# The Naked Scientists Forum

### Author Topic: Batteries  (Read 4902 times)

#### harryneild

• Full Member
• Posts: 88
##### Batteries
« on: 27/04/2006 16:35:56 »
How do the electricity indicators show me how much electricity ill be able to get out of my battery? Its the type where you have to press two buttons, at either end of a black strip, and the strip shows how much by a fraction of it turning yellow in a loading bar fashion.

How does this work?

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker
« Last Edit: 05/04/2007 22:01:00 by harryneild »

• Neilep Level Member
• Posts: 2175
• Scallywag
##### Re: Batteries
« Reply #1 on: 27/04/2006 16:56:22 »

The little disposable battery testers that you see on batteries or battery packages are a great example of combined technologies -- several existing technologies have been combined in a completely new way! Battery testers depend on two special types of ink: thermochromic and conductive inks. Thermochromic ink changes colour depending on its temperature. Conductive ink can conduct electricity. By applying layers of these special inks along with a layer of normal ink using a fairly normal printing press, it is possible to create an extremely inexpensive printed design that changes depending on the amount of electricity it receives.

What you do speaks so loudly that I cannot hear what you say.

#### daveshorts

• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Re: Batteries
« Reply #2 on: 27/04/2006 17:35:35 »
The thermochromic ink is put on top of a resistive strip that is narrow with a high resistance at the empty reading end and wide with a low resistance at the full end.

The power used to heat up the strip goes as I^2 R  so the narrow end will get hotter than the wide end for a given current

If the battery is almost empty, the small current will only heat up the narrow end enough for it to change colour, where as if it is full the larger current will heat up and change colour the whole strip so it will read full.

#### chris

• Neilep Level Member
• Posts: 5337
• Thanked: 65 times
• The Naked Scientist
##### Re: Batteries
« Reply #3 on: 28/04/2006 12:19:22 »
Dave - Can I clarify something in your answer which might, in its present form, confuse people:

The current flowing at any point in the circuit is the same, regardless of the resistance, or dimensions of any individual part of that circuit.

The test strip works by localised heating. Where the test strip is fattest (at the "Battery Full" end), there's a lot of metal to carry the current, and therefore dissipate the heat produced. As a result the increase in temperature - which is what triggers the ink to change colour - is small because the energy is spread out over a large area. This means that you need a big current flowing to make this part of the strip become hot enough to change the ink colour and signal that the battery is in tip top condition.

But at the "Battery Dead" end of the strip the wire is much narrower, so the heat energy produced by the current is confined to a small area, making it much easier for the temperature to rise and change the colour of the ink.

So, as you go along the wire conductor, it becomes progressively wider, spreading the heating effect out over a bigger area and therefore meaning that to trigger the ink at each point in turn requires a progressively higher current.

The current is driven by the voltage (potential difference) produced by the battery. As a battery "tires" the potential difference drops progressively.

Ohm's Law says V (voltage / pd) = I (current) * R (resistance). Since R does not change much (it'll increase a little due to heating), I is proportional to V. As V drops (as in a tired battery), I drops too, and hence the amount of heat produced by the battery tester drops at each point along the test wire.

Dave - any idea how much current flows through the tester? I'd think it's quite energy hungry. In that respect no wonder battery makers are so keen to add them to their cells, if they encourage you to flatten the batteries and then buy new ones!

Chris

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

#### The Naked Scientists Forum

##### Re: Batteries
« Reply #3 on: 28/04/2006 12:19:22 »