Rant about Ah and Wh...

[timselectric]

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

Compared to what?

 

[time2roll]

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]

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]

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]

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]

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

 

[timselectric]

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

 

[Rednecktek]

***COUGHCOUGHWALMARTCOUGHCOUGH***

 

[RV8R]

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]

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]

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]

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]

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]

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]

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 !!

 

[LBART]

I know this thread is about the cell level on a battery. But this is a rookie question about batteries.

If AH is the most important than and a 24v 200AH battery is 5120W and a 48v 100AH battery is 5120W they have equal output. Yet it appears some are saying the AH is more important. If the AH is more important or the deciding factor however it is looked at then the 200AH battery has twice the capacity of a 100AH battery? If I understand correctly both will keep the lights on the same amount of time baring wiring losses and such?

 

[Pierre]

You can only compare them at the same nominal voltage. If you run the 24v battery through a 48v boost converter they will be the same ( bar losses ) or otherwise you must use two loads e.g. 24v and 48v rated. That is why Wh are used for comparison - takes care of different nominal voltages.

 

[LBART]

I wasn't referring to a mixed system. How about 2 - 24 v in parallel for a 48v system and the same but in series with 2 - 48v batteries. Both would then be 48v and 200AH and 10240W. I understand nominal voltage which is why the watt rating is higher than 4800W.

 

[upnorthandpersonal]

If AH is the most important

That's not what I said.

a 24v 200AH battery is 5120W and a 48v 100AH battery is 5120W they have equal output.

Yes.

If the AH is more important or the deciding factor however it is looked at then the 200AH battery has twice the capacity of a 100AH battery?

No. When you get a battery, multiply the voltage with the Ah rating to get the energy estimate. That's not what I argue against. It's this:

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.

Which is absolutely correct. What is not correct is then take the Wh reading, divide by a 3.2V (instead of the nominal voltage your get by Wh/Ah in this case) to get a higher Ah reading than the one you just measured.

 

[upnorthandpersonal]

I’ve never seen an Ah measuring tool

Every single cell capacity tested does this: ZKE, CBA IV, those cheap ones with a fan, etc. It's the only thing they can accurately measure without separate voltage sense leads. The Wh is an estimate (or very wrong with those fan things), the Ah reading is (to an extend) accurate, because the current is the same in the circuit and does not care about voltage drops on the leads, losses in connections, etc.
The way the test works is you put a controlled load on the source and draw constant current until it drops off, measure how long it can do that. It's literally testing how many (constant) amps the cell can provide over time.