# How is battery capacity calculated ?

• posted

To all the electronics gurus on this newsgroup - how is battery capacity decided - for example if I buy a 6 Volt 5AH sealed lead acid battery, how did the manufacturer come up with the number

5AH ?
• posted

** You could easily Google that question you know.

FYI:

For SLA batteries, makers use the "20 hour rate" for the published AH figure.

So for 5AH, the battery is discharged at 250mA for 20 hours for a final terminal voltage not less than 1.7V per cell.

.... Phil

• posted

Who is the manufacturer? Some manufacturers provide discharge/charge curves. Bear in mind that the lead-acid batteries rapidly degrade at 100% charge / discharge cycle.

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"How is it calculated" is a question without a consistent answer, and one that's subject to cynicism.

"What does it mean" is better.

The term "battery capacity" itself means the useful charge that you can take out of the battery. In the case of rechargeables, "useful" usually means "without undue damage". In the case of primary cells, "useful" usually means "when it's sucked dry".

The _rated_ capacity of a battery is the manufacturer's promise to you that -- under some condition that's hopefully specified -- the battery's useful capacity will be as rated. For lead-acid batteries that usually means the portion of the battery's life between a few dozen charge- discharge cycles (lead-acid batteries don't achieve their full capacity until they've been used a bit) and a several hundred or thousand charge- discharge cycles.

Keep in mind that it's a promise, and that even if the manufacturer is cleaving to the letter of the law it only applies when you use the battery according to the specifications. This is where old hands may get cynical, because consumers just look at big numbers for the capacity and go "ooooh!" and buy, without thinking that (a) the manufacturer may not have rated it for their use, and (b) the manufacturer may be outright lying.

To actually answer your question: the manufacturer will have a good idea of how much capacity can be had from a certain amount of active material (which depends on the chemistry, but for a lead-acid is the mass of the plates), so they will design for a certain capacity. Then, if they're responsible, they'll either test any new battery designs to make sure they're up to snuff, or they'll over-design to make sure that they're at least as good as they say. Or, they'll induce cynicism in their customer base by just making something and stamping an approximate number on it...

A _really_ good battery manufacturer would test all cells coming off the line, or at least a representative sampling. If your rechargeable cell comes with a test certificate showing it's tested capacity, that's a cool thing. (If your primary cell comes with a test certificate showing it's tested capacity, well, maybe that's a bit too much diligence on the part of the manufacturer.)

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Tim Wescott
Wescott Design Services ```
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I doubt any consumer battery gets 100% testing other than whatever can be done quickly. Certainly not capacity testing. [I've mentioned the alarm clock tester enough times, but basically you pull on a battery with a constant current source and use a comparator to turn off the mechanical alarm clock when the discharge voltage is reached. Be sure the circuit can't retrigger since the battery voltage will rise when the load is removed.]

I don't own the book and couldn't recall the name anyway, but there are a number of electrical tests that can determine or help to guestimate the condition of a battery. All sorts of impedance sweep tests that relate to chemical reactions. At the time I was looking for a simple primary versus secondary cell tester as potential product or cell for products. It really isn't that easy, which is why devices that incorporate difference discharge limits do so by a menu section, assuming the customer knows the difference.

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If you find the book title please post it. I rarely need to know a lot about batteries, but when I do I'm always dismayed by the dearth of printed information to be found.

It does seem that judging battery charge by its terminal characteristics is, for the most part, a fool's game. If it weren't, there wouldn't be so many state of charge indicator chips that are basically just coulomb- counters.

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Tim Wescott
Wescott Design Services ```
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This can vary from manufacturer to manufacturer. But, typically, a given "battery" (which I am using to apply equally to cells *and* batteries) is discharged to a given depth of discharge over a fixed time interval and the total charge (discharge?) measured. Usually, this is on the order of 10 or 20 hours (depending on the "class" of battery).

At the "20 hour rate", you will often see a bit higher capacity than at the "10 hour rate" (which, in turn, would be higher than a "1 hour rate").

Your goal (in optimizing battery capacity) is to opt for a discharge rate that is closer to the self-discharge rate of the battery (though, obviously, *above* that else you have no usable energy! :> )

And, of course, how you use the battery will also affect it's lifespan (how often it can be recharged as well as how *well* it can be recharged).

Battery manufacturers have detailed catalogs (and app notes) for each battery chemistry they support. You can examine these characteristics to determine the "best" chemistry for your particular application (usage patterns, size/weight/power ratio, cost, etc.).

Gates, Panasonic, Everready come to mind. I'm sure I have other catalogs on the shelf.

HTH...

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Oh geez, it wasn't even my book. So I would have to track down the owner. I'd consider it a lost cause.

Battery impedance very state of charge and state of health is still an on- going research topic because everyone wants to know the future from the present state. [As you know, fuel gauging is using a history lesson to predict the future.] It didn't take me long to find a paper on this. Damn google hides the URLs, but do a search on "Capacity Measurements of Li-Ion Batteries using AC Impedance Spectroscopy". The paper is 2009.

The trouble is you really need to whack a battery to test the impedance, so doing such a test in a real product is silly. You need to incorporate the measurements in the product so they occur while you use the battery in normal operation.

It wouldn't surprise me if every electric car manufacturer has a few engineers working on such techniques. It makes for bad press when your \$70K electric car gets stranded by a NY Times reports doing an article on the technology.

Look at the conclusions in this paper: "The data obtained from the experiments shows that the battery capacity potentially could be estimated using EIS as a diagnostic tool. It is however not straightforward and requires a substantial knowledge of the battery in question."

Translation: this is kind of difficult!

Nobody does fuel gauging this way, but I think an approach would be to simply divide the battery pack into sections and drain them sequentially. If you did "quarter" packs, when you hit the last pack, tell the driver to go get a charge pronto.

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On Fri, 10 Jan 2014 23:51:22 -0800 in sci.electronics.design, miso wrote,

Of course that kind of deep discharge is much harder on the battery life than having one big battery and only discharging it part way.

>
• posted

The cell life would take a thump -- most battery chemistries do not like being deeply discharged.

It may be possible to instrument the cells themselves -- certainly with a lead-acid battery, if you could monitor the pH you could get a pretty good indication of the state of charge.

My bet is on coulomb-counting, though. Or maybe coulomb-counting with some of your fancy impedance-probing at the beginning of each recharge, to assess the actual state of charge of the battery (or better, of each cell).

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Tim Wescott
Wescott Design Services ```
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Capacity testing of VRLA Batteries. Yeah, I know its for large batteries, but the same procedures apply to smaller batteries.

This might be the book in question: It's mostly on the chemistry and construction of VRLA batteries. I couldn't find anything in the ToC on how the batteries are rated.

There's probably more on the topic, but I gotta crawl under my car and fix the clutch (again).

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com ```
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You have a good point here. The hybrids try not to do full cycles (charge and discharge) to extend the life of the battery. However I think for electric cars, they do full cycles because range is a selling point. You could rotate which cell gets to be the last quarter cell, so over time, all the cells would be exercised fully.

Battery management is a pain in the ass. It takes forever to prove anything if you do the tests at normal operating conditions, and if you don't, you need to know how to compensate for testing under non-standard conditions (i.e. accelerated testing). In the meantime, the battery chemistry changes and your measurements are now history.

There was a issue about battery burping back in the Nicad days and into the NiMH era. Burping is when you discharge a bit periodically during the charge cycle. It is patented with very little proof that it actually does any good. But it was a feature in some chargers and you looked less competitive without it. It would really take a long time to prove it does anything, or if it is not as good as discharge before charge, which is also dubious.

The holy grail is still this instantaneous state of charge/health measurement. It is kind of like cold fusion. You really really want to believe it can be done.

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I believe the purpose was to reduce the growth of dendrites. I think they do the same thing with electroplating to get a smoother surface.

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Burping battery chargers go back at least 30 years (maybe 40 if I looked really hard for references!) as I saw a published article describing its application to "car batteries" back then.

IIRC, it was originally intended as a means of getting more charge into the "battery" in less time -- by dislodging hydrogen bubbles forming on the plates (thereby effectively reducing their area).

After that, I believe some effort was made to *shape* the pulses (instead of just naively whacking the battery with the equivalent of a big load) in an attempt to dislodge sulfate formation on the plates (once the plates get "coated" with this stuff, they are effectively "insulated" from the battery's chemistry -- the battery is irrecoverably DEAD. If you intervene before the sulfate layer "hardens", the battery can be saved).

It has since moved on to be applied to other battery chemistries... for assorted reasons. :-/

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I think you might want to push that from 30 - 40 years to 40 - 50 years, i think i read an article about that in _Popular Electronics_ by the early

1970s, possibly the late 1960s.

Battery technology and plating technology share so much that there is a regular circulation of experts between the two.

?-)

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