Hello, I'm a beginner. My question is - when I tested several 'dead' 3 volt batteries with a multimeter the reading is 3 volts, so how come they're 'dead'?
Thanks for any info,
Gerard

 

When you test a battery by simply measuring its terminal voltage using a high impedance voltmeter it puts virtually no load on the battery. That is the voltmeter draws negligible current from the battery. Since the function of a battery is to supply current at some voltage a voltmeter only measures part of what it is suppose to do. So to really test a battery you need to measure the voltage while it is connected to a load drawing some realistic current. If the voltage is OK when it is supplying current to a load it is OK. Otherwise you will see the voltage drop down considerably from its specified voltage if you connect a load resistor across the battery terminals while testing the voltage.

 

Excellent, wmodavis, I can understand that. So, I have many old 3v coin cells, 1.5v AAAs, then it seems I should be able to use them for something that doesn't draw much current, maybe LEDs? I guess I'd need to find out how much current the batteries can supply - what's a good way to measure that?

 

Try measuring the current with about 360 Ohms resistance in series with a milliammeter.
A good 1.5v alkaline battery would measure around 4.0-4.2mA, NiCD around 3.8mA.

If your batteries are exhausted, you will hardly measure anything.

 

A good place to look is the manufacturer's data sheet. You can also read about using a DC load to test a battery here (also check the PDF given at the end of the document), but most hobbyists won't have a DC load, as they are expensive instruments (but handy when you need to do such things). You can build a suitable load yourself from a MOSFET, but it's a bit of a complicated project for a beginner.

Fortunately, a beginner can do some relatively simple things to load a battery. The easiest would be to have some resistors to hook across the battery as a load. You have to make sure the resistor would be capable of withstanding the power being dissipated in it. Plus, you'd want to be careful, as it's easy to get those little white burn marks on your fingers from holding a resistor that gets a bit hot (guess how I know ). Another reasonable load could be a flashlight bulb rated for the voltage of the battery.

Here's an example. A typical LED load is about 20 mA. For a 3 volt battery, you'd need a load resistance of (3 volts)/(0.02 A) = 150 ohms. The power rating of the resistor needs to be more than the current through the resistor times the voltage across it. Here, that's minimal at 3(0.02) = 60 mW. If the battery was unable to keep a voltage of around 3 volts across a 150 ohm resistor, it's time to put a tag on its toe and send it to the morgue.

 

Here's an example. A typical LED load is about 20 mA. For a 3 volt battery, you'd need a load resistance of (3 volts)/(0.02 A) = 150 ohms. The power rating of the resistor needs to be more than the current through the resistor times the voltage across it. Here, that's minimal at 3(0.02) = 60 mW. If the battery was unable to keep a voltage of around 3 volts across a 150 ohm resistor, it's time to put a tag on its toe and send it to the morgue.

That's fantastic; like I said, I'm a total beginner, so I think I'm just about following it - if the resistor is less than 150 ohms I guess that means that the LED is getting too much current and burns out? And the resistor has to be 60mW because that's how much power is in the circuit? Does that mean that the LED also has a power rating of 60mW? Sorry if my questions are too basic, I'm trying to learn this stuff myself and finding the right steps on the internet takes a LOT of searching I got the old Forest Mims "Getting Started.." book and I've ordered his kit from Radio Shack. I think getting a handle on basic electronics is considerably harder than basic computer programming!

 

You can learn this stuff too -- it's not that hard. I strongly suggest you head over to the AAC book on DC electronics and read chapters 1 and 2. Your questions can be answered by knowing that material.