Watt Hrs vs Amp Hrs
This article helps explain these ratings and raises a question wrt battery charging older vs newer LFPs too. Not sure if this also applies to other battery types or if it matters with any when you me...
Forum Discussion
road-runner wrote:pianotuna wrote:
amp-hours are voltage dependent
watt-hours are not voltage dependent.
That whole article could have been written around these 2 lines instead of overcomplicating it to confusion.
And that would be subtly incorrect. It's not that one is voltage-dependent while the other is not; rather, they're measuring fundamentally different things, which can be related to each other by voltage.
Amp-hours are a measure of charge, which is conceptually independent of voltage. One amp-hour is (by definition) 3600 coulombs. Watt-hours, on the other hand, are a measure of energy. If you want to know how much work you can get out of a battery, that's a question of how much energy is stored in the battery, and would most correctly be measured in watt-hours or some other unit of energy. If all you have is charge, then you need take voltage into account and, at least implicitly convert the charge to energy to get a valid answer.
There are times when charge, not energy, is really what you care to measure. How long will it take to recharge a battery at 10A charge current? If it's a 100 Ah battery, the answer is 10 hours (ignoring inefficiencies), and that's true whatever the voltage of the battery is. If all we know is that it's a 1000 Wh battery, there's no way of telling without knowing the voltage and converting the energy to charge. Here, it seems that Wh are voltage-dependent while Ah are not.
