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Topic: Amp Hours for AC appliances - Can you help clear this up?

Posted By: ZZSPIRAL1 on 03/08/12 02:33pm

Edit: The original post is an error, because my calcuation for the second example is wrong.

The first example suggests 10 amps per 100 watts. Based on their example, a 24 watt TV will use approx. 2.4 amps an hour using an inverter.

The second example suggests dividing 120 (120v TV) by 12 (12v battery) and multiplying the results (10) by the TV amps in ac mode (.20). Based on their example, the TV will use approx. 2.0 amps an hour using an inverter:
(24 watt, 120v TV is .20 amps)......(120 divided by 12 is 10).....(.20 amps multiplied by 10 is 2.0)


As you can see they're close. Better to error on the side of caution and use the first example. Sorry for the confusion.

____________________________________________________________________






I'm researching information in prep for a solar system, and I came across confusing information. Maybe these are both correct, but I can't see how. I have a 24 watt AC TV and I'm trying to determine how many amp hours (ah) it will demand using an invertor and a single 12V battery. Here's the information. Can you clear this up?
________________________________________________________________

Information Source One:

Put simply, for every 100 watts of AC power that your inverter is producing, it needs to draw about 10 amps from your 12 volt battery system.


Link: http://www.marxrv.com/12volt/12volta.htm

________________________________________________________________

Information Source Two:

One of the biggest mistakes made by those just starting out is not understanding the relationship between amps and amp-hour requirements of 120 volt AC items versus the effects on their DC low voltage batteries.

For example, say you have a 24 volt nominal system and an inverter powering a load of 3 amps, 120VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah).
However, in order to determine the true drain on your batteries you have to divide your nominal battery voltage (24v) into the voltage of the load (120v), which is 5, and then multiply this times your 120vac amp hours (5 x 12 ah). So in this case the calculation would be 60 amp hours drained from your batteries – not the 12 ah.


Link: http://www.12voltsolarpanels.net/deep-cycle-battery-solar-charging

_______________________________________________________________
Summary:

Unless I'm mistaken:

The first source suggests that a 24 watt TV requires about 2.4 amps per hour (24 watts divided by 10 amps = 2.4 amps per hour).


Assumming the TV is 120 volts, the second source of information suggests that a 24 watt TV will require about 12.4 amps per hour (120V TV divided by 12V Battery = 10). (2.4 amps per hour times 10 is 12.4 amps per hour).

What am I missing?Edit: The original post is an error, because my calcuation for the second example is wrong.

The first example suggests 10 amps per 100 watts. Based on their example, a 24 watt TV will use approx. 2.4 amps an hour using an inverter.

The second example suggests dividing 120 (120v TV) by 12 (12v battery) and multiplying the results (10) by the TV amps in ac mode (.20). Based on their example, the TV will use approx. 2.0 amps an hour using an inverter:
(24 watt, 120v TV is .20 amps)......(120 divided by 12 is 10).....(.20 amps multiplied by 10 is 2.0)


As you can see they're close. Better to error on the side of caution and use the first example. Sorry for the confusion.


* This post was edited 03/09/12 05:35pm by ZZSPIRAL1 *


Posted By: donn0128 on 03/08/12 02:37pm

Ohms law!
Watts divided by Volts = Amps
To invert 12VDC back to 120VAC is roughly 10 to one
So for every amp of 120VAC will require approx 10 amps DC
To make it really simply look up Ohms law.


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Posted By: NORM WADDELL on 03/08/12 02:44pm

For starters............

there is no such thing as 'Amperes per hour'

An Ampere is a given quantity of electrons per second
so, an Ampere per hour would be electrons per second per hour,
which is a measure of acceleration......

Now that this is solved (?), others will solve the problem.


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Posted By: Sailingnuts on 03/08/12 03:02pm

Responding to the OP.
24Watts @120VAC = 0.2Amps

If you want to use your inverter being fed by a 12VDC to produce the same amount of Watts, then you will need 10 times more Amps or 2Amps.

For every Amp consumed using 120VAC you will need 10 times more when using 12VDC.

So if you have a battery that is rated at 100ah and assuming you will use it to 50% discharge then you can use your TV on for 25 hours straight (50ah/2Amp=25hs).

I hope it helps.


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Posted By: tenbear on 03/08/12 03:02pm

The 24 watt TV will draw about 2 amps at 12 volts plus a little bit since the inverter is not 100% efficient, maybe 2.2 amps . Multiply that result by the number of hours you will use the TV and that will be the ampere hours.

If your TV is a 12v TV that uses an adapter to run off 120v you could run the TV directly off the battery and not have the inefficient inverter producing heat and wasting power.


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Posted By: vermilye on 03/08/12 02:51pm

As Don noted, divide your watts by voltage to get amps. 12V for the battery side, 120V for the output of the inverter. For amp hours, multiply the battery amps times the length of time you use the load.

In the case of your 24 watt TV, if it draws 24 watts, @ 12V that's 2 amps. (At 120V it is only drawing .2 amps) At 4 hours per day you will use 8 amp/hrs. Of course that doesn't include the loss created by the inverter, but unless you are using a huge inverter it won't be more than 5%-10%.


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Posted By: NORM WADDELL on 03/08/12 04:56pm

In reply to one responder

The Power Company does NOT charge you for the Amperes
used PER HOUR !!!!!!!!! The bill is for Amperes TIMES
Voltage times Hours.........Volt Ampere Hours......Volts TIMES Amperes times Hours......they call 1000 (volts times Amperes)
a KILOwatt............

there IS NO Amperes per hour .........I promise !


Posted By: 2oldman on 03/08/12 05:00pm

LarryJM wrote:

Thus they charge you for the amps used per hour averaged out.
No, that's like saying they charge you for water by the gallons/sec per hour used. Just doesn't make sense.

If what you said makes sense to you, fine, but you won't be taught that in college physics.


Posted By: 2oldman on 03/08/12 04:13pm

ZZSPIRAL1 wrote:

What am I missing?
Source two is for a 24v nominal system. Yours is 12v.

Sailingnuts has it right. And again, there in no such thing as "amps per hour".


Posted By: wgriswold on 03/08/12 04:14pm

ZZSPIRAL1 wrote:


Unless I'm mistaken:

The first source suggests that a 24 watt TV requires about 2.4 amps per hour (24 watts divided by 10 amps = 2.4 amps per hour).


Assumming the TV is 120 volts, the second source of information suggests that a 24 watt TV will require about 12.4 amps per hour (120V TV divided by 12V Battery = 10). (2.4 amps per hour times 10 is 12.4 amps per hour).

What am I missing?


As I read the two examples the first is using a 12V battery system to make its calculations and the second is using a 24V DC system to supply the inverter.

Everything starts with the TV at 24 watts. That requires 0.2 AC amps. The 12V DC system must supply about 10 times that or about 2 amps. The 24V DC system must supply about 5 times that or 1 amp.

I suspect that you have a 12V DC system so the first example is correct. The confusion comes from mixing the DC voltages.


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Posted By: wgriswold on 03/08/12 04:45pm

LarryJM wrote:



Not totally correct since all electric bills are charges is KWH which is basically amps time time over a certain period which is generally an hour. Thus they charge you for the amps used per hour averaged out.


Here is how I understand it. KWH is a measure of energy and the total on your energy bill is the total energy used over the billing period.
Amp-hr is also a measure of energy and is used to measure energy consumption in our 12V systems. They are not directly comparable because of he different voltages involved but both are a measure of energy.

I think you are saying that if the amps over any time period (say 30 days) were averaged and then that average was multiplied by that time period we would have the total of energy (Amp-days, which would be proportional to KWH) used in that time period. Does that make sense?

It is interesting how different people approach the same subject. I always enjoy these conversations because it makes me rethink my way of thinking.

On the other hand, maybe I am just wrong.


Posted By: wa8yxm on 03/08/12 04:50pm

It can indeed be confusing. Ideally, if Inverters were 100$ efficient then the second source would be correct, And correcting for 12 volts. a device that draws say 360 watts, would need 360/12 amps at 12 volts. the "Amp hour" load is the amps, times the numbers of hours it runs. (That by the way is 30 amps so for every hour it runs it needs 30 amp hours of battery at the ONE HOUR rate. More on that later if you choose to read on that far.

However Inverters are NOT 100 percent efficient, they are closer to 90. and by the time you figure in all the other losses, using 10 volts works out surprisingly accurate so Source 1 is the better source..

And that 360 watt device thus needs 36 amps and for each hour it runs 36 amp hours of battery.

Now, the part that I said you could skip... (Short version YOU NEED MORE BATTERY) (You may stop here if you please)

Tradition has the battery rated at the 20 hour rate. Though some companies rate 'em at other rates to make 'em look better Wal-mart for example uses the one amp rate. Thus a G-29 Wal-mart SHOWS a capacity that is bigger than a G-31 anybody else, This is because of something called by a name I can not remember, but basically it says that the faster you discharge a battery, the faster still it runs down.. Here is a web page that explains it but alas does not name it.

Click Here for Info

But the bottom line is if you take a 230 amp hour battery (common on RV's) and draw it down at 2.3 amps then it lasts about 10 hours before needing discharge. (you can use 1/2 it's capacity safely on a deep cycle, LESS on a Marine/Deep cycle)

But if you draw it down at 4.6 amps it does not last 5 hours as you would expect, but perhaps only 4.


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Posted By: LarryJM on 03/08/12 04:23pm

NORM WADDELL wrote:



there is no such thing as 'Amperes per hour'

An Ampere is a given quantity of electrons per second
so, an Ampere per hour would be electrons per second per hour,
which is a measure of acceleration......

Now that this is solved (?), others will solve the problem.


Not totally correct since all electric bills are charges is KWH which is basically amps time time over a certain period which is generally an hour. Thus they charge you for the amps used per hour averaged out.


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Posted By: LarryJM on 03/08/12 05:17pm

CLICKY

It's amperage x voltage x time pure and simple

Larry


Posted By: 2oldman on 03/08/12 06:47pm

wgriswold wrote:

I think our problem is one of semantics. In math talk per means divide
Amp-hour (amps x hours) has meaning. Amps per hour does not.

You're right, it probably is just semantics. But in physics classes those semantics are pretty important.


Posted By: 2oldman on 03/09/12 06:29pm

ZZSPIRAL1 wrote:

a 24 watt TV will use approx. 2.4 amps an hour
OK...


Posted By: MrWizard on 03/08/12 08:48pm

as usual the debate descends into minutia
for RV's using 12v batteries to power 120v appliances via inverter
know the watts per device
calculate the watts by hours of use TV watts per hours of use (24*4=96watts total)
devide watts by volts (96/12=8) and you have 8 AmpHrs of use removed from a 12v battery

if you have One grp 24 size battery, its total power is approx 72 > 80 AmpHrs
to help make the battery last as many cycles as possible it is best to try and not remove more than half of the total available power

so 36>40 AmpHrs of use = 18 to 20 hrs of TV,
but you have other power uses lights are a big one
each 11watt bulb uses approx 0.8 amps
just one fixture with two bulbs uses 1.6 amps, so that's 1.6 AmpHrs for every Hr that fixture is turned on

and there is the water pump that's about 5>6 amps avg so 10 minutes in the shower uses about 1amphr of battery power

its a balancing act, but not difficult once you have a real idea of your daily useage pattern


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Posted By: MrWizard on 03/08/12 08:51pm

moved from technology corner


Posted By: tenbear on 03/09/12 06:38pm

2oldman wrote:

ZZSPIRAL1 wrote:

a 24 watt TV will use approx. 2.4 amps an hour
OK...


Well, it will use 2.4 Amps.
In 1 hour it will use 2.4 Ampere hours
In 2 hours it will use 4.8 Ampere hours
etc.


Posted By: smkettner on 03/08/12 09:34pm

24w will pull 2 amps from the 12v connector. If you run it for an hour then you will pull 2 amps for an hour or two amp hours. If you use an inverter it might be slightly higher due to internal losses.

Voltage is nominal. Wh or more commonly kWh takes voltage into the equation.

* This post was edited 03/09/12 10:30am by smkettner *


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Posted By: wgriswold on 03/08/12 06:37pm

I am not sure how meters work but I suspect they use LarryJM's formula over a very short period (seconds/milliseconds?)of time and then sum up all the measurements for the billing period.

I think our problem is one of semantics. In math talk per means divide. In common talk per can mean a period of time. So to say amps are measured per second can mean that the amperage is sampled every second, not necessarily that amps are divided by seconds.


Posted By: wgriswold on 03/08/12 07:06pm

2oldman wrote:

wgriswold wrote:

So to say amps are measured per second can mean that the amperage is sampled every second, not necessarily that amps are divided by seconds.
How many of you 'amps per hour' people have ever taken a physics class?


If you are implying that I am an amps/hr person you didn't read my post carefully.

One can sum instantaneous amp measurements over an hour and get the sum of amps measurements over that hour. That measurement could be sloppily called amps per hour. If each instantaneous measurement was multiplied by volts and then the resulting curve integrated we would get the KWH for that period.

I found this description of how meters work on Wikipedia:

"The most common unit of measurement on the electricity meter is the kilowatt hour, which is equal to the amount of energy used by a load of one kilowatt over a period of one hour, or 3,600,000 joules. Some electricity companies use the SI megajoule instead.

Demand is normally measured in watts, but averaged over a period, most often a quarter or half hour."

So one could talk about watts per quarter hour but not mean that the watts were divided by 0.25 hrs but rather that the number is the measured watts over a quarter hour.


Posted By: wgriswold on 03/10/12 06:52pm

Amp hrs is a measure of energy and can be easily converted to the more traditional BTU, joules, Calories, kWh, or whatever if the voltage is known. It is just a simple easy to keep track or energy in the battery and the energy used.

on edit: I found a tool to make these conversions:

http://www.onlineconversion.com/energy.htm

* This post was edited 03/10/12 07:19pm by wgriswold *


Posted By: BFL13 on 03/10/12 07:13pm

Ok time to get real picky!

The capacity at a 2.2 amp rate will be more than at the bank's 20 hr rate of, say 11 amps for a pair of batteries

Inverter efficiency needs to be wrt its curves where peak is maybe at 70% of its watts rating, and you get worse efficiency at the top end and when the draw is low, such as when using a small TV on a big inverter.

While watching the TV you will get "inverter amps creep" as the battery voltage falls, the inverter draws more and more amps.


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Posted By: liborko on 03/10/12 07:25pm

Amperehours is a good and useful REPRESENTATION of energy but it is not energy until multiplied by voltage. 1Ah@12.6V=1Ah*12.6V=12.6Wh. Only Watthour is accurate representation of energy and that is what utility company uses to charge for electricity. Or equivalent in different units like Joules for example.


Posted By: liborko on 03/10/12 05:37pm

All the amps per hour guys need to go back to school. There is big difference between amps PER hour and amphours. Norm Waddell is correct. There is no amps per hour. AND amphours is not energy. Watthours(or kWh)is.

* This post was edited 03/10/12 06:08pm by liborko *


Posted By: liborko on 03/10/12 06:34pm

You are mostly correct, Gdetrailer. 24W:12.6V=1.90A. The inverter I installed today is 82% efficient. That will make it 1.90A:0.82=2.32A. So for every hour the TV runs, he will use 2.32Ah. In four hours that will be 9.29Ah. By the way, the only correct way to refer to a kilowatthour is kWh. Not Kwh or Kwhr. It is called "technical language" and every symbol has meaning.


Posted By: wa8yxm on 03/09/12 11:07am

NORM WADDELL wrote:

In reply to one responder

The Power Company does NOT charge you for the Amperes
used PER HOUR !!!!!!!!! The bill is for Amperes TIMES
Voltage times Hours.........Volt Ampere Hours......Volts TIMES Amperes times Hours......they call 1000 (volts times Amperes)
a KILOwatt............

there IS NO Amperes per hour .........I promise !


Uh... Wrong.. You left out a variable.

The meter is a WATT-HOUR meter, not an Volt-AMP-Hour Meter.

You described Volt-Amps, Watts and Volt amps are the same in a DC system, and in a pure resistive AC-system, but how many of us have one of those?


Posted By: Gdetrailer on 03/10/12 06:13pm

liborko writes "All the amps per hour guys need to go back to school. There is big difference between amps PER hour and amphours. Norm Wadel is correct. There is no amps per hour. AND amphours is not energy. Watthours(or kWh)is."

Electric Company BILLS using Kwh (Kilo Watt Hours).

Battery capacity is measured in Ahrs (Amp hrs) not Whrs or Kwhrs!

OP is wanting to figure out just how much battery capacity will be used by the TV.

I am not sure as to why everyone is concerned about Kwh since the electric company has NOTHING to do with the OPs question which is just how much BATTERY CAPACITY will be used by the TV.

So going back to the OPs ORIGINAL post they have a TV which is rated to use 24W at 120V.

Power/Voltage=Current (abbreviated as P/E=I)or in the OPs case 24W/120V=0.2A.

Now we KNOW that it TAKES at a minimum of 10 times the current at 12V so that will make it 2A at 12V (0.2A x 10 = 2A).

We KNOW that this CAN be checked by using the same power equation (P/E=I) by reversing the operation.

In this case we will make the equation as I x E = P or 2A x 12V = 24W.

We also KNOW that the inverter is not 100% efficient, general rule can be used of 10% loss can be factored in (assuming 90% efficiency of the inverter).

So this would look like this 2A x 10% = 0.2A additional loss.

This loss can now be added to the TV current at 12V so that would look like 2A (TV at 12V) + 0.2A (Inverter loss) = 2.2A at 12V.

We KNOW that if you draw 1A for 1 hr that it is 1Ahr of battery capacity used.

Now we KNOW that if you ran the OPs TV on battery for 1 hr the battery capacity used will be 2.2 Ahr.

So if you wanted to run your TV for say 4hrs you could figure on the TV using 9.6Ahr of battery capacity.

Not sure as to why it has been so difficult for such a simple thing...


Posted By: ZZSPIRAL1 on 03/09/12 05:02pm

Edit: The original post is an error, because my calcuation for the second example is wrong.

The first example suggests 10 amps per 100 watts. Based on their example, a 24 watt TV will use approx. 2.4 amps an hour using an inverter.

The second example suggests dividing 120 (120v TV) by 12 (12v battery) and multiplying the results (10) by the TV amps in ac mode (.20). Based on their example, the TV will use approx. 2.0 amps an hour using an inverter:
(24 watt, 120v TV is .20 amps)......(120 divided by 12 is 10).....(.20 amps multiplied by 10 is 2.0)


As you can see they're close. Better to error on the side of caution and use the first example. Sorry for the confusion. Thanks to those who responded.

* This post was last edited 03/10/12 07:50pm by ZZSPIRAL1 *


Posted By: NORM WADDELL on 03/13/12 03:19pm

Here is your problem

You do not have a 200 Ampere PER hour battery

You have a 200 AMPERE-HOUR (Amperes TIMES Hours) battery

O K ?


Posted By: NORM WADDELL on 03/13/12 01:11pm

An 'Ampere" (capitalized, a man's name) is defined as
6.241 x 10 to the 18th power electrons per second

(That's 6.241 with 18 zeros after it)

It is a RATE OF FLOW............electrons PER SECOND



To say "Amperes per second" is to say

"6.241 x 10 to the 18th power electrons per second PER SECOND"

(A measurement of something PER SECOND PER SECOND is a
measurement of ACCELERATION.......not speed)

So.....an AMPERE is by definition a movement PER SECOND,
and does not require the added "PER SECOND"

That's like saying I drove 50 miles per hour per hour.
Auto racing announcers often say "rate of speed", but
speed is ALREADY a rate, and does not need the added "of speed"

I hope this helps.


Posted By: 2oldman on 03/13/12 02:47pm

alcolby wrote:

what is the basis for this startling conclusion?
education


Posted By: Wayne Dohnal on 03/13/12 01:27pm

Using the common water analogy:

Amp-hours and gallons are a quantity measurement.
Amps and gallons per hour are a rate measurement.
Amps per hour and gallons per hour per hour are a bit fuzzy, but could be a rate of change measurement. Constant acceleration, for instance, can be measured in feet per second per second.

This is another example of terms that sound almost alike but are really quite different. Reminds me of comparing 30 and 50 amps service, which is a lot more complicated that it would first appear.


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Posted By: bryanl on 03/13/12 10:24pm

re: "Converting to watts is probably not worth the effort."

using the proper units for energy and power is a good start towards reducing confusion. It allows comparisons to common household usage as well.

Lead acid batteries have a usable energy density of about 12 watt hours per pound. That's a bit over 500 watt hours in a group 24 to maybe 750 or so in a T105. Usable means down to the halfway point which is generally considered the most cost effective discharge goal (see smartgauge.com.co)

A charging system capable of about 4 watts per pound of battery is needed for best battery health.

The most common 20 hour rating is at a power draw of about a watt per pound. Higher draws than this will reduce available energy capacity.

Solar systems need at least a watt of panel per pound of battery.

Typical residual loads in a modern RV run at about 10 watts. That's 240 watt hours per day.

A typical RV light is about 20 watts, the furnace about 100.

Typical household electrical usage is 30 kilowatt hours per day. A refrigerator uses about 1 kwh per day.


Bryan


Posted By: bryanl on 03/15/12 10:08am

re: "People don't typically weigh their batteries to determine the energy storage capacity. Why do that when you can just read the amp-hour capacity"

Weight can also be 'just read' and is a major factor in your RV loading.

by using weight as a base for many things battery, you can avoid confusions about battery bank wiring and components and sizing.

Look at the title of this thread! If you stick to watts, the whole "Amp Hours for AC appliances" question is answered whether you are talking 110v or 12v - and the wattage is usually listed on the appliance labels. All you need to do is to match that watt rating to your battery capabilities (with due consideration for efficiencies in conversion and transmission). That is very easy to do if you use the energy density of lead acid batteries to normalize everything to consistent units.

re: "It reminds me of models in chemistry, many of which are certainly not true" -- models can be useful if one keeps in mind their limitations and appropriate context. From these many discussions, it appears to me that many folks model batteries as simple electrical energy storage devices and then find out that that view doesn't work well in RV practice except for the very short term and special circumstance.

re: "I wonder why battery manufacturers chose to provide Amp-hours as the "energy" specification on their batteries?" -- good question, My guess is that it is part inertia and tradition and part laziness. Current is easier to measure and, if you are only talking about one battery, the voltage is assumed.


Posted By: tenbear on 03/13/12 01:34pm

To use an analogy to water flow, water flow is measured as gallons per minute or GPM. GPM is analogous to Amperes. To use Amperes per hour would be like GPM per minute, or, gallons per minute per minute.


Posted By: alcolby on 03/13/12 12:54pm

I don't know why you could not express usage in amps per hour..what is the basis for this startling conclusion?

Al C

NORM WADDELL wrote:

For starters............

there is no such thing as 'Amperes per hour'
Now that this is solved (?), others will solve the problem.



Posted By: alcolby on 03/18/12 05:44pm

Cedarhill,

Your example is specious.

Amp Hours has been a method of measurement for battery capacity for years,has it been recently changed?

An Amp certainly is a measurement of electricity.

Georg Ohm says they are all inter-related,amps, volts, watts.

Alcolby

Cedarhill wrote:

"Amperes", or amps for short, is a measure of current flow, not a quantity of electricity or energy. Suppose you asked someone how much fuel his gas tank held and he answered " 3 gallons per hour every hour". Would that be a sensible answer to your question?

alcolby wrote:

I don't know why you could not express usage in amps per hour..what is the basis for this startling conclusion?

Al C



Posted By: smkettner on 03/13/12 03:04pm

Nevermind

* This post was edited 03/13/12 10:30pm by smkettner *


Posted By: wgriswold on 03/10/12 08:21pm

liborko wrote:

Amperehours is a good and useful REPRESENTATION of energy but it is not energy until multiplied by voltage. 1Ah@12.6V=1Ah*12.6V=12.6Wh. Only Watthour is accurate representation of energy and that is what utility company uses to charge for electricity. Or equivalent in different units like Joules for example.


Upon reflection you are right. But we are looking at an easy way to compare battery capacity to appliance usage. We could convert to watts but we don't know the accurate voltage at each moment of usage and the amp draw is probably not accurate. Converting to watts is probably not worth the effort. Amp hours is proportional to energy and is sufficient for our problem.

My background is in chemistry and Einstein said "chemistry to too complicated for chemists". Our solution is to make assumptions that simplify the problem and makes it solvable. My tendency to do that used to drive my math friends crazy. I suspect that is what is happening here.


Posted By: wgriswold on 03/14/12 12:23pm

This discussion is interesting. It has helped me to a better understanding of our electrical systems. Thanks.

It reminds me of models in chemistry, many of which are certainly not true. For example, the Bohr model of the atom where electrons circulate around the nucleus like planets around the sun. That is not true but in many cases can be used for understanding chemistry as long as one is aware of its limitations.

Amps times hours is not energy and is useful only within a particular voltage system. None the less it is an easy way to estimate energy usage since it is proportional to energy and can be used in an energy balance calculation as long as the voltage is constant. No harm done as long as one remembers the limitations of the method.

I wonder why battery manufacturers chose to provide Amp-hours as the "energy" specification on their batteries?


Posted By: BFL13 on 03/10/12 08:16pm

The "300ah" bank is measured to be that at the 20 hr rate of 15amps. You can run a constant 15 amps load for 20 hours while voltage drops from 12.7 to 10.5 In real life you would not go down so far in voltage.

So if you want to try that in watts, you have a sliding scale of voltages. Calculus?


Posted By: liborko on 03/10/12 08:06pm

In this example the 24W TV running for four hours will consume 24W*4h=96Wh. If the battery voltage was 12.6V and did not change during four hours, that would translate into 96Wh:12.6V=7.62Ah(ignoring inverter efficiency factor). If, however the battery voltage was 12.0V, then 96Wh:12V=8Ah. So it is easy to see that watthours is a better way to describe energy consumption than the amperehours, because amperehours are voltage dependent.

* This post was edited 03/10/12 08:16pm by liborko *


Posted By: liborko on 03/10/12 08:14pm

Correct. Every hour it runs it will use 28.8Wh. So theoretically 3600Wh battery should support the TV for 3600Wh:28.8Wh:2=62.5 hours. You don't want to use more than half of the battery capacity before recharge.


Posted By: liborko on 03/13/12 11:26pm

To put another way what Norm Waddell already said:

Electrical current is flow of electrons. The electron has a charge of:

1e=1.602x10to-19C(coulomb).....1C(coulomb)=1As(ampersecond)

The question then is: The flow of how many electrons per second constitutes a current of 1 ampere?:

1A= 1:1.602x10to-19As/s...the seconds cancell out...6.2415x10to18 electrons.

The answer: The flow of 6.2415x10to18 electrons in one second is a current of 1 ampere.


Posted By: wa8yxm on 03/14/12 03:16pm

They do that because the word "Battery" means a collection of cells, The "Amp Hour" rating is for ONE CELL< now you can put a one cell "battery" or a 3 cell or a 4 cell or a six sell or even a 12 cell. (They make all those sizes) and you can put two 3 cells (For example) in series... No matter if it's one cell, or 100 (in series) the amp hour rating,, does not change.


Posted By: wa8yxm on 03/11/12 04:38pm

I am going to simplify for the O/P

The first example you gave (Watts/10) is the best .

That is the one I use in fact.


Posted By: ZZSPIRAL1 on 03/10/12 08:10pm

liborko wrote:

In this example the 24W TV running for four hours will consume 24W*4h=96Wh. If the battery voltage was 12.6V and did not change during four hours, that would translate into 96Wh:12.6V=7.62Ah(ignoring inverter efficiency factor). If, however the battery voltage was 12.0V, then 96Wh:12V=8Ah. So it is easy to see that watthours is a better way to describe energy consumption than the amperehours, because amperehours are voltage dependent.


I edited my post. Thanks for the information. Makes sense.


Posted By: ZZSPIRAL1 on 03/10/12 08:17pm

liborko wrote:

Correct. Every hour it runs it will use 28.8Wh. So theoretically 3600Wh battery should support the TV for 3600:28.8:2=62.5 hours. You don't want to use more than half of the battery capacity before recharge.


Gotcha...........Thanks for the info.


Posted By: ZZSPIRAL1 on 03/10/12 08:19pm

Ha..Calculas? I stopped paying attention from the 4th grade on.......


Posted By: ZZSPIRAL1 on 03/10/12 07:52pm

oooops


Posted By: ZZSPIRAL1 on 03/10/12 08:04pm

liborko wrote:

Amperehours is a good and useful REPRESENTATION of energy but it is not energy until multiplied by voltage. 1Ah@12.6V=1Ah*12.6V=12.6Wh. Only Watthour is accurate representation of energy and that is what utility company uses to charge for electricity. Or equivalent in different units like Joules for example.


I'm not exactly educated. Your point made me ponder and I have a question.

If a 24 watt TV is running on 2.4 amps off an inverter, your example leads me to believe that is equal to 28.8 watt hours (2.4amps x 12volt). Seems to me a 300ah battery bank is equal to 3600 watt hours (300amps x 12volt).

Or, maybe you multiply that 24 watt TV by 10 (120V TV divided by 12v inverter). That's 240 watts. Again, the battery is 3600 watt hours.

Either of these right, or should I quit now?


Posted By: Cedarhill on 03/13/12 01:14pm

"Amperes", or amps for short, is a measure of current flow, not a quantity of electricity or energy. Suppose you asked someone how much fuel his gas tank held and he answered " 3 gallons per hour every hour". Would that be a sensible answer to your question?

alcolby wrote:

I don't know why you could not express usage in amps per hour..what is the basis for this startling conclusion?

Al C



Posted By: Cedarhill on 03/13/12 10:49pm

People don't typically weigh their batteries to determine the energy storage capacity. Why do that when you can just read the amp-hour capacity from the label and then multiply by an average voltage of 12.4 or so to get watt-hours? I can't imagine that a calculation based on battery weight would be as accurate.

I have never even heard of anyone buying a battery charger according to the weight of the battery and chargers are rated in amps - not watts. Manufacturers recommend a charging profile for their products based on amps and sometimes volts but not watts.

Why confuse the issue by introducing oddball and inconvenient units like watts per pound?


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