Reader Rachel from Bay City, Michigan wrote in to ask, “Why do all of my things say not to mix old and new batteries or brands of batteries? What’s the worst that could happen?”
According to Rayovac, the performance of your battery-operated gadgets is limited by the weakest battery in the device. Mixing in an older, weaker battery can cause poor performance and reduced run time even if the other batteries are fresh and new.
A flashlight not shining as brightly or a remote control going dead faster don’t seem so bad, but there’s a bigger potential problem. According to safety materials from the Oak Ridge National Laboratory, mixing old and new batteries or different brands of batteries that have different performance levels causes the stronger battery to compensate for the weaker one, leading to overheating.
Meanwhile, as the weaker battery becomes exhausted, the stronger one can force it to discharge to very low or negative voltage levels, according to Energizer, which increases the chances it will leak.