What are the amp hours, really.

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What are the amp hours, really.

Postby Shadow Catcher » Mon Jan 23, 2017 8:15 pm

I know from what I have read manufacturers stated amp hours are may not have a great correlation with reality, also batteries lose capacity over time and with use. We have a Victron battery monitor and I know now it is not accurate.
I am looking for a way to find what the current amp hours are. There are a number of complex methods and the level of complication is daunting.
Any one have a simple method.
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Re: What are the amp hours, really.

Postby H.A. » Mon Jan 23, 2017 9:42 pm

Shadow Catcher wrote:I know from what I have read manufacturers stated amp hours are may not have a great correlation with reality, also batteries lose capacity over time and with use. We have a Victron battery monitor and I know now it is not accurate.
I am looking for a way to find what the current amp hours are. There are a number of complex methods and the level of complication is daunting.
Any one have a simple method.


How do you know your Victron monitor is not accurate ?
Last edited by H.A. on Thu Jul 06, 2017 4:24 am, edited 1 time in total.
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Re: What are the amp hours, really.

Postby tony.latham » Mon Jan 23, 2017 10:19 pm

I really like the GT Power meters.

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$18. https://www.amazon.com/GT-Power-Analyze ... B00C1BZSYO

Here's a review:



You should wire them with a cutoff switch so you can zero out the meter for each trip.

Tony
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Re: What are the amp hours, really.

Postby bdosborn » Mon Jan 23, 2017 10:26 pm

Are you trying to keep track of the amp-hrs that have gone in and out of the battery while you're camping or find the actual amp-hr capacity of your battery year-to-year? Measuring the specific gravity of the electrolyte is the only other way I know to measure state of charge but it's truly a PITA to do:

Using a Hydrometer

I second the question on the Victron, it's at least as accurate as the Trimetric I use and I'm happy with the amp-hr function of the Trimetric.

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Re: What are the amp hours, really.

Postby edgeau » Mon Jan 23, 2017 11:54 pm

Find a 1 amp load and time how long it takes to flatten the batt.

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Re: What are the amp hours, really.

Postby fotooutdoors » Tue Jan 24, 2017 7:25 am

edgeau wrote:Find a 1 amp load and time how long it takes to flatten the batt.

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The only issue with this method is most (all? I can't think of a situation where this isn't the case) loads will sag as the voltage sags.

Sent on my phone. Pardon the autocorrect.
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Re: What are the amp hours, really.

Postby Dale M. » Tue Jan 24, 2017 9:49 am

Not being smart A$$ but why do you need to know.... IF battery is functioning within your expectations it is ok... If it it does not meet your needs or loosing its charge sooner than last time it was exercised it is beginning to fail,,, Replace it.... Teardrop travel is for relaxation, not mentally fussing over something that is not really that critical... IT sort of like experience I had with my pickup truck... It sits a lot, and every once in a while I start it up and run it to keep battery up... Went out one day and it started just fine, about 2 weeks later, went to start it again, nothing, no click no lights, no whatever ( I know ECU drain) .. Checked all connections all good voltmeter test showed bad, put charger on it, got it started, drove to town, changed battery in FLAPS parking lot, all has been good since.... OLd batter has reached its "end of life".... Amp hour rating ( or CC amps) is merely a calculation for matching battery to "work" battery is expected to do at you initially purchase it...

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Re: What are the amp hours, really.

Postby Shadow Catcher » Tue Jan 24, 2017 10:05 am

One of the advantages of a battery monitor is it tells you what you SOC is, but that is based on a number of assumptions, which are probably wrong. That the manufacturer stated AH capacity is correct, that the battery does not change over time in its capacity... The only way to find out is to test. I know ours is a good bit less than the 150 AH (if it was).
Bruce I am also pleased with the function of the Victron but I know that our battery is no longer 150 AH and since it is a Lifeline AGM the hydrometer is out.
The life line manual http://lifelinebatteries.com/wp-content/uploads/2015/12/manual.pdf at 5.7 gives the way to check for capacity and I am having difficulty figuring out how I might do that.
I did find this article but it is above my level of incompetence.
From what I can see a constant current is necessary to test and I was looking at one of these http://www.ebay.com/itm/Constant-Current-Electronic-Load-Module-9-99A-60W-30V-Battery-Capacity-Tester-CL-/182388649741?hash=item2a7735ef0d:g:b6EAAOSw241YURIl

As I said this is exceeding my level of competence :thinking:
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Re: What are the amp hours, really.

Postby MtnDon » Tue Jan 24, 2017 10:28 am

Compared to its performance as new, if the battery can still perform all those tasks, then it is good. :)

The problem with the listed discharge device is its limit of 9.99 amps. You can not extrapolate the lower discharge rate to the 25 amps that the battery mfg specs. It might help you do a test at load rates under 10 amps, if that would mirror your use pattern.

A battery will supply more total amp-hours (watt hours actually) when discharged at 2 amps than it will when discharged at 25 amps.
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Re: What are the amp hours, really.

Postby Shadow Catcher » Tue Jan 24, 2017 11:44 am

The main goal of a battery monitor is to keep you above the 50% DOD (depth of discharge) which I understand is not as important with an AGM. The 150 AH Lifeline was an investment of about $400 and it powers our refrigerator (the big draw), as well as all the other items. Our fall trip which ended on the north shore of Lake Superior, had not much sun, shorter hours, less direct sun, shade, all of which made solar less able to keep the battery charged.
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Re: What are the amp hours, really.

Postby Bill n Robi » Tue Jan 24, 2017 12:12 pm

I wrote this up for a different forum but it might help explain.
So how to tell what you have in your battery? The number you need to read is RC - Reserve Charge. The Battery Institute came up with this number based on the formula of how long you can draw 25 Amps from your battery. Of course no one does that but it is a standard to rate the power. Since most people get stuck with an Interstate SRM-24, I am going to use that as an example. Please remember you need to check what you have and work the formulas yourself. I forgot, yes there will be math.
The SRM -24 has an RC of 2.5 hours – 25 X 2.5= 62.5 AmpHours. But you saw a battery that had an RC of 4, isn’t that better? If they rated it at 15 Amp x 4 hours = 60 AmpHours – look at the specifications, not everyone uses the correct standard. The lower draw, the more you get. The SRM-24 with a 5 amp draw actually gives you 82 Ah. Watch the numbers, do the research
What does this mean? Well, you can draw this much power until the battery is ruined. The rating is based on how much power you get until the battery drops to 10.5 volts. Sound familiar? – look at all the charts that are floating around – 0% left. That chart says ‘Green’ until 40%, so first math question – what is 60% of 80Ah? You got 48 AmpHours .
You can figure out how long you can run by using an amp meter in place of a fuse in your converter and measuring how much each device draws. This is my little guy results:
Nothing on - my ‘vampire’ draw is .13 A
Individual readings:
2 Reading lights - .4A
Ceiling Lights - .46A
1 Porch Light - .27A
Both Porch Lights - .43A
TV – 1.4A
Stereo - .65A
Stereo with CD/DVD – 1.2A (motor to spin disc)
Galley Lamp - .46A
Water Pump – 1.66A
I know what each item uses and try to maximize my battery life
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Re: What are the amp hours, really.

Postby bdosborn » Tue Jan 24, 2017 8:29 pm

I get it now, you want to set the battery capacity in the Victron to reflect a battery that doesn't have 100% capacity anymore. It would be nice to set it every spring so that you don't accidentally discharge an aged battery too deeply and damage it. Battery amp-hrs is a tough thing though to establish as capacity is depended on so many different things: age, cycles, temperature, DOD, etc. I use a wet finger in the wind method to do a rough check of my battery capacity. I look at the battery voltage and compare the meter battery capacity to the depth of discharge on this chart:

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So if the battery voltage says the battery is at 80% and I've used around 80% of my battery capacity as shown on the meter then all is good. I've noticed that the two values get farther apart as the battery gets older. I replaced my battery when the voltage would hit 12V after a couple of hours of typical (for me) use and the Trimetric said I had 70-80% capacity remaining.

FWIW,
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Re: What are the amp hours, really.

Postby featherliteCT1 » Tue Jan 24, 2017 9:39 pm

to: bdosborn

with much thanks for the temperature compensated battery SOC chart
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Re: What are the amp hours, really.

Postby Shadow Catcher » Wed Jan 25, 2017 10:33 pm

Thanks Bruce exactly what I need. One of the problems with the official ,this is how to find your real Amp Hours requires stabilizing the battery at a rather narrow temperature range that would be very difficult to get right now.
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Re: What are the amp hours, really.

Postby bdosborn » Wed Jan 25, 2017 10:49 pm

:D :thumbsup:
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