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Old 17-01-2014, 10:47   #31
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Re: Volts VS. Amp-Hrs

I been using the volt method for 40+ years and I still do. How do you think we did it before all this fancy monitoring stuff? Even with a small load, under 3 amps, you can get a basic idea. Once you know what amp draw/usage is then you can size the battery bank accordingly. We have three 8-D deep cycle house batteries, 660 amps. I donít go below 50% or when the needle drops below 12.4 volts, 330 amps. If you are trying to be to accurate/finite then you probable need a bigger battery bank. Some things where easier/better when analog rather than digital.

However, the biggest problem/concern is how to charge the batteries back up to at least 90%? To get closer take to long. High amps alternator, gen set, solar, wind? We use the multi/gang charging using one charger for each battery, but we have 3 gen sets, big main 10 KW, cruise gen 5 KW a portable 3 kw and we also reduced the amps used. I would be more concern how to chage the volts/amps back in the shortest time?
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Old 17-01-2014, 12:02   #32
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Re: Volts VS. Amp-Hrs

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Originally Posted by rw58ph View Post
I been using the volt method for 40+ years and I still do. How do you think we did it before all this fancy monitoring stuff?
Just like how we navigated before GPS, the old fashioned way. But like celestial vs GPS, using volts vs a battery monitor the new stuff will give you much more accurate information with a lot less trouble. To get even close using voltage while under load takes a good bit of experience with your system and batteries. Or you can shut down your system for a couple of hours to get a resting voltage to get a more accurate idea but often not very convenient. Or just check the monitor and know pretty darn close to where you stand.
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Old 17-01-2014, 12:27   #33
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Re: Volts VS. Amp-Hrs

"Question #1 was... if you have 640 amp hours available, and you go from 12.7 volts to 12.1 volts (for example), how many amp hours are you using?"

There is no direct correspondence between amp hours consumed and the instantaneous voltage. My 90 amp microwave load pulls 90 amps (an instantaneous reading) /60 minutes per hour*10 minutes or 15 amp hours from the battery bank ( 90/60*10 = 15) and the battery V sags to about 12.1 or 12.2 for a while. If I take 7 amps out for 2.5 hours I use the same amp hours but the battery V will stay above 12.5. You can not do much with non-resting battery voltage!

"We can't recapture the last 50 amp hours without hooking to shore power or running the engine for many many hours. We have no alternative energy."

That is the dilemma for living on the hook and is exactly the reason we have solar panels. It is very expensive to use the diesel to charge that last 5% of the battery capacity. And, that is a serious problem for you if you do not get your lead acid batteries up to 14.4 V or greater, adjusted for ambient temperature, because you are risking sulfation of your battery plates.

Quoting The Marine Electrical and Electronics Bible by John C. Payne.

"Sulfation is the single greatest cause of battery failure"

"If a battery is only 80% charged, this does not mean only 20% is sulfating: the entire plate material has not full converted and the entire battery and is sulfating"

"As long as some charging is taking place, even from a small solar panel, a chemical reaction is taking place and sulfation will not occur."

Quoting Nigel Calder Boatowners Mechanical and Electrical Manual:
"sulfation is a particularly acute problem with deep cycle batteries that are deeply discharged repeatedly and never brought back up to 100 percent charge"

"Sulfation needs to be dealt with before it hardens. Never leave batteries in a discharged state. The sulfate will harden and ruin the battery." Italics in the original text

Calder also points out that a battery which quickly shows a large voltage drop under moderate load, to below 12.2 in his example, when fully charged may well be in a state of sulfation.

When you report your battery is discharged to 150 amp hours after a couple days - that says to me you are rapidly sliding down the sulfation path.
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Old 17-01-2014, 13:25   #34
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Re: Volts VS. Amp-Hrs

I would disagree that the most accurate method of assessing charge is specific gravity. Even the battery makers will tell you that acid electrolyte builds local chemical differences, and it may take 24 hours to circulate and even out in a battery. So by all means test the sg, but oh, wait, you need to let the battery stand unused for 24 hours before that is accurate. Or, you need to swish around the around and try to blend it, much like shaking up an oil-and-vinegar salad dressing.

You've got a similar problem with voltage measurement, in that uneven electrolyte will still distort the reading. And an additional problem in that you don't know what is normal for a specific battery, since different brands and makes may be 12.6 or 12.7 or a different number when they are "full". You can find that out from the maker though.

But then there's a third problem, calibration. Unless you are using a calibrated meter, or you calibrate your own, if the meter is off 1 or 2 tenths of a volt, there goes the ball game. And that's not uncommon unless you own a good meter and keep it calibrated.

So voltage isn't perfect, but I'd rather reality-check a voltmeter than mess around with acid burns. I own (built) a calibration source now, so calibrating my meters just means buy a $2 battery every three or four years now, and remember where I left the thing. Way cheaper than buying new clothes after the acid burns. Or carefully packing away the bug hydrometer, so it doesn't break.

With the number of sealed (AGM, gel, Lithium) batteries on the market, even the battery makers are saying "Go electronic". There's a different learning curve, but acid is no longer king.
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Old 17-01-2014, 14:01   #35
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Re: Volts VS. Amp-Hrs

Well, our next step (in just a few weeks) is to get solar panels installed and replace our old wind generator, which is completely shot. It's a four winds II which is old old old in terms of wind generators. You guys complain about the sound of wind generators but you have never been next to one like mine It's like a helicopter taking off.
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Old 17-01-2014, 14:25   #36
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Re: Volts VS. Amp-Hrs

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Fair enough, wasn't intending it to sound like I'd debate it. I just typed what it said. It could well have been meant as generic info, for all I know, or an approximation of all 12v batteries writ large versus specific deep cycle units or whatever.

Good info from Trojan, and it obviously makes sense to use their knowledge if they made your batteries.

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Old 17-01-2014, 18:39   #37
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Re: Volts VS. Amp-Hrs

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Fair enough, wasn't intending it to sound like I'd debate it. I just typed what it said. It could well have been meant as generic info, for all I know, or an approximation of all 12v batteries writ large versus specific deep cycle units or whatever.

Good info from Trojan, and it obviously makes sense to use their knowledge if they made your batteries.

-Chris
Its so frickin' confusing! Tonight, my shunt is telling me I have an 80% SOC but the voltage is 12.10, and the amp hours used are way fewer than I should be getting before reaching 12.10 volts.

Tomorrow, I am going to hook a voltmeter up to the batteries and see if maybe my shunt is incorrectly reading the voltage. The ONLY thing that is hooked directly to the batts is my bilge pump and I only run that for maybe 5-7 seconds a day.
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Old 17-01-2014, 18:40   #38
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Re: Volts VS. Amp-Hrs

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Its so frickin' confusing! Tonight, my shunt is telling me I have an 80% SOC but the voltage is 12.10, and the amp hours used are way fewer than I should be getting before reaching 12.10 volts.

Tomorrow, I am going to hook a voltmeter up to the batteries and see if maybe my shunt is incorrectly reading the voltage. The ONLY thing that is hooked directly to the batts is my bilge pump and I only run that for maybe 5-7 seconds a day.
Your shunt does not read voltage , the battery monitor is.

Dave
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Old 17-01-2014, 19:02   #39
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Re: Volts VS. Amp-Hrs

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Your shunt does not read voltage , the battery monitor is.

Dave
Our shunt is a digital shunt not a piece of metal... it does read voltage. It then sends that data over canbus to our Masterview panel.

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Old 17-01-2014, 19:34   #40
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Re: Volts VS. Amp-Hrs

zboss,

I think your shunt is, indeed, of the usual type.....metal....but incorporates additional circuitry to calculate certain parameters, then converts these results to signals sent to the monitor.

A few thoughts which might help:

1. If you have 520AH capacity, then the 20-hour discharge rate is 26 amps. Any average draw more than 26 amps would reduce the effective capacity below 520AH, while any draw below 26 amps would increase the effective capacity above 520AH.

From the remarks you provided, it's clear that your average load is way below 26 amps (since 26 amps x 24 hours would equal 624AH per day). Thus, your effective battery capacity is more than 520AH...maybe as much as 600AH or more.

2. The only effective way to test the real capacity of your batteries is to do a controlled 20-hour test. A sophisticated internal inductance/resistance tester like the Midtronics series ($700 or more) is a quick way and is pretty accurate, but few cruisers have these. Maybe you can find someone nearby who has one and can test the capacity of each of your batteries.

Sulfation can occur if batteries are left below full charge or even if they are left on a float charge 24/7. They need to be bumped up to 14.8VDC or more periodically. And, as MaineSail said, they won't reach full capacity when new until they've been cycled a few dozen times.

I don't agree at all with the fashionable notion these days that voltage is not useful. If you know what you're doing and you know your boat, voltage is a very good way to estimate state of charge, and to guage how well your batteries are doing. I do research on batteries, I install marine power systems professionally, and have over 30 years experience with marine batteries on my own boats. I don't have a battery monitor on my boat, and I don't want one....though I've installed a number of them for clients who've drunk the Cool-Aid and think they're the cat's meow.

What I DO have is a good digital voltmeter, mounted where I can see it from anywhere in the cabin. At a single glance I can get a good estimate of the SOC of my house batteries (six golf-carts), because I know in my head what I should be seeing before I look. I know what to expect in the morning upon waking, when motoring or charging from the diesel genset, when retiring at night, etc. I can see how long it takes to get to 14.8VDC charging, and how long the DC ammeter remains at 80A when running the engine or generator. This is all the info I need.

Don't be seduced by what your monitor is saying. As the retired process engineer said above, use independent tools to measure what's really going on. A good multimeter and a good clamp-on AC/DC ammeter is all that's necessary. You do need to inventory your boat, though, and calculate closely what each piece of equipment draws. Develop your own energy budget on a spreadsheet.

Most promising thing I've seen in all the above discourse is your statement of intent to fit solar panels to your boat. GREAT. That's the only real solution to keeping your batteries fully charged.

Get panels with at least 270-watt total capacity and don't skimp on a good MPPT controller. Forget the wind genny....too much trouble, too much noise, and the solar panels alone, properly installed, will likely do the trick.

Good luck,

Bill
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Old 17-01-2014, 19:39   #41
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Re: Volts VS. Amp-Hrs

Shunts measure voltage drop over a specified distance and the meter converts that to current through the shunt.

I think after all these posts it has finally occurred to me what the problem is. Your battery monitor will accumulate errors over multiple discharge/charge cycles, as they all do. This error will automatically be cancelled when the batteries reach a full charge, typically when the charge current is less than 2% of the bank size in AH. Away from shore power most do achieve a full charge from solar or a wind gen, or occasional long (8 to 10 hour) runs under engine power. The alternative is to reset the meter manually when you know the batteries are fully charged. The situation you are in with engine charging only will not bring the batteries to the point that the monitor considers the bank full. Because of this the errors are adding up from the last time the AC powered shore charger was in use.
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Old 17-01-2014, 19:42   #42
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Re: Volts VS. Amp-Hrs

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Originally Posted by zboss View Post
Our shunt is a digital shunt not a piece of metal... it does read voltage. It then sends that data over canbus to our Masterview panel.

MasterShunt 500 | Mastervolt Marine
Still a standard shunt inside with a convertor to canbus

Dave
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Old 17-01-2014, 20:14   #43
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Re: Volts VS. Amp-Hrs

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""As long as some charging is taking place, even from a small solar panel, a chemical reaction is taking place and sulfation will not occur."



This is very important and far too often overlooked. I'm constantly telling folks to pickup a little cheapy $25 solar charger even if they aren't ready to install a real solar system just to fight against sulfation.
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Old 17-01-2014, 20:50   #44
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Re: Volts VS. Amp-Hrs

This is not exactly true for all lead acid batteries.

Batteries begin to hard sulphate at 12.4 volts.

They don't begin to soften the sulphate until they reach 14.4 volts.

So adding some current will not ultimately stop a bat from sulphating.

It all depends on the time.

Left at less then 12.4 resting voltage the hard sulfate will be detrimental.

Enough current must pass during charge to keep the bats, above 14.4 volts to soften the the lead acid reaction, until the plate equalize. else they harden.

Bats by design are to have a charge, the discharge, then re-charge.... all before sulpahting has a leg up.

Lloyd





Quote:
Originally Posted by TacomaSailor
""As long as some charging is taking place, even from a small solar panel, a chemical reaction is taking place and sulfation will not occur."



Quote:
Originally Posted by Aussiesuede View Post


This is very important and far too often overlooked. I'm constantly telling folks to pickup a little cheapy $25 solar charger even if they aren't ready to install a real solar system just to fight against sulfation.
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Old 18-01-2014, 09:00   #45
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Re: Volts VS. Amp-Hrs

What I did today was make sure that my shunt was actually reading the voltages correctly. During charge cycle the other day I noticed a big difference between the balmar setting (14.7) and my actual charging showing on the shunt during charging (14.3 to 14.43 during absorption).

So I hooked a voltmeter up to the batts during charge and the shunt matched the voltmeter exactly. Then, I compared the shunt to the voltage coming off the alternator and found the alternator was reading 0.10 volts higher than the shunt. There are about 4 feet between the alternator and the batts. I believe the wire is 4 AWG. We have a Balmar AT 175 amp with a serpentine pulley system.
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