replacement batteries information: When the AA or AAA batteries in electronics seem to get low why does it temporarily revive them when you open the cover and swap them around?

Monday, February 18, 2008

When the AA or AAA batteries in electronics seem to get low why does it temporarily revive them when you open the cover and swap them around?

The voltage from a small battery is rather low, typically 1.5v when new and gets lower as they 'wear out'. This continues until the voltage produced gets so low that the device can no longer work.

Low voltage is mostly caused by the battery wearing out, but there is another thing that comes into play in this example.

The other thing is a small amount of oxidization between the batteries and the spring clips that hold the batteries in and make electrical contact. Over time this layer of oxidization builds up between the spring clips and the batteries. Just like a penny is all nice and shiny when new, and gets less shiny when old. This layer of oxidization is not very 'conductive' to electricity and makes it harder for the power to get from the battery into the device.

Here is how this layer of oxidization comes into play. All the time that the batteries are getting weaker (and their voltage is getting lower and lower), this layer of oxidization is getting bigger, and it is causing a further reducing in the voltage that finally makes it to the device. It is actually a combination of the batteries lower voltage (as a result of wearing out), and the increased additional voltage drop through the layer of oxidization that finally reaches a point where the total supplied voltage is too low for the device to work.

So, buy swapping the batteries (actually, you just need to move them around a bit), you physically rub off this layer of oxidization and eliminate the small voltage drop that cause by it. Often, this is just enough to allow the device to work for a little while longer until the battery further discharges.

Oxidization is a well known issue. And it can be avoided several ways: Passing a high current through a connection will 'burn through' any oxidization. Hence, all of the above tends to apply to very low current devices like TV remote controls and not your car's starting battery. (Although, if oxidization gets bad enough, it can effect how your car starts...) Some metals resist oxidization, like Gold. Hence you see 'Gold Plated' connectors on very high end audio cables (note that unless both sides are gold plated, the plug AND the socket, it does not really help :-) Finally, one can try and keep air away from a connection. Every see those special 'wire clips' the telephone guy uses? The ones filled with a jelly? Idea here is to keep air away from the joints and avoid oxidization.

So bottom line, when I am too lazy to get up and change channels on the TV, and the batteries on the remote are wearing out, and I am too lazy to get up and get new batteries. Pulling the battery cover off, and 'moving the batteries around' will often get me though the moment, all without even leaving my chair :-)

A battery behaves like a perfect voltage source in series with an internal resistance, this resistance increases with age and limits the life of the battery. When the battery is taken off load for even a little while, this internal resistance lowers significantly. When the battery is loaded again the resistance rises to at least the same resistance over a short period. Almost everyone has used a torch with an almost flat battery, that seems to light well for a second or two and then dims rapidly. Switching it off then on gives another short burst of brightness. This is electrically similar to removing the batteries from any gadget.

Oxidisation is often used as a scapegoat for battery related problems, however I have never known it to cause a problem in any equipment that is reasonably well cared for, usually and resistance due to a layer of oxide is insignificant compared to the internal battery resistance. A more likely cause is poor spring tension, this can be witnessed by shaking a lit hand torch violently along the axis of the batteries.

Unless the user has very hot hands and changes the batteries very slowly indeed there would not be any significant transfer of thermal energy to the batteries, so heat is unlikely to be the cause.

Please don't throw them in the fire. They really hate that and will seek revenge by maiming you.