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Old 08-07-2018, 05:08   #1
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Measure present Full Battery Capacity (as it declines)

How to Determine the Current Capacity (Size) of Batteries in AH or %Capacity
Why is this needed? For the Ah Counter parameter for battery capacity.

From an earlier thread (involving John, I think):

Put a 9 amp load for one hour, so 9 amp-hrs consumed. Read a voltage at the end of that period of say 12.63, which the Battery State of charge chart says is a state of charge of 95%, then C=9/(1-0.95) = 180 amp-hour capacity at full charge. In this case the battery is still as new.

Now use this battery for 3 years (or 3 months) and after 9 amp-hours, you read 12.42 volts, which state of charge chart says 80%. C=9/(1-0.80) = 45 amp-hour capacity at full charge.

The other alternative is a 20 hour Discharge Test down to 10.5 volts by first charging up to Full then discharging it at a specified rate. Few people do this.

The 9 amps out of this old battery is much greater than the c20 rate, so redo the test 45/20=2.25 amps and you will get a better capacity.


Are there other methods?
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Old 08-07-2018, 10:44   #2
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Re: Measure present Full Battery Capacity (as it declines)

Only the 20-hour test is accurate if the goal is to compare with the published when-new rating

Maine Sail gives a poor-man version on that BM howto page, if you set a benchmark at commissioning time, can then compare later long as its variables are repeated identically, give a rough idea.

But forget the voltage charts, bank SoC relative to voltage varies by

Each mfg and model, size

true resting V (48-72 hours) vs relative C rate of dis / charge

and the relationship itself changes over time as the batt ages.
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Old 14-07-2018, 10:54   #3
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Re: Measure present Full Battery Capacity (as it declines)

If the only way to measure the battery capacity is by 20hr test down to 10.5v, then how could Smartgauge possibly be accurate at this task?

What is it's methodology such that it is accurate?
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Old 14-07-2018, 11:15   #4
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Re: Measure present Full Battery Capacity (as it declines)

Isn't the partial discharge technique better than nothing? I need a practical way of resetting my Link10 capacity setting.
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Old 14-07-2018, 11:18   #5
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Re: Measure present Full Battery Capacity (as it declines)

Maybe the most practical approach is to purchase a smartgauge.
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Old 14-07-2018, 13:13   #6
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Re: Measure present Full Battery Capacity (as it declines)

There are battery testers that use the internal AC impedance of the battery to determine the actual remaining state of capacity.

https://smile.amazon.com/Clore-Autom...ery+tester+ba9

That's one of the more common cheap models. A battery distributor will have a much better one, and there are cheaper models out there as well. One alternative to trying actual discharge cycles.
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Old 14-07-2018, 13:57   #7
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Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by rgleason View Post
Isn't the partial discharge technique better than nothing? I need a practical way of resetting my Link10 capacity setting.

rg,


Sometimes we can tend to overthink theses issues, I know I have. This is primarily because the equipment, like my Link 2000, gives us the "cool" numerical readouts to 85 decimals places!


Either do a 20 hour test correctly, or take a "deduction" in how you operate your electrical system. Either reduce the capacity input to the Link, or just start recharging at an earlier time. One can do this [a tad more] yearly.
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Old 14-07-2018, 14:30   #8
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Re: Measure present Full Battery Capacity (as it declines)

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If the only way to measure the battery capacity is by 20hr test down to 10.5v, then how could Smartgauge possibly be accurate at this task?

What is it's methodology such that it is accurate?
It is not **that** precisely accurate.

Just more so than the rest.

Gibbo's sensor readings + database + algorithms are all proprietary, I've seen high-level screeds full of speculation how it may work but all gobbledygook to me.
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Old 14-07-2018, 14:33   #9
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Re: Measure present Full Battery Capacity (as it declines)

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Isn't the partial discharge technique better than nothing? I need a practical way of resetting my Link10 capacity setting.
If you start with a standardized benchmark when new you can guesstimate.

Otherwise just make sure it is set lower than reality, it will catch up soon enough.
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Old 14-07-2018, 14:36   #10
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Re: Measure present Full Battery Capacity (as it declines)

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Maybe the most practical approach is to purchase a smartgauge.
Yes, if the cost seems worth it **to you**, once it has settled down finished its learning, use that to calibrate the Link.

But with anything use endAmps to set a benchmark for 100% SoC, whatever you choose, but be consistent across the board.
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Old 14-07-2018, 14:40   #11
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Re: Measure present Full Battery Capacity (as it declines)

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Originally Posted by hellosailor View Post
There are battery testers that use the internal AC impedance of the battery to determine the actual remaining state of capacity.

https://smile.amazon.com/Clore-Autom...ery+tester+ba9

That's one of the more common cheap models. A battery distributor will have a much better one, and there are cheaper models out there as well. One alternative to trying actual discharge cycles.
Those gizmos are good to measure CCA (cranking amps) to start a big engine; they are noy very helpful to measure Ah capacity at slow (say 20h) discharge rates, which is the "capacity" measure that matters for a house bank. It is very very easy to do a (roughly) 20 hour test provided you have batteries in parallel so that you can use one while you test the other.
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Old 14-07-2018, 14:43   #12
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Re: Measure present Full Battery Capacity (as it declines)

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Originally Posted by hellosailor View Post
There are battery testers that use the internal AC impedance of the battery to determine the actual remaining state of capacity.

https://smile.amazon.com/Clore-Autom...ery+tester+ba9

That's one of the more common cheap models. A battery distributor will have a much better one, and there are cheaper models out there as well. One alternative to trying actual discharge cycles.
Yes that is part of what SG may be doing.

But Maine Sail couldn't catch it on his scopes.

The consensus is none of the sub-lab-quality gear is worth messing with for residual capacity.

20-hour test or guess.

With an AH counter *and* a SG, working backward from 100%, with patience could maybe work out a reasonable estimate.

IMO setting good benchmarks from new, when you see performance declining, time to replace next time it's convenient if you're cruising to underserviced locations.

Pushing past 75-80% SoH, the odds of "unexpected failures" gets too high for my taste, better to be proactive.
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Old 14-07-2018, 19:04   #13
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Measure present Full Battery Capacity (as it declines)

I think you can get a reasonable idea with an amp counter and a Smart Gauge, the longer and deeper You discharge likely the more accurate the guess will be. Ideal of course is to discharge over 20 hours to a Smart Gauge reading of 50%,then look at how many amp hours were used, double that.
I end up more in the overnight discharge to 75% number and multiply by four of course.
I think itís good enough, not as accurate as a real 20 cap check, but good enough.

Lifeline publishes an excellent chart of SOC and discharge rate, plus of course the open circuit resting voltage. However for whatever reason my bank is much higher voltage than the chart says it should be at a particular SOC, so itís not real applicable for me, for some reason.
I find my Smart Gauge is only even close to accurate in the morning, for the first several hours of discharge, it reads way higher than my amp counter does, however in the morning, they agree very close to each other, within a percent or two.
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Old 14-07-2018, 19:44   #14
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Re: Measure present Full Battery Capacity (as it declines)

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discharge over 20 hours to a Smart Gauge reading of 50%,then look at how many amp hours were used, double that.
Best to stick to the 20-hour rate e.g. 500AH bank would be a 25A load.

So if 50% is the target, should with a new batt take around ten hours.
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Old 14-07-2018, 19:47   #15
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Measure present Full Battery Capacity (as it declines)

Your right, just a habit to think of 50% as being fully discharged, itís not of course, just I have never gone below 50%.

However the point is with a Smart Gauge and an amp counter, I think you donít necessarily have to do a real cap check, just pay attention to see if 25% or whatever number your usually at in the morning starts being a smaller number of amps, then maybe equalize if you can.
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