London - Ever spent ages swapping batteries in and out of remote controls to find out which ones still have power in them?
Next time, just try dropping them on a hard surface and see if they bounce.
This is because spent alkaline batteries are more likely to bounce than fresh ones, according to an American electrical engineer whose video demonstrating the phenomenon is becoming an internet sensation.
In his YouTube video, Lee Hite tests a series of batteries that have expired against a set which have not been used.
He demonstrates that a used battery bounces far higher than one that has only just been taken out of the packet.
A good battery, he explains, contains a gel-like substance which solidifies as the battery discharges its electricity.
While the gel is in a semi-liquid form, it absorbs the energy when the battery hits the surface. An anti-bounce hammer, which contains an internal core of moving buckshot, works in the same way.
But when the gel in the battery has solidified it cannot move and the whole battery bounces, the same way that a solid hammer bounces off a nail.
The theory was greeted with caution by academics in Britain, as chemistry and physics departments across the country tried to test the idea for themselves.
Many said the theory made sense – but few were able to replicate Mr Hite’s results. A spokesman for the Institute of Physics said there was “sound thinking behind the idea” but added: “Experiments need to be reproducible for results to be reliable. We’re struggling to replicate the experiment over here.”
Dr Lee Banting, a senior lecturer in chemistry at Portsmouth University, said the video had convinced him. “If the gel in the battery is in a liquid state it absorbs that kinetic energy – all the energy is gone and the battery does not bounce back,” he said. “But if it turns into a solid state the kinetic energy pushes it back away from the surface and it bounces.”
Dr Fred Davis, a chemist at Reading University, said: “This video has prompted me and my colleagues to dig out dud batteries from all over the place to try it out for ourselves.” He added: “The energy output of a battery arises from chemical changes, which means the material in the battery at the start will be chemically different from that when discharged.
“It may be that these changes would influence how it behaves when dropped on to a desk. But I remain to be convinced.”
A spokesman for battery maker Duracell said: “This bounce test is certainly not an accurate way to measure battery life.”
He added: “Yes, while the density of the materials does change as the battery is discharged, it is not indicative to power delivery.
“As batteries are discharged, the internal volume increases due to the expansion of materials. This often makes the battery swell and can play an important role in how it will balance when dropped.” - Daily Mail