A few years ago I bought this little battery tester at Amazon for about 12 dollars. It takes AA batteries, like shown here, AAA, and 6 Volt. It must have saved me hundreds of dollars in battery cost by now.
Every time, before I am about to discard a battery, I test it first.
When the wall clock falls behind, it must be the battery. Before I go hiking, I replace my AAA batteries in the headlamp so I don’t run out. When my wireless keyboard or mouse act up, the first thing I do is replace the AA battery.
I have found that about half of the time, the battery that I was about to put into the recycle container was perfectly good, sometimes at 80% or better. I have started to save batteries at 60% or better in a little box from which I take my “new” batteries when I need them. I believe I buy half the batteries now compared to what I used to before I had this tester.
The battery in the tester in this picture shows 100% full, since all 5 LEDs are lit. This is a perfect, fully charged battery, that I was going to throw out.
This tester is great value for the money.