Hi there.

I would be really grateful for some advice on testing all the old non-rechargeable batteries that I have lying around.

Having watched a few videos, I am aware that I need to use a resistor to put the batteries under load when testing them with my multimeter.

However, I have a few further queries:

1. Should I place the resistor in series or parallel, when testing or does it make no difference?
2. I will be be testing 1.5V, button batteries and 9V batteries. What resistor rating should I use for each?
3. What is the threshold (for each of the three battery types) at which I should throw them out?

Thanks very much.

 

Welcome to AAC.

The best, easiest way to test batteries or cells* is… with a battery tester. These are very inexpensive and have an internal load with a meter scaled to a go/no-go green and red. They have various arrangements for holding the device to be tested some better than others.

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I have several of these around where cells and batteries are kept so it’s possible to check if loose ones are good or just discarded in place when they were changed. You can certainly get cheaper ones, but these have been accurate and reliable. I strongly recommend getting one or more (of these or a well-reviewed cheaper alternative) to make life easier.

On the substance of your question:

When you place a meter in series with a load (the resistor) you will measure the voltage drop across that load. This is a reasonable test for the cells SoC (State of Charge).

When you place the meter in parallel, while you are still measuring a voltage, it is a voltage proportional to the current flow in the circuit. Ohm’s Law can be used to derive the current in Amps. This is not a good way to test cells.

The choice of resistor size is a bit of a controversy. The size of the resistor will determine how much current the cell will have to deliver into the circuit. Different cells and batteries used for different purposes are ideally tested with a load that matches reasonably well to the load presented by the application. That way, the voltage drop represents the voltage you’d expect to see if it was in the device it is supposed to power.

The tester I have recommended uses a 5Ω resistor on the 1.5V/3V types, so a pretty aggressive test current of ~300mA for the former and 600mA for the latter. For the 9V test it uses a much less aggressive 210Ω resistor giving a ~42mA current.

The 1.5V seems high(ish) but the good section of the meter is a range, and the cell is good anywhere in it. The coin cell (of the 3V type) tries to produce a 600mA current which is really quite high—but empirically, it seems to be OK, as the testing works. The 9V (PP3 type) has a very low current test, but 9V batteries are very low capacity so this is sensible.

As far as voltages—this will depend on your application because the device will determine how much voltage/current is “enough”. The best way would be to find out what your devices need because some cells no longer useful in one case may very well have additional useful life in another application.

The best source of general information is the datasheet of the cell or battery you are testing. If you search for the brand and part number, and “datasheet”, you will usually find a document with a lot of useful information.

*A little pendanticism, but something that aids thinking when working with cells and batteries. A cell is a single device that produces electric current through a chemical process. A battery is a collection of two or more cells. We get the name “battery” from Ben Franklin who, not generally known, was a genuine electricity researcher with an international reputation. The arrays of cells, which increased the power available for experiments reminded him of an array of artillery pieces—called a battery—which increased the fighting power in a similar way. He is also responsible for making a big confusion of the names of the positive and negative terminals on a cell or battery because he made a guess and lost a 50% bet—but we won’t get into that here.

 

Most of my batteries are used in low current applications, such as clocks and sensors were they normally run a year or more.
I find that just measuring the open-circuit voltage of a battery is generally a reasonable indication of its charge condition for such applications.
Thus an alkaline voltage of 1.2-1.3V shows it still has about 40-50% charge remaining, and it's essentially dead at <1V.

Of course for higher current applications, such as a flashlight, measuring under load gives likely a better indication.

 

For 1.5V batteries, I use a 1.5V incandescent flashlight bulb as a load.
I built this for my kids. Now I use it for testing batteries.

 

Measuring battery voltage alone is not a good indicator of remaining battery life. I save weak batteries because I may be able to use them in a different application.

By observing the intensity of the light bulb along with the voltmeter indicator, you can get a visual indication of the remaining power. If you want to go the trouble, what you need to measure is the internal resistance of the battery.

 

Measuring battery voltage alone is not a good indicator of remaining battery life.

In my experience it is for low current applications.

 

In my experience it is for low current applications.

Even if it wasn’t, there are a lot of appliances out there which use voltage to determine if the battery is discharged and whether to operate or not.
That is what determines whether the battery is going work in anything more complicated than a torch, not necessarily how much charge remains.

 

Even if it wasn’t, there are a lot of appliances out there which use voltage to determine if the battery is discharged and whether to operate or not.
That is what determines whether the battery is going work in anything more complicated than a torch, not necessarily how much charge remains.

True, but I have some devices that want a new battery when the voltage drops to about 1.2V, and I find those will still operate other devices, like a clock or my wireless mouse, for many more months.
I like to get all the energy out of a battery before discarding it.