There are a lot of guides that state that this is caused by internal resistance of the source but not why. Here's the formula V = emf − Ir.Does increasing current increase resistance so the resistance is not a constant?

Thank you for your expertise!

Ohm's Law: E = I*R.

You're asking a very astute question. The simple answer: The more resistance you have, the more voltage it takes to force current through it. When you measure a voltage, you're not measuring what you have- you're measuring what you're losing across some path to get current to flow.

If too much current flows, voltage sags-- if this continues unabated, you have a short-circuit. If not enough current flows, voltage stays high. Regulating this balance between the flow of electrons (current) and resistance or impedance slowing those electrons is the most basic aspect of electronics- and how we make it do work for us. Just the right amount of work causes just the right amount of current to flow, without voltage dropping- this is the secret to circuit design.

You will learn more about these concepts with Ohm's Law, Kirchhoff's Laws, and Thevenin's Theorem.

Meanwhile, check this book out:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

 

Ohm's Law: E = I/R.

E = I * R

 

My soldering iron has controlled temperature so it is never too hot incinerating the rosin in the solder like cheapo soldering irons.
Then my smoke detectors never beep until I test them so their battery lasts for more than 1 year. When I replace their batteries the used ones measure 8V to 8.5V without a load so I use them for testing LEDs and for other things.

Just now I looked in my smoke detector to see where its Energizer battery was made but the smoke detector is stamped 2019 and has TWO Duracell 9V alkaline batteries (not For Resale) in it that say Made In Malasia. I do not have any new Energizer 9V alkaline batteries but the AA ones I have say Made In USA.

Energizer AA Ni-MH batteries say Made In Japan and Duracell ones say Made In China. Cheapo Super Heavy Duty batteries also say Made in China.

I found a Super-Heavy-Duty 9V battery stamped 06/2020 so it is almost expired. I don't know what it came in. It measures 8.78V with no load and drops to 6.87V with a 100 ohms load (69mA). It says Made In China but it is not leaking yet.

 

My soldering iron has controlled temperature so it is never too hot incinerating the rosin in the solder like cheapo soldering irons.
Then my smoke detectors never beep until I test them so their battery lasts for more than 1 year. When I replace their batteries the used ones measure 8V to 8.5V without a load so I use them for testing LEDs and for other things.

Just now I looked in my smoke detector to see where its Energizer battery was made but the smoke detector is stamped 2019 and has TWO Duracell 9V alkaline batteries (not For Resale) in it that say Made In Malasia. I do not have any new Energizer 9V alkaline batteries but the AA ones I have say Made In USA.

Energizer AA Ni-MH batteries say Made In Japan and Duracell ones say Made In China. Cheapo Super Heavy Duty batteries also say Made in China.

I found a Super-Heavy-Duty battery stamped 06/2020 so it is almost expired. I don't know what it came in. It measures 8.78V with no load and drops to 6.87V with a 100 ohms load (69mA). It says Made In China but it is not leaking yet.

Poor quality products can arrive from many different directions, but some places have got themselves "a reputation."

 

E = I * R

Yep... sorry about that. I've corrected it.

 

"Super-Heavy-duty" batteries from Asia use 110 years old carbon-zinc technology. They have low cost, low current and short life.

You do know the "heavy duty" name on a battery means that it is the old type non-alkaline don't you? No matter what brand or where they are made.

Quote,
Zinc-chloride "heavy duty" cell
The zinc-chloride cell, frequently referred to as a heavy-duty, extra-heavy-duty, or even super-heavy-duty battery, is an improvement on the original zinc–carbon cell, using purer chemicals and giving a longer service life and steadier voltage output as it is used and offering about twice the service life of general-purpose zinc–carbon cells, or up to four times in continuous-use or high-drain applications.[1] This is still a fraction of the output of an alkaline cell, however.
Alkaline batteries[6] offer up to eight times the battery life of zinc–carbon batteries,[7] especially in continuous-use or high-drain applications.[1] From - https://en.wikipedia.org/wiki/Zinc–carbon_battery

 

Yep... sorry about that. I've corrected it.

No need to apologize - we all make typos sometimes.

 

Time changes things.
Not very long ago people on the roads were driving horse-pulled wagons but not today. Airplanes today instead of old ships?

I can barely remember the original old carbon-zinc battery cells. Maybe I used them 64 years ago? They were American.
Now most battery cells from China are "super heavy duty" and are garbage. Most people today use alkaline cells.