How many amp hours are left

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How many amp hours are left

Postby Shadow Catcher » Sun Mar 18, 2018 7:38 pm

Have an aging AGM and it is no longer what it once was. I am bumping up against my aspergers (subset of autism) in trying to get some handle on what remains. I am mind blind to math and similar kinds of organizational things.
I would think that a device could be cobbled together that would get an accurate Amp Hour reading
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Re: How many amp hours are left

Postby tony.latham » Sun Mar 18, 2018 7:43 pm

Shadow Catcher wrote:Have an aging AGM and it is no longer what it once was. I am bumping up against my aspergers (subset of autism) in trying to get some handle on what remains. I am mind blind to math and similar kinds of organizational things.
I would think that a device could be cobbled together that would get an accurate Amp Hour reading


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I think you can tell the battery's condition by how high the voltage is after it's rested post-charge for a few hours. But I don't think you can measure amps in a battery.

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Re: How many amp hours are left

Postby Shadow Catcher » Sun Mar 18, 2018 9:33 pm

That tells the SOC state of charge, which is relative. It does not tell the AMP hours the battery is capable of holding. It started out as 150AH some years back and i=even that may not have been accurate. I have a Victron battery monitor which needs to be re-calibrated.
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Re: How many amp hours are left

Postby tony.latham » Sun Mar 18, 2018 10:12 pm

Shadow Catcher wrote:That tells the SOC state of charge, which is relative. It does not tell the AMP hours the battery is capable of holding. It started out as 150AH some years back and i=even that may not have been accurate. I have a Victron battery monitor which needs to be re-calibrated.


I don't think you can. But let's hope I'm wrong.

I partially base that on the fact that the higher the discharge rate, the less power you'll get out of a battery. Peukart effect. :frightened:

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Re: How many amp hours are left

Postby Pmullen503 » Mon Mar 19, 2018 5:52 am

There are watt meters that will measure exactly what you want to know. They are used by us RC airplane and drone pilots. Search for RC airplane Watt meters.
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Re: How many amp hours are left

Postby Aguyfromohio » Mon Mar 19, 2018 8:23 am

Shadow Catcher wrote:That tells the SOC state of charge, which is relative. It does not tell the AMP hours the battery is capable of holding. It started out as 150AH some years back and i=even that may not have been accurate. I have a Victron battery monitor which needs to be re-calibrated.


You could test it and know the capacity of the battery any given day. But you inevitably need a bit of electronics lab equipment to get any results, and you need expensive lab equipment to get good results. Most batteries are specified at the "20 hour rate". That means you pull as many amps as required to get the battery down to 10.5 volts in 20 hours.
Of course, you need to guess that rate the first time. If I was doing this, I would start with the specifications of the battery when it was new; divide the advertised amp hours by 20, and use that amperage load. Like this:

(150 ) amp hr / (20) hr = 7.5 amps

So I would run the first test at 7.5 amps. Since we know the voltage, we can use ohm's law to turn that into a resistance:

R = V/I
= (12) volts / (7.5) amps
R = 1.6 ohms

That's a pretty oddball resistor, but the electronics folks have high power adjustable resistors just for this purpose.
They are cheap, under $20.

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Of course, everything is shifting and changing as the test runs. The wire in that resistor gets hot, and its resistance value in ohms changes significantly between hot and cold, so you should really adjust it when it's hot. And the voltage of the battery drifts down during the test, so you should adjust the resistor constantly during the test as the battery voltage drops.
Somebody makes a piece of specialized automatic test gear for everything, and there are such constant-current load banks targeted at testing batteries, just like you need.
Here's one for $11,995. For twelve grand you can know the answer to your question with certified high precision with the push of a button and dowload the resulting test data.

https://www.tequipment.net/Storage-Batt ... gKRMvD_BwE
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Re: How many amp hours are left

Postby MtnDon » Mon Mar 19, 2018 8:53 am

How many amp-hours any battery contains is also dependent on the rate that the energy is used. A slower drain of power will normally provide more amp-hours than a faster drain. Which is why battery capacity is rated at a "hour rate"... 20 hour rate is norm for rating RV / off grid batteries.

So back to the question. The RC meters that were mentioned are likely your best relatively low cost solution. There are many available these days. One is here. . Then you would need to pick a device or two that would consume approx the average amps that typifies your use pattern. Turn it on and watch the battery voltage. The meter provides the info: Current (A), Voltage (V), Watts (W), Amp-hours (Ah), Watt-hours (Wh), Peak Amps (Ap), Minimum Volts (Vm), Peak Watts (Wp)

Run the battery down, from full to your comfort zone and read watt-hours or amp-hours consumed and that is the effective number of WH or AH you have. You can use that as a calibration point.

Or just buy a new battery if the old one is 4 to 5 years old or older. I can't seem to get more than 3 years out of the batteries in the RV's we've had. Those have all been flooded golf cart 6 volts. The same batteries, but more of them, have lasted me 9 years at the cabin. (Next year when we hit 10 years I may replace them; perhaps with lithium-ion LFP.) The obvious difference is stationary vs mobile, but the ones at the cabin also don't get discharged as deeply. maybe 25% discharge maximum, often less before a recharge the next day. The RV batteries get discharged more deeply because I don't want to cart around more weight. A trade off.
Last edited by MtnDon on Mon Mar 19, 2018 7:33 pm, edited 3 times in total.
Our 6x12 deep vee nose cargo trailer camper conversion... viewtopic.php?f=42&t=58336

We have a small off grid cabin we built ourselves in the NM mountains; small PV solar system; 624 watts PV, Outback CC & inverter/charger ... http://countryplans.com/smf/index.php?topic=2335.0
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Re: How many amp hours are left

Postby tony.latham » Mon Mar 19, 2018 10:31 am

Pmullen503 wrote:There are watt meters that will measure exactly what you want to know. They are used by us RC airplane and drone pilots. Search for RC airplane Watt meters.


I use the G.T. Power watt meter in my teardrop. They're good to 160 amps, accurate, and cheap. But they will only measure what's coming out of your battery (including totals) or what's going in. They can't measure how much is actually in the battery and I think that's the question at hand.

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On my next build, I'll put one coming off the solar panel too. For $18, why not?

Tony

https://www.amazon.com/T-Trees-G-T-Power-Precision-Analyzer-Screen/dp/B01ELEQJA0/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1521474828&sr=8-1&keywords=g.e.+wattmeter
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Re: How many amp hours are left

Postby Pmullen503 » Mon Mar 19, 2018 12:16 pm

You would have to test your battery to see what it's actual capacity is. Charge it up and place a typical load on it. Then watch the voltage until you get to whatever safe voltage you select. The meter will tell you how much power was consumed. You can also buy battery dischargers that record the Ah used and will automatically shut off but those are limited to around 150W discharge load.

RC charging equipment is very advanced: if you screw up charging or discharging LIPOs they burst into flames.
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Re: How many amp hours are left

Postby Shadow Catcher » Mon Mar 19, 2018 8:49 pm

Pmullen503 I am looking at some of those dischargers along with a digital voltage cutoff and a digital timer. All of the items seem precise and doing a short term test, a fraction of the 50% discharge would/should be representative?
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Re: How many amp hours are left

Postby John61CT » Tue Mar 20, 2018 11:21 am

The only real way to measure residual AH capacity, aka State of Health without spending thousands is a 20-hour load test.

​Search for 20 hour capacity test here https://marinehowto.com/programming-a-battery-monitor/ Better yet read it all,  and check out the rest of the site.

Industry standard is 20% decline is scrap time, maybe push to 30% but "unexpected failures" then much more likely.
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