Problem is having a testing standard that all adhere to. Take for instance a vehicles MPG (miles per gallon or kilometers per liter if you are metric) rating. Without an EPA in the US you could have rather divergent claims for what any vehicle had. This can be a big deal when people shop for a car.
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That's only true if the voltage is constant over the period of time you do the measurement, or if you actually measure the average voltage. Just taking the nominal voltage is not correct since the nominal voltage is only equal to the average voltage at 0.2C.
That's the reason batteries have mostly been rated at amp-hours - and because you can check this easily with relatively basic equipment. If you for example have one of these:
Your Wh test will be way off, but your Ah test will be pretty spot on.
Again, my main issue is that if you test with the above meter and come to an Ah rating, that the vendor then tells you that your measurement is wrong.
That's only true if the voltage is constant over the period of time you do the measurement, or if you actually measure the average voltage. Just taking the nominal voltage is not correct since the nominal voltage is only equal to the average voltage at 0.2C.
That's the reason batteries have mostly been rated at amp-hours - and because you can check this easily with relatively basic equipment. If you for example have one of these:
Your Wh test will be way off, but your Ah test will be pretty spot on.
Again, my main issue is that if you test with the above meter and come to an Ah rating, that the vendor then tells you that your measurement is wrong.
Interesting and informative thread, and some good points raised by many.
I get your point but in the context of home consumers it's just way to complicated.
Some of us understand. And agree.
I made the exact same points after Sun Fun Kits was busy using V x Ah = Wh math to back into whatever answer they wanted. I got blow-back from several people, and decided they just didn't understand my point.
A Wh measurement is far less accurate. Not only do you have to measure amps over time, but also voltage over time. Your measurement error more than doubled. This is the culprit/issue. They are convincing people that their cells are better by telling people to use the 3.2V nominal to divide which has been stated in this thread will always make it look like you have more Ah's then you really do and therefore you should pay them more for their "automotive grade" cells when in fact they aren't different then "regular" cells you can get cheaper from others.
Because how much Work (i.e. energy) a system can actually perform is usually what most people are interested in knowing. It's the end result of discharging the battery, and tells us what is required to charge it back up.
First off: I think most of us would agree that Watts and Watt-hours are the correct things to design and judge your system by. In that regard, you SHOULD use Wh when sizing the battery for your system.
The OP said this was about both cells and batteries.
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.
