Amps v Amp hours v Amps/Hr

[jwcolby54]

Originally Posted by Privilege

Thanks for the thread Stu. I'm not challenging anyone here, I'm just checking out my understanding.

I've recently built a freezer and wanted to work out its energy usage. There's little use in knowing how many amps it uses whilst running as it cycles frequently. Also, the cycling periods will depend on ambient temperature, how many times I open it, etc... This will also vary from hour to hour so again, there is little use in knowing the amp/hr useage. What I'm really interested in is the average useage or amp/day. This is the same for my water pump...it will depend on how many showers I take, how often I flush the fresh-water head etc... So, if I can work out my amp useage per day (amp/day) I can work out how much I need to get out of my solar panels without going into the red.

Is that about right?

The problem with this is that it is not amps that are consumed, but watts. Or in fact amps ARE consumed but at a specific voltage, which taken together turns into watts. So it is incorrect (or not useful) to say that "my fridge drew 6 amps". Was it drawing 6 amps at 12 volts (6*12 = 72 watts) or was it drawing 6 amps at 14 v (6 * 14 = 84 watts).

What you REALLY want to know is how many WATTS (how much POWER) does my fridge consume when it is cooling. Then you still have to measure how many minutes / hour or hours per day to discover how many watt hours are consumed.

This matters because a battery stores electrons. It does not store amps or volts. Those are units used to express how many electrons the battery will have in it when it is fully charged.

So a battery may provide 25 amps at 12.75 volts or 318 watts (just pulled out of thin air as an example) but watch it with a volt meter and you can see the voltage drop. So if the load is held constant (same resistance) and the voltage drops from 12.75 volts to 12.65 volts in 60 seconds (a number pulled from thin air) the AMPS drop because the load is staying constant while voltage across the load is dropping, and so WATTS drop as well.

The watts consumed is the area under the curve across time. IOW, E (voltage) drops, I drops as well and thus Watts also drops. Unless you have an intelligent meter that is watching E and I coming out of the battery, getting exact Watts consumed is tough.

Which is when "close enough" comes into play.

In any event, simply asking "how many amps is my refri drawing" is like asking "how fast is the plane flying when it is going 100 miles."

If the battery is being charged and kept at 13.5 volts (thin air, middle of the day on solar) and the refri kicks on and draws 6 amps for 3 minutes, the answer is entirely different than if the battery is down to 12.2 volts (middle of the night, no solar) and the refri kicks in and draws 6 amps for 3 minutes. The power consumed from the two scenarios are pretty different.

And BTW the cooling results will likewise be very different. You are pumping heat, and (within limits) you will pump more heat per minute (hour) at a higher voltage. Assuming a compressor motor the horsepower goes up as the voltage goes up, allowing you to compress and pump more coolant.

[Stu Jackson]

Originally Posted by fryewe

1. They do calculate KW-hrs used, but only in conjunction with equalizing charges and test discharges, when determining actual capacity and battery health. Calculating KW-hrs or watt-hours used continually would be of little utility.

2. Knowing the A-H ratings of my batteries and the discharge rate, with knowledge of the discharge rates used to determine the ratings, I have a pretty clear running approximation of the states of charge and can compare my mental estimates with the battery monitor as a backup to it. Using watt-hours would not be useful.

1. I understand "they" to mean the electricians on board submarines. However, I might point out that "they" could also be our ubiquitous coulomb counter ah meters / (slash) battery monitors. The specs on my Link 2000 unit state that the algorithm measures the kW per hour and simply translates that to amp hours.

2. I agree.

[sailorchic34]

Originally Posted by fryewe

...you're just messing with StuM, aren't you?)

Cough... Mayyyybe.

[sailorchic34]

Originally Posted by jwcolby54

So a battery may provide 25 amps at 12.75 volts or 318 watts (just pulled out of thin air as an example) but watch it with a volt meter and you can see the voltage drop. So if the load is held constant (same resistance) and the voltage drops from 12.75 volts to 12.65 volts in 60 seconds (a number pulled from thin air) the AMPS drop because the load is staying constant while voltage across the load is dropping, and so WATTS drop as well.


And BTW the cooling results will likewise be very different. You are pumping heat, and (within limits) you will pump more heat per minute (hour) at a higher voltage. Assuming a compressor motor the horsepower goes up as the voltage goes up, allowing you to compress and pump more coolant.

Hum I hate saying this, but for a induction motor load, should the voltage drop, the amps consumed could actually increase, at least up to a point.

For a DC direct drive motor Increasing voltage will increase RPMS, while amp draw stays the same and watts would increase as voltage rises. For a AC motor a change in voltage down or up would effect amp draw and the speed would be constant.

In a marine DC fridge, the motor is actually an AC induction motor and the little black box is an dc to AC inverter. So different voltages effect amp draw, but does not increase the speed of the motor. For an AC motor you change the frequency.

[jwcolby54]

Originally Posted by sailorchic34

Hum I hate saying this, but for a induction motor load, should the voltage drop, the amps consumed could actually increase, at least up to a point.

For a DC direct drive motor Increasing voltage will increase RPMS, while amp draw stays the same and watts would increase as voltage rises. For a AC motor a change in voltage down or up would effect amp draw and the speed would be constant.

In a marine DC fridge, the motor is actually an AC induction motor and the little black box is an dc to AC inverter. So different voltages effect amp draw, but does not increase the speed of the motor. For an AC motor you change the frequency.

LOL. Oh.

Well I was kinda under the impression we were discussing DC (batteries). I must say I have never inspected a DC refri except the RV kind which are actually phase change thingies and just use heat generation. Is Phase Change thingie technical enough for this conversation?

I have a peltier cooler from WalMart on my boat. Which is a solid state heat pump which would (I assume) use DC calcs.

[StuM]

[QUOTE=jwcolby54;1935828actually phase change thingies and just use heat generation. Is Phase Change thingie technical enough for this conversation?[/QUOTE]

Generally know as a eutectic fridge or freezer.

[valhalla360]

Originally Posted by Cottontop

But I really wish everyone would use watts and watt-hours for power and energy.

When people talk about their 120v fridge using 10 amp-hours each day, I am left confused by not being sure if they are talking about 120v amps, or 12v amps.

The standard for cruising boats dealing with battery storage is 12V, but as long as the voltage is the same going in and going out, voltage doesn't matter (if you use 5amp-hr at 120V and replace 5amp-hr at 120V you still have a system that recharges).

Yes, wattage is more precise but it takes an extra step each way to convert to watt-hrs when it doesn't matter unless you are switching between voltages (ie: using an inverter to change 12V to 120V)

[valhalla360]

Originally Posted by sailorchic34

I'm not so sure its me that's missing the point.

0.1gph/day would be the same as saying 2.4 GPD (Gallons per day). Where 0.1 GPH would be a tenth of a gallon per hour. Both are volumetric. FPS, MPS, measures velocity. Feet per second or meters per second is a Speed rate. GPH or GPM is a volume rate.

Now say 2GPH for 30 minutes and its a single gallon volume moved not a speed rate of flow. Or 2GPM/HR would be 120 gallons. 2GPH/hr is a bit redundant. Neather give you the velocity of flow.

0.1gph = 2.4gpd

0.1 gph/day = 2.4 gal/(day^2)

gph is a flow rate not a volume measure.

gal/(day^2) is the rate of change of the flow rate.

Let's say you have a hose filling a tank. Without additional information, if you have 0.1 gph/day, you know neither the flow rate nor the volume in the tank without adding more information.

If you honestly have an engineering background as you claim (in the recent related thread)...I weep for the engineering world.

Sorry if that is harsh but several of us have tried to explain a very simple terms how this works and you clearly don't understand the concept no matter how much experience you claim.

[sailorchic34]

[QUOTE=valhalla360;1935888
gph is a flow rate not a volume measure.


LOL, So funny.

How do you calculate a flow rate. You need the volume of flow to even get GPH. It's gallons per hour so 10 GPH is 10 gallons moved for one hour. A gallon has volume, mass, etc. There are 8.33 pounds per gallon of water and 7.48 gallons per Cubic foot of water at STP. There is mass, and volume in that. It's volume x time. Yes its a rate but that rate is in gallons per hour. Easily calculated. And engineers do this day in and day out.

If you know GPH you can calculate tank volume, pump HP, pump efficiency, velocity (with pipe diameter), etc. I do this many times a week.

[LakeSuperior]

Originally Posted by valhalla360

0.1gph = 2.4gpd

gph is a flow rate not a volume measure.


If you honestly have an engineering background as you claim (in the recent related thread)...I weep for the engineering world.

Sorry if that is harsh but several of us have tried to explain a very simple terms how this works and you clearly don't understand the concept no matter how much experience you claim.

valhalla360 - Harsh words to get it wrong yourself. gph is a volume flow rate and if the fluid is identified then it is equivalent to a mass flow rate (density X volume). This stuff has been cut and dried for 100's of years. If you don't have the formal training don't try to make this stuff up as it will only make you seem foolish.

A gallon is a measure of volume it is defined as 231 cubic inches. Get it? gph - volume per hour, volume flow rate. I just looked in my Halliday and Resnick, vol 1, to verify.

[Cottontop]

Originally Posted by valhalla360

The standard for cruising boats dealing with battery storage is 12V, but as long as the voltage is the same going in and going out, voltage doesn't matter (if you use 5amp-hr at 120V and replace 5amp-hr at 120V you still have a system that recharges).

Yes, wattage is more precise but it takes an extra step each way to convert to watt-hrs when it doesn't matter unless you are switching between voltages (ie: using an inverter to change 12V to 120V)

Read a domestic fridge v. marine fridge thread. You'll see amp-hours being used as a proxy for energy use in ways that thoroughly muddy the water.

Is 12v still the "standard" for boats? Maybe. But when the discussion involves more than one voltage, a little precision is useful, I think.

[StuM]

Originally Posted by sailorchic34

LOL, So funny.

How do you calculate a flow rate. You need the volume of flow to even get GPH. It's gallons per hour so 10 GPH is 10 gallons moved for one hour. A gallon has volume, mass, etc. There are 8.33 pounds per gallon of water and 7.48 gallons per Cubic foot of water at STP. There is mass, and volume in that. It's volume x time. Yes its a rate but that rate is in gallons per hour. Easily calculated. And engineers do this day in and day out.

If you know GPH you can calculate tank volume, pump HP, pump efficiency, velocity (with pipe diameter), etc. I do this many times a week.

I have a 60 gph pump. Please tell me my tank volume,

[StuM]

Originally Posted by LakeSuperior

valhalla360 - Harsh words to get it wrong yourself. gph is a volume flow rate and if the fluid is identified then it is equivalent to a mass flow rate (density X volume). This stuff has been cut and dried for 100's of years. If you don't have the formal training don't try to make this stuff up as it will only make you seem foolish.

A gallon is a measure of volume it is defined as 231 cubic inches. Get it? gph - volume per hour, volume flow rate. I just looked in my Halliday and Resnick, vol 1, to verify.

One more who doesn't appreciate the critical difference between "volume measure" and "volume flow rate" ?

It's the same as the difference between miles and miles per hour.
Miles is "distance measure". miles per hour is "speed" or "distance movement rate".

It's when you and SC use these two different concepts as though they are synonymous that all the trouble starts.

[jwcolby54]

Originally Posted by StuM

I have a 60 gph pump. Please tell me my tank volume,

Easy if we know the time required to fill.

[sailorchic34]

Originally Posted by StuM

One more who doesn't appreciate the critical difference between "volume measure" and "volume flow rate" ?

It's the same as the difference between miles and miles per hour.
Miles is "distance measure". miles per hour is "speed" or "distance movement rate".

It's when you and SC use these two different concepts as though they are synonymous that all the trouble starts.

Your arguing with a physic's text Stu. It's not the same as miles and mph. Miles and mph are distance measurements. Gallons is a unit of volume and gph is a unit of volume per hour. You know, I've worked with GPM and GPH every day for a really long time, in that engineering thingy. You can insult me all you want but that does not at all change physics.

As to the size of your tank I don't really care what size it is. But I have used GPM and GPH to size water and fuel tanks, and for determining pump size. mechanical engineers do this all the time. It's actually on the professional engineering exam for goodness sake.