Sometimes a battery-operated device (like my Walkman) runs out of juice and quits working. But before I replace the batteries, the next time I try it, it works. Not for long, but it works. How can batteries be so dead that something doesn’t work, but if you let them rest for a while they can cough up a little more power?
Batteries are chemical devices.
When the reactants are used up, the battery is “dead”.
In devices that use alot of current, the reactants are used faster than they can migrate around inside the battery. For example, most batteries have dissimilar conducting plates with a liquid electrolyte. The electrolyte is caustic and chemically disolves the conductive plates. During hard use the electrolyte very close to the plates will be “more depleted” than the electrolyte elsewhere in the battery. Letting the battery rest gives osmosis (probably) a chance to even out the strength of the electrolyte around the interior of the battery.
With fresher electrolyte in contact with the plates, the battery will be a bit stronger for a while after a rest.
Hope that made sense.
This is a common phenomenon in car batteries. If you leave the lights on and then cannot start the car, let it rest for a while (with the lights off) and try again.
I’ve also noticed this effect. In addition to the “chemistry explanation” provide by scotth, I wonder if the battery’s source resistance has anything to do with it? Under heavy usage the battery heats up, thereby increasing its source resistance (it has a positive temperature coefficient) and decreasing available current & power. Allowing the battery to cool reduces its source resistance, which increases the amount of current and power it can produce. I’m confident this is a factor, but just not sure how significant it is.
I use a cordless drill at work a lot, and when the battery goes dead, usually at the worst possible time, I take the battery out of the drill and shake it violently for a few seconds, replace it in the drill, and voila, it works again, for maybe a minute or two. Any explanation for this?
Unless it’s a lead-acid cell (which I doubt), I would venture to suggest that the shaking does little except keep your arm busy for a while; have you tried taking it out, not shaking it and putting it back?
Crafter_Man: I’m pretty sure that a few drops of extra power can be squeezed out of most batteries by actually warming them up because the rate of (most) chemical reactions increases as the temperature rises.
whoa…and i was thinking along the lines of hydrogen build up that decreases the reaction rate so it needs to rest and let the gas excape…guess i’m nowhere close to the answer…am I?