Looking for accurate information on "which store-bought batteries are best?" You'll need to consider the source and their test methods. Not all reviewers are doing their testing correctly!
Technical Basics
Two common terms related to specifications of rechargeable cells are voltage (V) and current capacity (mA*h). The capacity is also available for disposable cells but less prominent. Let's talk about these measures, starting with some related terms.
Cell = One electrochemical vessel by itself, with an anode (+) and cathode (-)
Battery = Multiple cells in series. Also commonly used for just one cell.
Primary Cell = Not rechargeable. One-time use. Disposable.
Secondary Cell = Rechargeable. Can be used a second time after recharge.
Voltage describes how strong a cell is chemically. The chemistry causes an electro-potential between anode and cathode, ready for delivery at the terminals. The open-circuit voltage is measured directly on the terminals when to connected. The loaded voltage is the reserve strength of the cell when under load. This happens when electrons are flowing and the cell chemistry is restoring electro-potential to the terminals.
The loaded voltage is always lower than the open circuit voltage. Over time, the chemistry gets weaker, and both the open circuit voltage and loaded voltage decreases. The loaded voltage has the more significant decrease and is more "telling" of the battery age.
Battery/Cell Quick Check for Health
Lots of people estimate how much life a cell has left by placing a multimeter across the terminals and measuring the open circuit voltage. If it's close to or higher than the marking on the cell, such as 1.5V, they call it good. A better test involves adding a moderate load resistor. A good resistor to choose will put approximately C/10 load on the cell. Alternately, a battery tester with appropriate current settings can be used.
Well, what is "C"? This is the capacity of the cell. More technically, it's "the amount of electrical charge a cell can store and deliver". For small cells it's usually given in milliampere hours, or mA*h. Larger cells might be in A*h. Charge in motion is measured as current, so measuring total charge moved requires multiplying by time.
A big, wide "D" cell has a lot of capacity compared to a skinny "AA" cell. Chemically identical, both have 1.5V nominal voltage. But when you put the same load on both cells, the voltage of the "AA" cell will drop more quickly than a "D" cell.
Let's calculate the proper resistor to test a "AA" cell at C/10 test load.
We see from the datasheet that an alkaline AA cell is rated 1.5V and 2000 mA*h (or 2.0 A*h).
To determine the test current C/10, we drop the hours "*h" and divide the current value by 10.
Therefore, the resistor should request 200 mA (0.2 A) from the cell during our loaded voltage measurement.
R = V/I, so we choose a resistor (1.5V) / (0.2A) = 7.5 ohms.
The physical size of the resistor needs to handle the wattage, so we use P = V * I, or (1.5V) * (0.2A) = 0.3W.
Rounding up, we want to use a half-watt resistor around 7.5 ohms resistance to load test an alkaline AA cell.
Attach a voltmeter to the resistor legs, and then temporarily attach the same resistor legs to the terminals of the battery. We compare the voltmeter reading to the rated voltage when under load and how many hours the cell can operate.
Referring to the datasheet, we see a new/charged cell will read above 1.3V and a old/discharged cell will read below 1.1V. This estimates how much capacity (C) is left without actually doing a discharge test.
Mistakes using "Quick Check" method
Discharge curves are presented by each datasheet, showing voltage versus time. Every battery size, chemistry, and brand will have a different curve. The cycle tester assumes you are adhering to the appropriate discharge curve. However, it can't stop a person from using the wrong one.
For instance, let's say somebody does our battery quick-check with the 7.5 ohm resistor we calculated for a AA size, and mistakenly uses it on a small 1.5V watch battery which is only 20 mA*h. You'll find the measured voltage is extremely low! A 750 ohm resistor would have been the appropriate sized resistor for this size cell. User error will lead to the conclusion the coin cell is bad, when it really isn't.
Battery/Cell Full Cycle Testing
Units that are made to fully cycle rechargeable cells include products from Maha and Opus. You’ll frequently see these these used on YouTube.
Why do full-cycle battery testers exist? These testers are exclusively sold for charging and discharge evaluation of rechargeable cells such as Nickel-Metal Hydride (1.2V) and Lithium-Ion (3.7V). How so?
Secondary/rechargeable cells have several hundreds of cycles useful life. One full cycle under controlled conditions will not significantly impact the life of the cell.
The test result "mA*h" will be compared to the rated capacity on the cell manufacturer datasheet to determine useful life remaining.
The test result "mA*h" requires the user must use the cell manufacturer's recommended discharge current and terminal voltage to be correct.
Pay special attention to the last item. The "mA*h" measurement will be incorrect if the wrong datasheet is used, such as the wrong sized cell, the wrong cell chemistry, or the wrong battery manufacturer. It will also be skewed if the improper current draw is used, such as higher or lower than the specification sheet recommends.
In general, most rechargeable cells of similar level quality have discharge curves almost exactly the same as each other. Therefore, comparing mA*h across different brands of rechargeable cells has become common, and leads to reasonably accurate conclusions at low current draws. At higher current draws, the internal construction of the cells may lead to more severe differences.
Mistakes using "Cycle Test" method
"Influencers" on YouTube are using cycle testers designed for rechargeable cells for comparison of different disposable battery brands and chemistries. Why are their results invalid? For starters, they are ignoring the item’s purpose as stated in the sales literature and device instructions. They are headed straight into accumulating data with no understanding what they are measuring.
Each cell chemistry and construction leads to a different discharge curve at any given current draw. This means each cell will maintain a different voltage level (strength level) through the discharge cycle. This is especially true for different chemistries like carbon-zinc, alkaline, and lithium primaries. Strength is important!
The testers they are using throw out the voltage value and only report mA*h. The influencers will not understand this fact and describe the result as power. The problem is all devices are extremely dependent on voltage while current is being drawn. The reason cycle testers report mA*h is that rechargeable cells are typically specified with current capacity, not power capacity.
Most modern consumer devices will adjust the current consumed by how much voltage is available to maintain constant power consumption. This means a battery with a higher voltage through the discharge curve will last much longer than a capacity reading will predict. Viewed another way, when the device can't get enough voltage, it increases current to compensate, using up their capacity more quickly.
Capacity testers force the cell to maintain a constant current, regardless of the voltage. By doing so, the test equipment is not behaving like a typical consumer device that the disposable battery would be installed. The time it takes to discharge a cell does not directly correlate with the time the cell will last in a device because voltage is ignored.
Capacity testers were only ever intended for secondary cells. To use the device to compare primary cells against each other is ludicrous. Don't believe me? Read forward to a few analogies.
The Automobile Powertrain
The power an engine produces depends on horsepower. Horsepower is derived from Torque * RPM. The rear wheels of the car is like your device, a flashlight, toy, radio, camera. The gearing is like the input circuit to your device, setting the power used.
Voltage = Torque
Current = RPM
Power = Horsepower
Now, to determine how much power the engine has sent to the wheels over time, would you report RPM*h? Of course not, the reading on the tachometer isn't measuring power. You would need to report hp*h.
If someone compared a Corvette engine to a Metro engine using RPM*h, you wouldn't take them seriously would you? Both could be turning 3,000 RPM for an hour but the power output is quite different. Same with comparing different batteries using mA*h.
Just because you can measure RPM on your tachometer and time expired with a stopwatch, it doesn't become a valid comparison tool across different engines. This is why dynamometers exist.
The Bicycle
Two bicyclists pedal their bicycle crank 1000 times in 5 minutes until they are equally tired. The better rider goes a full mile, the weaker rider travels only half a mile.
Here's what's happening at the crank of the bicycle:
Voltage = Leg Force
Current = Rotations/minute
Power = Force * Rotations/minute
Capacity tester would say both riders performed the same thing, 1000 crank rotations for 5 minutes until tired. Nobody in their right mind would say both riders are equal capabilities to go a certain distance. But that is what the capacity tester would claim!
So the better rider can put the bicycle in a higher gear, going a greater distance. It's the same as the better battery lasting longer. This is because the device cares about power at the wheel, not how fast the crank is being rotated.
The difference in total leg strength exerted over the period of time would be entirely ignored. It's not possible for the cell tester to measure or even estimate the road distance traveled. The "gearing" inside the test unit is not tracked.
By ignoring the strength of the legs of the bicyclist, they ignored how capable the rider is to perform, both instantaneously and over the distance. This is what happens when ongoing voltage is not tracked by a capacity tester.
The capacity tester is reporting mA*h which defines how well a cell tried to deliver power during the test, not how well it actually performed. How long it can last in actual device is not measured.
Correcting The Test Mistakes
If the influencers truly want to compare different brands and chemistries, they really need to do one of two things, or both.
Obtain a tester which does not discard voltage and truly measures power. These testers measure mW*h, which integrates BOTH voltage and current over time. This measure has a direct correlation to how well the cell will perform in a device.
Compare longevity in actual devices people use.
Reviewers who are improperly using ampere-based capacity testers for cell comparisons are completely misunderstanding fundamental electronics and power delivery systems. They often confuse the ampere with power.
They did not take the time to understand the capacity discharge device and how it is intended to use. Often they ignore discharged curves, and march forward with publishing incorrect data.
They believe the numbers on their testers are somehow fact. While sincere in their efforts, they have misled themselves and their reviewers on the outset, because they are graphing data that is practically meaningless.
For instance, disposable lithium cells last 5X as long as alkaline cells in a trail camera. Many reviewers have proved this. However, the mA*h rating is only 2X, as proven by reviewers using capacity testers. They can't understand the cost benefit of lithium cells and claim we are being taken advantage of by the battery manufacturers. Trust the trail cameras! Most hunters are enjoying the benefits of long runtime and low overall cost using lithium primary cells.
My hope is these types of errant reviews are fact-checked more often, and the reviewers either move to actual device testing or mW*h testing. They are putting bad information into the hands of consumers, leading them to make bad consumer decisions.
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