Battery Capacity

[mark]

Why is it that rechargeable batteries state the capacity,eg., 2500 mAh but the 'use once' sort don't?

mark

[The Natural Philosopher]

becaue..they dont actually HAVE a capacity in tesame sense.

A rechargeable is generally rated at a 10 hour discharge rate: with decent cells that capacity is broadly constant across anything from 10 minutes to about 100 hours. Dry cells are not like that. They rapidly polarize under high currents, but then 'recover' if left alone.

[robert]

If you look in the technical data on the manufacturers sites you will find mAh data. ie

[John Rumm]

I suppose its a selling point and also indirectly an indication of quality on a rechargeable - they are probably also bought by more clued up users.

Dry batteries also have a stated capacity - but often only on the makers data sheets rather than boasted on the side of the battery.

Note the discharge characteristics will be different - you may not be able to draw the full capacity from a dry cell in quite such as linear was as you can from a rechargeable.

Some of the figures are available though:

[Dave Plowman (News)]

Probably because it's not in their interest. Better to show bunnies going on and on...

But some pro suppliers do give these details.

[meow2222]

Also when one question is answered, the next one is raised. And the next one would be why do so many one shot batteries give only a fraction of their rated capacity in real life apps. Some questions are better left unasked.

NT

[robert]

Often I would suspect because some devices cease working before the terminal voltage used in the capacity calculations is reached.

[Man at B&Q]

I haven't bothered with theirs for quite a few years.

MBQ

[Man at B&Q]

Time to mention

again?

MBQ

[Man at B&Q]

Is it really true that a primary cell takes more energy to manufacture than you'll get out of it, or just an urban myth?

MBQ

[Andy Burns]

If it was false, wouldn't it best to power the factory making them, from lots of them?

[Dave Plowman (News)]

I'd bet it is true - and by a factor of many. Then there's the energy used to transport it etc.

[Bob Mannix]

Of course it's true, otherwise the laws of thermodynamics would be in a pickle and the universe would probably explode fairly quickly as entropy would be reversed!

[Mark]

No. My perpetual motion machine runs off them. ;-)

[John Rumm]

I still have a 1995 Maplin catalog on the shelf - kept because it was the last one to include lots of technical information and device pinouts etc.

[Rod]

I have recently had a set of Energiser Lithium AA cells in a camera. They lasted *far* longer than 'ordinary' alkalines which I usually use. Maybe they are sufficiently good to be worth paying their hefty premium? But I do wonder how much is simply loss of voltage rather than actual energy content.

[robert]

Quite - if the camera or device's battery life detector can't be set up for the different discharge characteristics of the available batteries then there is a good chance that the device will close down while there is usable energy in the battery.

However looking at the Ultimate Lithium discharge curves - they seem flatter than alkalines and thus shouldnt suffer from early close down.

[Rod]

Thanks.

[dennis]

Now that explains what was before the big bang. Have you suggested it to that Hawkins fellow?

[meow2222]

There was a supposed perpetual machine that did. It used 2 battery piles disguised as just supporting pillars to electrostatically attract a clock pendulum, and this slight force was enough to keep the clock running.

NT