A VERY Basic Question - Page 1
Forum section: electronics / testgear

author: Sierra
author_rank: Contributor
timestamp: July 12, 2023, 03:48:53 pm
content: Believe it or not, I've searched and am still turning to you for an answer to a very basic, completely rudimentary question.How would I convert amp read on a meter to amp hours?The underlying question is how often is the current I'm measuring actually being drawn and how does it relate to amp hours?As an initial inquiry, meter reads 1.5AA) If it's drawing it every hour the circuit would simply be using 1.5AhB) If it's drawing it every minute, would we then multiply 1.5 by 60 = 90AhC) If it's drawing this at every fraction of a second (which makes sense) how do we compute Ah needed to run it?On and on it goes. So the basic question is how do I convert read current into current hours?I hope the above makes sense and thanks in advanced.

author: bdunham7
author_rank: Super Contributor
timestamp: July 12, 2023, 03:58:22 pm
content: Quote from: Sierra on July 12, 2023, 03:48:53 pmTo start, it is currentXhours, not current/hours. Think of it as a total charge transferred, sort of like gallons of water.An ampere is a rate of flow equal (almost) to one coulomb (a large number of electrons, or charge carriers as the case may be) per second. So one ampere-second is one coulomb. An ampere-hour is literally one ampere for a period of one hour. So one ampere-hour (amperes multiplied by hours, not divided) would be one ampere multiplied by 3600 seconds, or 3600 coulombs.You convert an ampere reading to ampere-hours only by observing it for some time and adding up, or 'integrating' the readings. For example, you could observe it every second, add up all of your observations for one hour and divide the result by 3600 and you would have the number of ampere-hours, or pretty close to it as long as the current wasn't varying wildly.

author: metrologist
author_rank: Super Contributor
timestamp: July 12, 2023, 04:12:26 pm
content: Quote from: bdunham7 on July 12, 2023, 03:58:22 pmWhat a coulombic post. I like the water flow out of a hose analogy.

author: Sierra
author_rank: Contributor
timestamp: July 12, 2023, 04:17:30 pm
content: Quote from: bdunham7 on July 12, 2023, 03:58:22 pmThanks for the information. The current/hours wasn't intended to be division but instead a statement of current per hours. Edited above for ease of understanding.So, if we were to take 1.5A recorded every second (assuming it does not fluctuate). We would have 1.5 * 3600 (seconds in an hour) = 5400 which we would then divide by 3600 to arrive at 1.5Ah.Alternatively, the current you read on a meter could then be (assuming no fluctuations in current) simply expressed in hours for its Ah. That is a meter reading of 1.5A (assuming the current doesn't fluctuate in an hour) is consuming 1.5Ah, yes?

author: Fungus
author_rank: Super Contributor
timestamp: July 12, 2023, 04:28:57 pm
content: Quote from: Sierra on July 12, 2023, 03:48:53 pmYou wouldn't.Imagine a circuit is battery powered. A meter can't know how much energy is left in the battery by measuring the current. All the meter knows is how many electrons are coming out of the battery every second ("Amps").The "energy in the battery" is measured in Amp hours and it lets us know how long the battery will last.Quote from: Sierra on July 12, 2023, 03:48:53 pmOK, so 1.5 Amps is theconsumptionof our circuit.If the circuit runs for an hour then we can say the battery had 1.5 Amp hours of charge in it.If it runs for two hours then we can say the battery had 3 Amp hours of charge in it.

author: bdunham7
author_rank: Super Contributor
timestamp: July 12, 2023, 04:29:37 pm
content: Quote from: Sierra on July 12, 2023, 04:17:30 pm"per" implies division. Miles per hour means you take the total number of miles you drove and divide by the number of hours it took you. The fact that we use MPH to describe instantaneous speed can be misleading--feet per second would be easier to understand. One MPH = 1.467 feet per second.QuoteNo, I think maybe you're still missing it. 1.5A recorded continously for an actual hour means that there was 1.5Ah over that period of time, which means that 5400 coulombs of charge was transferred. Amp-hours is an aggregate, not a rate. Amperes are a rate.Water analogies are a rather limited tool for understanding electricity, but at this level they work. Consider a coulomb to be a gallon of water. An ampere of current is like a pump that delivers one gallon of water per second. An ampere-hour is like 3600 gallons of water. Of course with electricity (and pool pumps) the water circulates and doesn't actually accumulate someplace, so dont' worry that there isn't a giant tank of coulombs someplace.

author: dmmartindale
author_rank: Contributor
timestamp: July 12, 2023, 04:31:04 pm
content: Perhaps a better way to think of it: Amps are an instantaneous rate of something happening. Amp-hours is the total amount of "something" that happened over a period of time.For example, when driving in your car, your speed might vary between 30 and 60 km/hr. That's the instantaneous rate you are moving. Over the period of an hour, you cover some longer distance, perhaps 50 km. Amps are like your speedometer - it tells you the rate that electricity is flowing right now. Amp-hours are like your odometer - it tells you the total amount of charge has passed through the meter since you started measuring.If you know calculus, amp-hours are just the integral of current in amps with respect to time, with time in units of hours. Equally, amps are the derivative of amp-hours with respect to time.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 12, 2023, 04:33:58 pm
content: To make it more complicated, the Ah rating on most secondary batteries is only valid at a specific discharge rate; at a different rate the Ah will be different. Generally speaking, the lower the discharge current, the higher the AH rating will end up.Generally, not always.

author: Fungus
author_rank: Super Contributor
timestamp: July 12, 2023, 04:55:16 pm
content: Maybe the trick is to realize that "Amps" is a composite unit. It's really "coulombs per second".The car analogy would be "speed". Your speedometer tells you how fast you're going and from the number it shows you can work out how long it would take to travel 100km (for example).If you travel at 100 "speeds" then it takes you one hour to arrive.If you travel at 50 "speeds" then it takes you two hours to arrive.That sounds weird but it's no weirder than saying "amps". Try looking at your multimeter when it's measuring amps and say "coulombs per second" instead. See if that helps.

author: Sierra
author_rank: Contributor
timestamp: July 12, 2023, 05:23:14 pm
content: Some of the above is understood and some remains unclear while it's also apparent my phrasing certainly wasn't tight enough.Perhaps asking the opposite question would help solidify the understanding.As an example:1) I have an energy source with 3Ah of capacity (assuming it's all usable and we have a constant voltage)2) A meter reads current of 1.5AThe units above would then imply a run-time of 2 hours. Correct?

author: dmmartindale
author_rank: Contributor
timestamp: July 12, 2023, 05:28:48 pm
content: Quote from: Sierra on July 12, 2023, 05:23:14 pmYes, that's correct.(Just be aware that a lead-acid battery that says "3 Ah" doesn't mean you will have 3 Ah at the 2-hour discharge rate. The battery is 3 Ah at the 20-hour discharge rate, and probably less than 2 Ah at the 2-hour discharge rate. And if you want the battery to last a long time, it is best not to discharge below 50%, meaning the useful capacity for you might be 1 Ah for a "3 Ah" marked battery.

author: Fungus
author_rank: Super Contributor
timestamp: July 13, 2023, 01:09:41 am
content: Quote from: Sierra on July 12, 2023, 05:23:14 pmYes.