Ah or Wh

[Doctorbass]

Ah or Wh

There is a big question about using Wh instead of Ah.

Many person use the term Ah instead of Wh to discuss about the energy reserve they have about their EV.

Personally each time I see that I don’t understand why it’s like that.

Actually, many rechargeable battery have a wide range of nominal voltage depending on their type of chemistry.

That mean if you talk about Ah with the lead acid battery to a guy that have lithium battery, you can’t really compare the energy your ev have because energy is more relevant to :

THE NOMINAL VOLTAGE x THE NOMINAL CAPACITY

V x Ah = energy in Wh

It is easy to see here that lead acid that have 50Ah at 2V per cells is 100Wh energy
And that lithium of also 50Ah at 3.7V is 185Wh

Then for the same Ah, a lithium have a lot more energy than the pb battery.

I mean that people talk about energy when they use Ah to compare the range that their EV can get…


So why people still talk about Ah when they refer to the energy they have???

Maybe Ah is used to refer to that because lead acid existed for long time and new battery chemistry are recent and the term have not been adapted?

Doc

 

[Ypedal]

Specially in ebike talk, given that we use such a wide range of voltages.. AH starts to get mis-used to represent range... i agree.

There is a larger percent of the general population that understands AH than WH.. so for simplicity's sake it's more popular.

Good point tho.

my 72v 20ah lithium pack has 1440 wh

vs

my 36v 12ah SLA pack that had a hard time with 250 wh lol..

 

[safe]

My SLA's have 1368 Wh, but after Peukert you get about half that...

I prefer Watt Hour.

1368 Wh of SLA cost me $138.

 

[jondoh]

Why not killowatt hour? If you read your electric bill, you understand you get charged by the killowatt hour. My battery pack is approximately 0.5 kwh which is about 6 cents of electricity depending on your rate. Depending on my speed, i get between 10 and 20 miles range with 0.5 kwh.

 

[Drunkskunk]

Watt hours is more accurate, but its a dimensionless number for most people.

I could also tell you I have a 234,000wh fuel tank on my truck, but its easier to relate to that information by telling you its 26 gallons.

But If I tell you its 26 gallons, I also need to give you another demsion. Like to tell you its gas, or diesel, or ethinol. None of those have the same watt hour rating.

Its the same with batteries. 10 amps of SLA doesn't have the same wh as 10 amps of lithium. Sure, the numbers add up the same, But If I tell you its SLA, you know that its only good for 60% or so of that 10 amps.

By telling you I have 10 amps of SLA, I'm giving you what my friend Sarah calls "Layered Information". I've told you not just what my capacity is, but in part, also what my weight is, and what my over all bike design is like.

 

[EMF]

I go by watt hour since it is voltage independant. As someone else already mentioned, we pay for our power usage by the watt hour and this is why. It makes sense to use watt hour to judge where you are at with your pack. Watts is power and we want to know how much power we have used or have left, so we don't get stuck somewhere..

 

[TylerDurden]

I go by watt hour since it is voltage independant.

It can often be an issue of context, like when we use pronouns.

If I want to describe my setup, I say: "48Ah of 24V Emoli". That describes V, Ah and chemistry.

Later, I may assume the other people in the discussion will know I'm operating at 24V and I will simply use the Ah as a metric for capacity.

 

[Link]

I do it the way Tyler does. Whr doesn't tell you the size or voltage of your pack. Ah @ V is more descriptive.

 

[TylerDurden]

...and if you can't brag about the size and voltage of your "pack" , well... why buy batteries at all??
8)

 

[jjackstone]

I also use WHrs for energy description...but the AmpHour rating is much better for charge/discharge characteristics.
JJ

 

[Dalecv]

1 coulomb is the amount of electrical charge in 6.241506 × 1018 electrons
1 ampere represents the rate of 1 coulomb of charge per second
1 ampere-hour is equal to 3,600 coulombs (ampere-seconds)

The amp hours of a cell describe how many electrons are available for use from that cell. I think it is fairly easy to visualize as a unit of measurement because it can be described as a volume of electrons.

Trying to visualize watt hours is more confusing for some because it is a combination of two basic units of electricity, amp hours multiplied by volts.

Neither amp hours nor watt hours are a true measurement of a battery's capacity for work because of additional variables. A 48 volt 50 amp hour rated battery has 2,400 watt hours of potential energy no matter what the battery chemistry is.

How fast we can get the electrical charge out of a cell (the C rate) along with the volume of electrons available is the real measurement we are all interested in and attempting to use amp hours or watt hours to describe these capacities is lacking.

We should have a rating that takes amp hours multiplied by the continuous discharge C rate multiplied by the volts to express a battery's capability and maybe we could call it "A_CV" rating

 

[safe]

Power output is really about "total energy supply" which is best considered in terms of Watt Hours then that total figure needs to be proportioned in terms of an Efficiency Value.

In the case of SLA the Watt Hour figure might be high, but it's ability to be efficient with it's energy is low in real world riding conditions.

So this way people would think in two separate mindsets... one in absolute terms and another in relative terms based on their bike. Things like motor configuration (volts verses amps) do not always translate into a simple efficiency calculation. Even the idea of more volts is not an absolute because you have cases like the PMG 080 that gets relatively high efficiency and power at only 24 volts.

Other factors like aerodynamics factor very large in any efficiency calculation because (like in my case) my SLA batteries are inefficient, but my aerodynamics are good enough that I get a yield out of SLA's that most people would not expect. That aerodynamic component is all about efficiency and has nothing at all to do with the battery.

 

[Toorbough ULL-Zeveigh]

1 coulomb is the amount of electrical charge in 6.241506 × 1018 electrons

*nitpik correction*
x 10^18

Any idea for the basis of this number or how this fudge factor was derived?

 

[Frank]

Wikipedia says that:

"Historical note

The ampere was historically a derived unit—being defined as 1 coulomb per second. Therefore the coulomb, rather than the ampere, was the SI base electrical unit.

In 1960 the SI system made the ampere the base unit. [1]"

http://en.wikipedia.org/wiki/Coulomb

 

[Freddyflatfoot]

When talking about Whrs, is it more correct when talking about the capacity of a LiFePo4 pack as 3.2v x 10 AHr x 8 = 256 whr, or 24 v (nominal) x 10Ahr = 240whr? Might be splitting hairs, but there is a difference, and that difference will get much larger as the number of cells increase.
Or should we just be using the KISS principle, and talk about nominal capacities?
I was thinking about this, because we often talk about having, say a 24, 36 or 48 volt pack, and then talk about the volatges we have above that, usually another 1.5 - 3 volts. To my way of thinking, that is actually extra capacity, or am I barking up the wrong tree?

 

[Link]

When talking about Whrs, is it more correct when talking about the capacity of a LiFePo4 pack as 3.2v x 10 AHr x 8 = 256 whr, or 24 v (nominal) x 10Ahr = 240whr? Might be splitting hairs, but there is a difference, and that difference will get much larger as the number of cells increase.
Or should we just be using the KISS principle, and talk about nominal capacities?
I was thinking about this, because we often talk about having, say a 24, 36 or 48 volt pack, and then talk about the volatges we have above that, usually another 1.5 - 3 volts. To my way of thinking, that is actually extra capacity, or am I barking up the wrong tree?

No, you might be right. And consider that every chemistry isn't EXACTLY what it's specced at. And then you have to factor in discharge rates, how well the batteries can handle high discharge rates, internal resistance, how cold it is, how old the batteries are, etc., etc., etc....

It pretty much becomes impossible to measure EXACTLY how much energy you're going to get for any one ride, so you have to make do with an approximation.

Anyway, getting back to the actual question, I can only think that perhaps its because it seems more relative to their EV. Since the voltage is more or less fixed (friggin' SLA ), you just skip multiplying that with amperage. I do. It's easier to think, "Okay, if I keep my amperage draw to 20 amps and I have 10Ah usable I can ride for another half-hour." Just seems that way to me, anyway.

 

[safe]

1 Watt-Hour = 3600 Joules

http://en.wikipedia.org/wiki/Joule

Then after that it's all about efficiency and there are so many factors individual to the bike that it's too complex to calculate. The Watt Hour or Joule is the "ideal" amount that the battery can achieve in perfect conditions. Everything else is just relative to that.

So for the "idealized" Watt Hour or Joule count you would likely test at 0.1C, but the "real world" usually means 1C to 10C and is worse.

They pretty do it like this right now, so I'm happy with the way things are. You always assume 0.1C as perfection and then calculate DOWN from there.

 

[Dalecv]

1 coulomb is the amount of electrical charge in 6.241506 × 1018 electrons

*nitpik correction*
x 10^18

Doh! I can't even copy and paste good sources.