Why not KWh rating for batteries?

[ddw36]

I'm an electrical/electronics "innumerate." I've read much (maybe not enough) trying to understand DC, esp. batteries. I know batteries are rated in volts and Ah, but why not express their output in Wh, since volts x amps =watts? Does it matter whether a battery is 12v, 50 Ah (600 Wh)or 24v x 25 Ah (576 Wh)? It may be an AC thing.



The Tesla Semi battery is rated in Kwatt hrs:


https://www.teslarati.com/tesla-semi-underestimated-500kwh-battery-convoy-mode/


So, if those batteries are rated in basically watts, why can't boat, etc., batteries be rated the same way?

 

[Sabre602]

They can be, but it doesn't provide enough information. The voltage is key and there are such a variety of systems that use different voltages. Once you have established the system voltage you need, you can then choose batteries that suit your needs. Converting to kwh does help keep comparisons simple if you're looking at alternative configurations. For example, the common use of 6 volt flooded cell batteries, most often wired as 12 volt banks. Many boaters look at alternative battery technologies in replacing them, and the use of kwh makes capacity comparisons easy.

 

[STB]

This gets a little nuanced...

A watt is a measure of the rate at which something uses or produces energy, i.e. Joules/second.

In a simple DC system, the Wattage is equal to Volts*Amps, i.e., is equal to a Volt-Amp.

This gets more complicated from the get-go with an AC system, where the voltage varies over time. In the most basic cases we need to take the average magnitude of the voltage, i.e. the RMS voltage, times the current.

But, then it gets more complex from there, what if the current curve varies, just like the voltage curve?

Worse, what happens if the current and voltage are oscillating out of sync?

Worse, yet, what if the oscillation of each occurs at a different frequency?

Or, even worse, each occurs with a curve representing a different combination of frequencies?

In these type of "reactive" circuits, the math gets fun and finding the wattage supplied or consumed isn't as easy as multiplying peak voltage times peak current, or even averages.

Watts can be added up linearly, as they've baked in the complexity. Volt-Amps, not so much, because of the phase stuff -- but adding them will never give a result that is larger than the watts, so is useful as a bound.

Technically speaking, Amp-Hours is a unit of charge, which is what batteries actually store. Watts-hours are a unit of energy, the capacity to do work.

So, when Tesla puts a Watt-Hour rating on a battery, they are really commenting about more than the charge the battery can store, they are implicitly commenting about what they can achieve with it.

 

[jhall767]

This gets a little nuanced...

A watt is a measure of the rate at which something uses or produces energy, i.e. Joules/second.

In a simple DC system, the Wattage is equal to Volts*Amps, i.e., is equal to a Volt-Amp.

This gets more complicated from the get-go with an AC system, where the voltage varies over time. In the most basic cases we need to take the average magnitude of the voltage, i.e. the RMS voltage, times the current.

But, then it gets more complex from there, what if the current curve varies, just like the voltage curve?

Worse, what happens if the current and voltage are oscillating out of sync?

Worse, yet, what if the oscillation of each occurs at a different frequency?

Or, even worse, each occurs with a curve representing a different combination of frequencies?

In these type of "reactive" circuits, the math gets fun and finding the wattage supplied or consumed isn't as easy as multiplying peak voltage times peak current, or even averages.

Watts can be added up linearly, as they've baked in the complexity. Volt-Amps, not so much, because of the phase stuff -- but adding them will never give a result that is larger than the watts, so is useful as a bound.

Technically speaking, Amp-Hours is a unit of charge, which is what batteries actually store. Watts-hours are a unit of energy, the capacity to do work.

So, when Tesla puts a Watt-Hour rating on a battery, they are really commenting about more than the charge the battery can store, they are implicitly commenting about what they can achieve with it.

Since Watt-Hour is how much a battery can put out a 100AH 12V lead acid battery should have a different (lower) KWH rating than a 100AH 12V Lithium that can be discharged much further.

So if I'm selling lead acid I want to talk about AH but if I'm selling lithium I want to talk about KWH

 

[STB]

Since Watt-Hour is how much a battery can put out a 100AH 12V lead acid battery should have a different (lower) KWH rating than a 100AH 12V Lithium that can be discharged much further.

So if I'm selling lead acid I want to talk about AH but if I'm selling lithium I want to talk about KWH

I'm not a battery expert, so I could be very wrong. I am standing by to get schooled.

But, my understanding is that the Amp-Hour rating for a battery is specified either for a specific rate of discharge, specified either as amps, e.g. 4A, or via a C-rate.

A 1C discharge rate would fully discharge the battery in 1 hour. A 5C discharge rate would fully discharge the battery in 1/5 of 1 hour or 12 minutes, etc. And that, if neither is specified, a 1C discharge rate is likely for common battery types.

My understanding is that a "full discharge" leaves the battery unable to deliver the "minimum useful voltage" for any longer. For lead acid batteries, I think this is, by convention, something like 1.5 volts per cell, or roughly 3/4 of the full voltage.

I don't think that is a level of discharge most of us would like to subject our batteries to. But, that isn't really the idea. The idea is really to provide a way of comparing similar batteries -- not different chemistries.

At any rate, my understanding is that the Amp-Hour rating we see for a common battery is the number of amps that it can deliver in 1 hour at a rate of discharge, such that, at the end of that hour, it leaves the battery unable to further satisfy the minimum useful voltage, however that is defined for the type of the battery. I don't think it considers the battery lifetime.

Having said that, I think regular batteries can discharge as fast as you let them, even to the point of a "kaboom event", but many advanced lithium batteries have protection circuits to limit the discharge rate to a safe rate, or even to one that ensures significant longevity.

 

[DDW]

Amp hour ratings for deep cycle batteries are traditionally given in the number of amps it can deliver when being discharged at the "20 hour rate" - the rate at which it will be depleted in 20 hours (not 1 hour). Start batteries are rated differently, CCA or Reserve Minutes, which are at a high discharge rate appropriate to starting motors.

For most purposes on a boat, the Ah rating and the KWh rating are directly related by a constant (the voltage) so neither is complete, and one offers no more information than the other. What might be interesting is to have them rated as KWh-life, that is how many KWh will they produce in their life cycle. That is where you would see the differences in chemistries and manufacturers, and would implicitly include depth of discharge information. Even then, a lot of information is lacking.

 

[pjtemplin]

Very valid point. I often say to people, half-jokingly, "would you ever shop for a house based on how wide it was? No, you shop based on square footage. Same thing applies here."

 

[twistedtree]

I think there are a few reasons for the differences, even though both measures are mathematically related and interchangeable.


The first reason is just habit. For deep cycle batteries, they have just always been measured in Ah.


The other reason is what Sabre602 mentioned which is that it's an easy way to compare batteries of the same voltage, and historically that's been 6V and 12V, and multiples thereof.


In contrast, with a car, the voltage is invisible and irrelevant to a buyer, so kWh is the only meaningful comparison. It's also starting to appear more and more in the specs for LFP batteries, as Jhall767 points out, since in daily use much more of the rated Ah capacity is usable with an LFP matter vs a lead battery.