I see a lot of battery testers for sale but I don't think you can trust them. So if you are going to build one for 1.5V cells what kind of load should be and what's the range of voltage for good? Also do you need different load for different battery size like a D vs AA?
Hi there,
Generally the cheaper they are the less you can trust them. However, as others have pointed out they are all only so good because they are usually only made for certain batteries of a certain size. There is a huge difference between testing an AA alkaline and a automobile battery for example, but there is not too much difference between testing an AAA battery vs an AA battery, as long as they are the same type like alkaline vs carbon zinc.
The nice thing is, we don't really have to have a battery tester that can test every possible type of battery under the sun. Most of us only use a few different types of batteries. This makes it a little easier, and we can also gather our own data set as we test them as the days go by.
The best load is based on the voltage and capacity. The voltage is of course measured in Volts (V), and the capacity in Ampere Hours (AH or Ah), or even in milliampere hours (mAh). Knowing these two can help choose a load size.
The idea is to put the battery under enough stress so that it can show a problem but not too much so that it overloads the battery. For an example, if we have a 1.5v alkaline AA battery the rating might be around 2000mAh, and a good test would be a load that draws 100ma which is C/20 and that is the mAh rating divided by 20, which here is 2000mAh/20=100ma, and you only apply the load for a few seconds while measuring the voltage. The resistance would be about 1.5/0.1=15 Ohms.
When you do this test with a new cell you get an idea what to look for. You write down the cell type and age, and the resistance you used and the voltage before and after the test. This becomes the start of your data base. Next, you use the cell in the intended application and note the hours used, then test again, then write everything down again. Once you do this a few times you start to get an idea what to look for.
Car batteries are very different though because you have to have a really heavy-duty load which you can purchase and that will have the instructions on how to use it.
An AA carbon zinc cell is different than an AA alkaline because it is not rated as high as an alkaline. it could be as low as 500mAh at the same voltage 1.5 volts. For this cell, 500/20=25ma and 1.5/0.025=60 Ohms, so this cell requires a higher resistance than the alkaline which was 15 Ohms.
I should point out that this explanation is sort of a 'best bet' method as you can use almost any resistance to get some idea what is going on, it just may not be the best test. For example, since we know now that 15 Ohms is good for AA alkaline and 60 Ohms is good for AA carbon zinc, we might compromise and use 30 Ohms for both tests. This still gives us a decent idea what is going on. It's just important to log all the test results until we have enough results to show us what we need to know.
Next, we also have to include the application sometimes. If one application draws 100ma and another draws just 1ma, we can expect to see differences in the everyday use of the cells. That's because there is another factor referred to as the "P" factor that varies between battery capacities. It means that the more we load it, the less overall capacity it will show. That means that at 100ma it will look like the very same cell has less capacity than a cell used at 1ma. This means that we need to include the application in our logs. It will show us that if we test a cell as dropping too much in voltage for one application, it may still work in anther application that draws less current.
This might sound like a lot to take in, and in a way it is, because batteries are fairly complicated devices. It's not like a light bulb were if you know the voltage and the wattage you know if you can use it or not. However, doing these little tests and logging the results eventually you get a good enough data set that you can start to make really good predictions about the health of your batteries. Take the time to do some of these tests and you'll find it works really well after maybe 10 to 20 tests on a given battery type. Luckily, we don't have to test all the batteries that were ever made just the ones we use a lot.
If you do these tests you can report back here some of the results and we can go farther with this if you like.
It also helps to have a good voltmeter and some resistors you can use as loads. If you are interested we can go deeper into that too.
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