Watt Hrs vs Amp Hrs

[BFL13]

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 measure your charging by AH instead of using power. Important--read all the comments too!

It also makes clear why this conversion needs the volts.

https://www.linkedin.com/pulse/amp-hour-ah-watt-hour-wh-how-properly-spec-energy-alkuran-ph-d-#:~:te...

https://convert-formula.com/ah-wh

[pianotuna]

BFL13,

My watt meter (120 volt) does show instantaneous load and can count watt-hours as well.

I prefer watts because when there is voltage sag--it still shows me the amount of energy I'm consuming.

That's how I found out that the roof air, on a hot day, after running continuously for several hours, draws up to 1920 watts.

That caused me to abandon my idea of running the air conditioner from solar. I simply don't have room for that many panels on the roof--and I'm at 102 inches in width--so I can't "do" awnings made from solar panels.

I could have done a double layer rack with actuators....but the cost prevents that.

[BFL13]

"The “amps” or “watts” display measures the rate of energy going in or out of your batteries so you can check your charging systems, and also to see how much energy each of your appliances use to help you conserve energy. "

So you look at the amps (or watts) display, and it says 10. That is the rate of energy transfer (power) that appliance is at.

But he also says "10" shows how much "energy" that appliance uses. Should that read, "power"? Seems the energy the appliance uses depends on how long you leave it on?

I don't know if the Trimetric does Watt hours to go along with the watts it can show. if it did you would use the battery capacity when full as your starting point for watt hr counting and SOC.

Some confusion over the watt hr rating of a battery exists so what to use as your starting point could be tricky. One LFP at 100AH says it is rated at 1280, but that is because the "nominal" voltage is 12.8

Its full SOC voltage is listed as 13.6v so now that would be 1360

If the rating is based on the max draw what does that make it?

(I will stick with AH to preserve my sanity 🙂 )

[DrewE]

Energy is defined (scientifically) as "the ability to do work", which admittedly does not in itself really make things clear. It's basically the oomph required to get something done: the amount of heat that must be added to a tea kettle to get the water inside from room temperature to boiling, or the exertion required to haul a weight up a hill, or whatever. A measure of energy does not tell anything about how fast the work happens, just what it takes to make it happen.

Power is the rate of energy transfer. More power means more energy is expended or used or whatever in a given period of time, and the work can be done more rapidly. A small, one horsepower engine could move an RV up a hill, very slowly; a 300 horsepower engine also can get it up the hill, but at a much more rapid rate. (It won't be 300 times as fast, but that's due to increased frictional losses from air resistance, etc. The amount of energy required to overcome gravity, ignoring other losses, is the same in either case.)

Watts are a measure of power, or rate of energy usage. The quoted blurb gets that reasonably correct, especially if you disregard the reference to amps (which, of course, are measuring electrical current flow--and in this particular case that is approximately an equivalent measurement, in practical terms, because the voltage remains more or less constant-ish.)

[FWC]

Watts (and Amps if you assume a fixed voltage) are a measure of power. Power is the rate at which energy is being consumed or produced. For a battery, the maximum discharge current * voltage is the power that battery can produce.

Watt hours (and Amp hours assuming a fixed voltage) are a measure of energy. Energy is the ability to do work - in the case of batteries we are specifically referring to chemical potential energy. For a battery the stored Watt hours is the energy the battery has available.

A battery can be capable of producing a lot of power while having relatively little energy (eg the LiPo batteries in an RC car) or can produce a modest amount of power but store a lot of energy (eg a deep cycle battery).

[BFL13]

My Trimetric 2025 will show watts instead of amps, but I have not used that. I can't figure out where it would use Watt Hours instead of AH though.

http://www.bogartengineering.com/wp-content/uploads/docs/TM-2025%20Users%20Instructions062017.pdf?bo...

It says the watts reading is to show "energy" I am lost on "energy" vs "power" as used in the various blurbs for batteries.

"The “amps” or “watts” display measures the rate of energy going in or out of your batteries so you can check your charging systems, and also to see how much energy each of your appliances use to help you conserve energy. "

[valhalla360]

BFL13 wrote:
How do you operate in Watt Hrs? Do you have a battery monitor that does that instead of AH?

Yes, many do provide watt-hr.

[DrewE]

For rough operation in Wh, you can simply multiply Ah by the nominal voltage of the battery (as opposed to whatever its current voltage is), assuming one is working with a single voltage and a single battery.

For accurately measuring energy usage, you'd integrate the instantaneous product of voltage and current over time--which is basically what an electric meter does. Of course, with AC power, phase relationships and power factors must also be considered; but for a DC meter, there are no phase relationships to worry about.

To determine the state of charge of a battery, simply integrating net charge or net power is too naïve to be useful, since the response of a battery is hardly linear thanks to the Peukert effect, among other things. To figure that reasonably accurately requires more complex math. I suspect in the majority of cases, determining the state of charge is really the goal, rather than just how many amp-hours or watt-hours have gone into or out of the battery.

[BFL13]

How do you operate in Watt Hrs? Do you have a battery monitor that does that instead of AH?

How do you get started? You have to enter a "full" amount for capacity at 100%.

An LFP rated at 100AH says it is rated at 1280 Watt Hrs . Turns out that is using the "nominal voltage" of 12.8v BUT

they also say the resting voltage at 100% is 13.6v which would be 1360 watt hrs. They say 40% SOC is 13.1v. 40 x 13.1 = 524 Watt hrs, and
524/1360 = 38.5% so about 40.

Meanwhile a 100AH 12v FLA batt says resting "full" is 12.73v so that is 1273 Watt hrs. 40% SOC is 11.96v = 478 watt hrs and 478/1273 = 37.6%

I don't know the rated watt hrs for the FLA, but it seems your watt hr monitor should be using the resting voltage when full as the capacity.

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BTW, that Battle Born LFP claims their battery has "2 to 3 times the power of the same size FLA."

I don't see how 1360 is even 2 times 1273 so that claim is sort of amazing? They also claim the same for its "energy density"--which is fine in that the same size FLA weighs more. 50 lbs vs 31 lbs so not even half the weight but whatever.

Is that as bogus as it looks to me, or am I just not understanding what they mean?

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So what do folks who monitor by Watt hrs really do? Does your monitor keep track of the battery's actual voltage? How? It needs the resting voltage, but you always have a loaded voltage of some amount while camping.

It would need the shunt ammeter to keep track of the amps, but what does it do with the time? The AH counter uses amps and time, but how would the monitor track watts vs time matching the amps with the voltage (which it only has the loaded voltage for)?

[MEXICOWANDERER]

Though this is true the pump will pump nearly the same gallonage at 12 as at 14. the current will change. It it may well increase as the voltage DROPS (or the other way around. Now you could read total watt hours and yes that would work.

Caution! Joke Follows

Is this why I hear no-starts crank faster and faster as the battery wears down?

DC is simple, less of one equals fewer of the other.

[jaycocreek]

Almost all of the so called solar generators are rated in watt hours not amp hours.

[larry_cad]

Here is my $.02 FWIW.

My favorite analogy involves working on an old car motor:

You have a big, long wrench to loosen a rusted head bolt. You put the wrench on the bolt and pull for all you are worth for about 10 minutes (until you are sweaty and sore) but don't move the bolt at all. THAT IS THE EQUIVALENT OF AMP HOURS.

Then you get smart and spray on some PB Blaster and let it set overnight. Next day you put the wrench on and with one pull, you loosen the bolt. THAT IS THE EQUIVALENT OF WATT HOURS.

In the first example you did NO work. You may think you did. You may feel like you did, but you didn't.


In the second example, you did work.

The word "WORK" here is the scientific or physical definition of the word

[valhalla360]

BFL13 wrote:
One of the comments mentions that Watt Hrs are good when the voltage stays the same as in the stick house.

With a battery falling in voltage as you draw from it, does that not make using AH better for seeing how much capacity is left in it?

Actually, AC voltage doesn't stay the same. It will sag when big draws come online...if on shore power, this can be in nearby RVs also.

Watt-Hr is better as Watts = Amps * Volts and both Amps & Volts can vary. If you aren't stressing your batteries too badly, Amps by itself is a fair but less acurate estimate of Watts.

[wa8yxm]

MEXICOWANDERER wrote:
100 amp hours @ 12 volts -- 24 volts 120 volts.

Amp hours is a shortcut. It is an assumption figure. You know how much energy you require. To say pump 100 gallons of water. Is the pump going to deliver the same gallonage at 13.6 volts as it does at 12.1 volts?

Though this is true the pump will pump nearly the same gallonage at 12 as at 14. the current will change. It it may well increase as the voltage DROPS (or the other way around. Now you could read total watt hours and yes that would work.

But let's say you have a 12 volt system
Ok, two GC-2 Six volt batteries, to make math easier let's round 'em down to 200 amp hours at six volts or 1200 watt hours. Put 2 in series and now you have 2400 watt hours... but it's still 200 amp hours.

That is the point of it all..

And there is so so much more when you go beyond the basic.

[BFL13]

I guess watts are ok if you know what you are doing. I like amps and AH.

I could care less about energy and power. All I care is that running a 10 amp load for 3 hours is 30 AH out of my AH allowance, and that I know when I have used up my allowance of AH, so it is time to recharge.

I like seeing how many AH I have restored during recharging so I know if I have restored enough.

Energy smenergy , it just hurts the poor old brain! 🙂