Rant about Ah and Wh...

Horsefly · Mar 20, 2023

Supervstech said:
I guess I don’t understand.
If, at .2C a 1280Wh battery outputs 1280Wh, is that not a valid test of the Ah of the battery?

If it outputs 1320Wh would that not be more than a 1280Wh battery, thus more than 100Ah?

Well, there are two problems.

First, to get from Wh to Ah you have to divide by some voltage. What voltage do you use? If you use 3.2V per cell, just know that the answer may be "good enough" for whatever you are doing, but not really accurate since the voltage was changing some throughout the test from 100% to 0% SoC. Like I said, if you are just designing a system, stay with Wh and it will be good enough.

Second, a measurement of Wh is measuring the energy that is going through an entire circuit. So that will include the load are using for the measurement, but also the cables, terminals, and bus bars, etc. These all will have some resistance, and so will be included in the watts you actually take out of the battery, but they will be over and above what you measure.

If you are measuring amps the losses in the circuit don't matter. All of the amps go through the whole circuit, no matter how small the wires or how bad the connections are. So Ah is more repeatable, and can be both more accurate and precise than any measurement of Wh.

As long as you either (1) don't care much about the precision of the measurement, or (2) know exactly what the watt losses are in your measuring system and include that in your Wh calculation, or (3) have low enough watt losses that it doesn't matter you can do just that. Fact is, option #3 is the same as #1 since you will always have some losses.

Supervstech said:
If it outputs 1320Wh would that not be more than a 1280Wh battery, thus more than 100Ah?

Maybe I'm being dense, but I'm not sure what you are trying to say here. If you are only saying that 1320 is more than 1280, then obviously I agree. I don't think you really know what the Ah rating is on either of those two Wh measurements, so I don't think the number 100Ah fits.

Supervstech · Mar 20, 2023

Horsefly said:
Well, there are two problems.

First, to get from Wh to Ah you have to divide by some voltage. What voltage do you use? If you use 3.2V per cell, just know that the answer may be "good enough" for whatever you are doing, but not really accurate since the voltage was changing some throughout the test from 100% to 0% SoC. Like I said, if you are just designing a system, stay with Wh and it will be good enough.

Second, a measurement of Wh is measuring the energy that is going through an entire circuit. So that will include the load are using for the measurement, but also the cables, terminals, and bus bars, etc. These all will have some resistance, and so will be included in the watts you actually take out of the battery, but they will be over and above what you measure.

If you are measuring amps the losses in the circuit don't matter. All of the amps go through the whole circuit, no matter how small the wires or how bad the connections are. So Ah is more repeatable, and can be both more accurate and precise than any measurement of Wh.

As long as you either (1) don't care much about the precision of the measurement, or (2) know exactly what the watt losses are in your measuring system and include that in your Wh calculation, or (3) have low enough watt losses that it doesn't matter you can do just that. Fact is, option #3 is the same as #1 since you will always have some losses.

Maybe I'm being dense, but I'm not sure what you are trying to say here. If you are only saying that 1320 is more than 1280, then obviously I agree. I don't think you really know what the Ah rating is on either of those two Wh measurements, so I don't think the number 100Ah fits.

I see now. Ah is a test measurement that tracks the amperage output of the battery, where watt-hour is a test of watts over the measuring tool.
If the tools are both directly connected to the terminals of the battery, I can measure the Wh of the system, but that will NOT be an accurate Ah measurement of the battery.
I’ve never seen an Ah measuring tool…

Ampster · Mar 20, 2023

i agree with the original post. Since i think in terms of Watts and Watthours i rarely bother to even do the conversion. The only time I care about Amps is when I am sizing wire, fuses or breakers. When buying bstteries or cells the metric I use is price per Watthour. As long as I am consistent with the voltage used that metric is useful for me.

Warpspeed · Mar 20, 2023

Supervstech said:
I’ve never seen an Ah measuring tool…

They certainly exist. I have a couple of Turnigy power meters, they display instantaneous volts, amps, and watts, as well as amp hours, watt hours, and elapsed time.
https://www.amazon.com.au/Turnigy-180A-Meter-Power-Analyzer/dp/B0778J8V8R.

Amp hours are very easy to measure.
All you need is a current shunt and a voltage controlled oscillator, and a pulse counter.
The oscillator goes faster the higher the current, and the longer the time, the higher the number of total output pulses.

A crude example, if 1 amp = 1Hz after a minute you get a count of 60 at one amp.
If there are 2 amps, you get a count of 120 per minute. So this counts up in amp minutes.
If you divide by 60 you get amp hours.

Of course the oscillator is proportional. 3.015 amps = 3.015 Hz

Supervstech · Mar 20, 2023

Oh, I wasn’t doubting their existence, just stating I haven’t seen one.
I’m a fairly new electrician, batteries are mostly automotive with me.

Warpspeed · Mar 20, 2023

For the price, those Turnigy power meters have surprisingly high accuracy and very good resolution.
Highly recommended.

Horsefly · Mar 20, 2023

Supervstech said:
I’ve never seen an Ah measuring tool…

Most of the capacity measuring devices measure Ah. The relatively cheap little boards with a big heat sink and fan do, and the more expensive ZKE EBC-A40L does too. They both may also show an estimate of Wh, but I've never even looked at that because I wouldn't trust it for the reasons I've explained.

Ampster said:
When buying bstteries or cells the metric I use is price per Watthour. As long as I am consistent with the voltage used that metric is useful for me.

That's the key. Wh is fine for you to use, and it is what you should use when designing / sizing your system, as long judge them all consistently (using the same voltage as you described). I really don't like it when cell brokers try and use Wh to back into a Ah rating that makes their cells look better than they are.

wattmatters · Mar 20, 2023

Horsefly said:
Context matters. What he said is absolutely correct. Trying to get Ah by dividing the Wh of a cell or battery by some "nominal" voltage is foolhardy, and will give you an arbitrary and wrong answer. However, nearly all of the rest of his post was about when people are trying to determine the capacity of a cell.

I refer you to my initial post on this thread:

Rant about Ah and Wh...

Ok, this has been grinding my gears to the point where I'm annoyed with it, so I just wanted to write it down here. I see statements like this all over the net when it comes to cell capacity testing: - "you need to divide the Wh reading by the nominal voltage of 3.2V to get the real Ah...

diysolarforum.com

and the OP's response:

Rant about Ah and Wh...

Ok, this has been grinding my gears to the point where I'm annoyed with it, so I just wanted to write it down here. I see statements like this all over the net when it comes to cell capacity testing: - "you need to divide the Wh reading by the nominal voltage of 3.2V to get the real Ah...

diysolarforum.com

I think I get the context just fine.

timselectric · Mar 20, 2023

Supervstech said:
I’ve never seen an Ah measuring tool…

I test all of my battery Ah capacities with a Victron smart shunt.

Lt.Dan · Mar 20, 2023

timselectric said:
I test all of my battery Ah capacities with a Victron smart shunt.

I tried the same, but mine just seemed like it was never accurate, always +/- 3-5% it seemed. But no proof to that, and never tried to adjust any settings

timselectric · Mar 20, 2023

Lt.Dan said:
I tried the same, but mine just seemed like it was never accurate, always +/- 3-5% it seemed.

Compared to what?

time2roll · Mar 20, 2023

AH is fine on a simple one voltage system. kWh is more for larger systems. Either way having accuracy beyond 5% to 10% is overrated anyway.

timselectric · Mar 20, 2023

I think that everyone has gotten lost from the purpose of this thread.
It's about how some vendors want you to measure and calculate the capacity of the batteries or cells that they are selling. In order to change the results in their favor.

time2roll · Mar 20, 2023

Lead-acid used to have testing standards set by BCI such as the AH rate of discharge in hours or the constant current RC rating.
Has all that just gone away? Or has our interpretation gone sideways?

timselectric · Mar 20, 2023

This thread is about LFP, not lead acid.
Ah is the standard rating.
The testing standards for LFP are......
Charge to full (3.65 per cell)
Discharge at 0.2c
Until empty (2.5 per cell)
Resulting Ah should at least meet the advertised rating.

time2roll · Mar 20, 2023

Is that the BCI standard? Or just a manufacturers definition/spec?

timselectric · Mar 20, 2023

Industry standard, as far as I know.
With the exception of a few shady vendors, recently.

Rednecktek · Mar 20, 2023

***COUGHCOUGHWALMARTCOUGHCOUGH***

RV8R · Mar 20, 2023

Supervstech said:
I see now. Ah is a test measurement that tracks the amperage output of the battery, where watt-hour is a test of watts over the measuring tool.
If the tools are both directly connected to the terminals of the battery, I can measure the Wh of the system, but that will NOT be an accurate Ah measurement of the battery.
I’ve never seen an Ah measuring tool…

I’ve never seen an Ah measuring tool

I have a few Victron BM712 Smarts. I assume these shunt based little computers measure volts, amps, & time ( or hours ). I assume they record the amps & hours, then show how many Ahs have passed thru the shunt.

Maybe you have seen an Ah measuring tool ?

Lt.Dan · Mar 20, 2023

timselectric said:
Compared to what?

Compared to a half dozen different BMS's, the Sol-Ark's internal shunt, and the fact that it says I have 10-15% battery remaining sometimes when the voltage is at 48v. I don't remember exactly, but it was things like that.

Horsefly · Mar 20, 2023

time2roll said:
Lead-acid used to have testing standards set by BCI such as the AH rate of discharge in hours or the constant current RC rating.
Has all that just gone away? Or has our interpretation gone sideways?

A bit off-topic, but one of the things that drove me crazy about lead-acid was that the manufacturers used such odd-ball specs and then gamed the system. Most batteries were rated in for a "20h C-rate", meaning that the Ah rating was an indication that if you drained it that Ah divided by 20, it would last that many hours. Whaaaa? I'm sympathetic in that peukert showed the battery will deliver fewer Ah at higher A rates (unlike LiFePO4, which are mostly constant), but this just made it so no one really knew how much they could get from their batteries. Worse, people could just claim a 20H rate and who would ever check to see? Arghhh. It's frustrating to think of the money I've wasted on lead-acid.

timselectric · Mar 20, 2023

Lt.Dan said:
Compared to a half dozen different BMS's, the Sol-Ark's internal shunt, and the fact that it says I have 10-15% battery remaining sometimes when the voltage is at 48v. I don't remember exactly, but it was things like that.

BMS's are terribly inaccurate. Not sure about Sol-Ark, but I would trust Victrons accuracy over anything else.
I assume that most would agree.

Lt.Dan · Mar 20, 2023

timselectric said:
BMS's are terribly inaccurate. Not sure about Sol-Ark, but I would trust Victrons accuracy over anything else.
I assume that most would agree.

What about the 10-15% capacity when the battery clearly is near dead?

I have no idea, as I have nothing more accurate, so I can't prove.

timselectric · Mar 20, 2023

Lt.Dan said:
What about the 10-15% capacity when the battery clearly is near dead?

At 48v, each cell is at 3v. 10-15% is about right.
I would say 10%.

Pierre · Mar 21, 2023

Supervstech said:
I see now. Ah is a test measurement that tracks the amperage output of the battery, where watt-hour is a test of watts over the measuring tool.
If the tools are both directly connected to the terminals of the battery, I can measure the Wh of the system, but that will NOT be an accurate Ah measurement of the battery.
I’ve never seen an Ah measuring tool…

I recently pulled a 48v 100Ah pack apart that I built and used with a Growatt SPF 5000ES for about 8 months. Wanting to sell the the cells I had to determine the capacity for the buyer. All in all I had to test 24 cells ( had a few spares ) . Using the ZKE EBC-A20 I tested each cell as follows : Charge the cell up to 3,6v , constant current of 5A , until the tail current dropped to 0,7A , rest the cell for about 10 - 15min. Discharge cell to 2,8v at a constant current of 20A until the ZKE turned off. After, the test the results are displayed on the screen in Ah and Wh. You can then calculate the nominal / average voltage by Wh/Ah. All cells tested between 93 - 102 Ah . Could also have plotted all the graphs on the PC but needed it elsewhere. Took a few days to do !!

Rant about Ah and Wh...

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