As one might imagine, hundreds of students in labs each semester means we use a lot of batteries. Just a ton of ’em.
Sometimes we need them at full capacity, but certain uses can be fine if they’ve started to develop some of that internal resistance after a while. Certain situations call for the higher potential current output of alkalines, while others need lithiums to keep the voltage from dropping too low. And at other times, the relative calm of a zinc-carbon battery is just perfect, like when students are building basic circuits and there’s a risk of shorting the batteries.
Much lower risk of booms and burns with the zinc-carbon.
Still, unless a battery’s in real rough shape, you never know how much juice they’ve got left just by looking. Pop one on the tester to see. When they’re almost new, you can hear the needle tap the far side!