Why/how does a dead battery regain a small amount of power?

I'm sure all of us at one time or another had a battery powered device/toy and experienced what happens when the battery is drained. After waiting for a while, the device/toy will operate some more albeit at a lower level of efficiency and not for very long but there is a noticable difference. Anyone know why/how that is?

I think it's because the toy that you're using is "used" to the charge that's coming from it.

For example, I'm playing with my remote control car, and the battery was half full. The toy is able to use the energy coming from the battery as it should, but then, as the battery is running out, not as much juice is there for the car to use, so it stops. But, the battery never really died. So, if I go back later, the car just "readjusts" itself to use what's left more effciently, albeit with less umph.

I hope that that makes sense.

What is happening inside a battery is a chemical reaction between a chemical and two metal pieces. As you use up the battery the area immediately around the pieces of metal can become depleted resulting in a sudden drop in power.

Even though there may be a major depletion just around the metal parts, the battery itself may still have a bit of power left. Let the battery sit for a while and the chemical potential inside the battery evens out and you can use the battery again for a short while.

Jon

It's magic, simples.

I don't really know actually, I used to be told that you can put dead batteries under your armpits, or even down by your groin to get some body heat into them... I guess it's to do with the chemical reaction working better at a higher temp.

I do like to get the most out of my batteries though, take a kids remote controlled car for instance, it needs a lot of juice... When those batteries are 'dead' they will make the remote contro for the TV/DVD etcl work for another couple of weeks because the remote draws a lot less - the down side is that the remote stops working quite often, but the upside is you know you squeezed that battery dry!

[edit on 1/6/2009 by Now_Then]

reply to post by octotom


Uh, no. Toy cars don't have brains to readjust themselves. Unless you can equip them advanced sensors and chips. Which would make them expensive.

As Voxel pointed out - it's a chemical process.

It is far more observable in the dry-cell cadmium batteries. The voltage potential degrades rather steadily with time - a nice linear slope. When the voltage falls below nominal levels, there is still quite a bit of energy that can be re-distributed from the more chemically isolated regions of the battery.

It's not so noticeable with alkaline batteries - their voltage potential remains rather consistent, then drops off rapidly as they approach the end of their life. When alkaline batteries 'die' - there's not much chemical energy left.

Lithium-Ion batteries are somewhat similar, but they are far more temperamental than any of the other batteries. They also have a 'real lifetime' - from manufacture, the battery self-erodes and will degrade regardless of how many times it's charged or used. Buying a spare Lithium-Ion battery should only be done if you anticipate the need to use two batteries (if you do a lot of mobile computing and may need two batteries for your laptop, or something).

Keeping batteries in the refrigerator/freezer will slow their chemical processes and extend their shelf-life. Heating them up will increase their voltage and current capabilities. I wouldn't recommend heating them - as many batteries will experience a thermal run-away and explode (or vent hot nasty stuff all over....), or in the case of Lithium-Ions... become a flaming box of doom.

You may find that this effect is not present in lithium Ion batteries. Actually you will find that unlike most types of batteries who's voltage drops off somewhat gradually, a lithium ion battery will shut off instantly when it is depleted.

This is because if a lithium ion battery is discharged too much it will be damaged so there is a internal circuit board with a processor that determines when the battery is about to be discharged beyond its minimum safe capacity and then disconnects it.

Lithium ion battery technology is extremely fragile so the processor is also needed to babysit many other conditions as well. For instance it will not allow a battery to be charged if it is below -15 degrees Celsius and wont allow fast charge rate until the low charge rate has warmed the battery up to 0 degrees Celsius. It also limits the maximum charge rate, the maximum charge amount, the maximum charge temperature, the maximum discharge rate, and if the manufacturing company is a jerk it is also programed to fail.

Originally posted by octotom

But, the battery never really died. So, if I go back later, the car just "readjusts" itself to use what's left more effciently, albeit with less umph.

I've tried this on a plain electric motor, same results so your theory is wrong. The car/motor doesn't readjust itself to the smaller charge.

I'd rather choose the chemical reaction theory Voxel pointed out. It happens on most kinds of batteries I've tried including non-rechargeable types except lithium ion and lithium polymer (perhaps this is due to the sophisticated energy management system of many gadgets using lithium ion like mobile phones that prevents the device from functioning once the power falls below a threshold, not even waiting for the battery to go fully out.