each Battery (Trojan T-105) is rated at 225 AH (20 hr Rate).
When I'm not at the dock I charge using a generator until the (Outback Inverter) meter reads 14.1 volts.
I shut off the gen and let it set and it drops to 12.3
I have a draw (again from the Outback panel) of one amp constant (Inverter is supplying a ice maker).
I also have a DC draw on my DC meter of < 5 amps.

After 6 hrs my voltage is down to 12.2 At 10 hrs it's down to 11.9

So, if I HAD 225 AH * 6 batteries I should have had 1350 AH (Assuming the 20 hr table) correct?

If I'm using 6 Amps (metered) and drew my batteries down to 50% would that mean that I should have been able to run the 6 amps for: 1350 AH total / 6 amps draw * 50% capacity level = 112 hrs?

If I replaced all the batteries with new, how would I calculate the total time it would take to bring the battery bank down to 12.1 volts assuming a 6 amp draw?
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Old 06-02-2009
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Yes, I know it's linear - that's why I specified the 6amps continuous.

BUT, YES I DID FORGET the Outback display is showing amps at ~115 volts. so you are right, my actual usage is the 5 amps from the DC meter plus the 10 amps from the AC. So I'm drawing 15 amps total (big difference) - thanks for catching that.

Series parallel always throws me. You're right, 675AH @ 12VDC at the 20-hour rate which at 50% would bring me down to 337capacity / 15use = 22 .5 hrs theoretically.

This helps A LOT - THANKS.
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One more question…
What's the difference? Costco has 6V golf cart batteries with white cases, 220 AH @ 20 hrs. Their specifications, dimensions and weigh 66# are the same or close to both Interstate and Trojan. They cost much less but should be about as tough as marine. My trawler doesn't go fast enough to pound (unless a bayliner goes by) so there should be no exceptional need should there?
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varvendive a bunch of people use the costco 6 volt batteries with out problems. they might not last as long as trojans but i seriously doubt they will only have 50 % of the life, they might last as long or maybe 80 %. nobody really knows if they will last the same or not. i have walmart deep cycles that cost me 62 bucks for 115 amps hours and 12 volts and have been happy for the last 5 months with em.
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Old 06-03-2009
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Someone like btrayfors is going to know a lot more about this, but ...

I've had okay success with off the shelf deep cycle batteries from Walmart and other box stores, but I have found that for myself the main factors that decide how long they last are very simply - (1) If I have a solar panel hooked up to them, and (2) If the bank is much bigger than I actually need.

I can only assume that (1) keeps them at float much of the time so they don't go bad as fast, where as if I don't have them on a solar panel I tend to leave them deeply discharged between charging sessions, and (2) Same thing, if they don't get deeply discharged in the first place they don't have as much of a problem. I think THE BIGGEST thing is having at least one solar panel hooked up, because having just one panel changes the whole situation from having periods of full charge between discharges into having periods of discharge between full charges, and that makes all the difference. I think having a much bigger bank than you need helps the bank survive because you don't draw them down as much at night

Of course, insuring they have water in them is important to, I suppose, but I can't comment on that as a factor in their life expectancy because I always keep water in mine and I haven't had any go bad for that reason.
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glad to help.... there's a huge difference between 22.5 hrs and 112 hrs.
Quote:
Originally Posted by carvendive View Post
Yes, I know it's linear - that's why I specified the 6amps continuous.

BUT, YES I DID FORGET the Outback display is showing amps at ~115 volts. so you are right, my actual usage is the 5 amps from the DC meter plus the 10 amps from the AC. So I'm drawing 15 amps total (big difference) - thanks for catching that.

Series parallel always throws me. You're right, 675AH @ 12VDC at the 20-hour rate which at 50% would bring me down to 337capacity / 15use = 22 .5 hrs theoretically.

This helps A LOT - THANKS.
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a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Quote:
Originally Posted by sailingdog View Post
there's a huge difference between 22.5 hrs and 112 hrs.
Unless you are listening to someone talk about Catalina's, after the first 5 minutes it is all pretty much the same ..
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