this post was submitted on 10 Sep 2024
684 points (79.2% liked)
Memes
45746 readers
1709 users here now
Rules:
- Be civil and nice.
- Try not to excessively repost, as a rule of thumb, wait at least 2 months to do it if you have to.
founded 5 years ago
MODERATORS
you are viewing a single comment's thread
view the rest of the comments
view the rest of the comments
I believe it actually has to do more with historical conventions in electronics or math. (This is just what I remember from heresay when I was in university as an electronics engineer), but there is also a mathematical reason.
history hearsay theory
The easiest way to measure power draw is by measuring current draw (voltage across a sense resistor) way back before there were affordable, quality ICs to measure voltage and current and pretty much joule count.
To add to this, current sensors are much easier and cheaper than test machines that do the calculations for you.
When lithium batteries and NiCAD batteries became standard compared to the earlier lead-acid (which are measured in Wh), they had an extremely flat voltage curve compared to lead acid. They could be considered to be at a constant voltage.
Now cheaper electronics were being made and if a designer wanted to know how long a battery would last, they could take the nominal battery voltage that the battery would be at a vast majority of the time, and they could just measure the current draw over a short time of the circuit, 10s of calculations, and you have your approximate battery life. There is a joke that engineers approximate π to 3.
Even designing electronics today, everything is specced to current draw, not power draw. ICs take X current in mA during Y operations. Your DCDC converters have Z quiescent currents and from there you can calculate efficiency. It is much easier to work in current for energy running through the circuit.
Math units
Ah is a measure of electrical charge.
Wh is a measure of energy
Batteries and capacitors hold charge so are measured in Ah, generators that power the grid generate energy and use of that energy is measured in Wh (it also isn't a "constant" voltage source like batteries as it is AC)
The thing is, it does not matter how much charge the battery holds, it does matter how much energy it holds. Without knowing the Voltage the Ah is useless.
Sorry, but you are simply wrong. Simple math says that you are wrong.
You can buck or boost convert nearly any voltage to any other voltage.
Then measure the current output of the battery, boom you have battery life.
Also electrical charge can be used in many, many very valuable calculations without involving voltage at all.
Let's take an arbitrary example with an arbitrary battery powered device. Let's say the battery is somewhere between 1V and 10000000V. You can't measure it because you might blow up your multimeter.
You know that the battery is 5000mAh. You can safely measure that all of the circuitry is draining 1000mA because sense resistors or contactless magnetic current measurements don't have anywhere near dangerous voltages. You know that the battery will last about 5 hours. What is the voltage? Doesn't matter.
Yes, charge and the flow of charge is not the entire story, but to say it is useless or does not matter is just a straight lie. It is fine if you don't understand electronics, but then don't spit out misinformation.
Yes Watt-hours would give a more complete picture to slightly tech-inclined consumers (makes 0 difference for 99% of consumers), but then it returns to not mattering because you can do the 5s calculation yourself because single cell lithium batteries are overwhelmingly 1 nominal voltage.
Literally 90% of calculations related to efficiency are JUST as valid using mA as W.
Your device uses 12mA at idle with a 5000mAh battery has the same relevance as your 18.5Wh battery using 45mW at idle.
I am ONLY speaking from a consumer position and for those Wh is more useful.
The consumer looks on device a and on device b and then determines how often he can recharge its device. With Ah you cannot do this unless you know the Voltage, with Wh you can make this decision without any further knowledge.
Yes this does not include battery life or conversion of efficiency. But a cunsumer measures nothing he looks at the lable.
Btw. no need to insult me. I have never put out misinformation, I may have not stated enough that I am viewing this as a consumer.
Please explain to me what the difference is between battery life if you have a 5000mAh battery and an 18Wh battery.
Please state the calculation that you would use to "determine how often you have to recharge" that is valid for Wh and not for Ah. I am all for it. If you can cite a single source where the manufacturer gives a specification that would give battery life in Wh, and not in Ah, I will concede the entire argument and say that you were right the whole time in every comment make a note that you were right. Please show your calculation work.
This is patently, objectively misinformation and completely false. That is a direct quote of your words, today. That was your last comment. I have already laid out multiple examples of how Ah is a useful measurement and what you can do with it. Therefore, it is misinformation. It is not disinformation, but stating untrue things as fact is misinformation, even if you have no idea you are wrong.
Basically every Laptop manufacturer.
https://www.dell.com/en-us/shop/dell-computer-laptops/latitude-5550-laptop/spd/latitude-15-5550-laptop/s0035l5550usvp?ref=variantstack
Lol, you literally quoted me, didn't actually read what you quoted, and then did something completely different.
Do you know that battery life ≠ battery capacity? That is not the same measurement as I have already tried to teach you 3 times.
What is its idle power draw? What is its power draw under load? Playing video? Sleep mode? That source gives nothing which determines battery life. All it gives is a nearly useless capacity number, just like all other manufacturers. So not valid at all. You still have exactly 0 more information about battery life.
If I am wrong, please state your calculations of what the battery life is with that 54Wh battery.
Your entire argument was "Ah is useless and Wh gives consumers the information to determine battery life" So go ahead, determine the battery life.
How is this any different at all if they said that it is a 5.8Ah battery? They don't give any current or power draw.
As an exercise:
can you tell me the battery life difference between an arbitrary Laptop A with a 54Wh battery and Laptop B with a 27Wh battery?
Not quit sure where battery life is comming from, thats not my argument. To restate and also better phrase my argument: Stored energy is the better measure for consumers for the quantification of the battery size in a consumer device compared to ~~Ah~~ stored charge.
Now i can cross compare devices based on that and do not have to worry about the Voltage of the battery of any other devices.
I never claimed that this is possible. I wrote "can recharge" not 'have to". I am referring to devices like a power bank which i can calculate with a simple:
powerbank has 100Wh and phone has 25Wh so 100/25=4 -> i can recharge my phone 4 times using that powerbank.