why does a dead button battery still put out full voltage?

[TheUnicorn]

Went to turn on my kitchen scales yesterday, nothing on the screen whatsoever. Put a replacement battery (a 2032, which was like for like) in, from a packet that had been sat around far too long, again nothing. A ran a multimeter over both batteries and they were both putting out a solid 3 volts. Went to the shop today, new battery, scales works no problem.

I think I'm right in saying that in alkaline batteries rated at 1.5v (AAAs and the like) a battery is normally considered dead when it reaches 1.2v (roughly, obvously some variation according to the device they are in), so by that logic 2 x AAA would put out 3v new (possibly slightly higher in reality?) and would stop being useful at around 2.4v.

So why is a lithium button battery that is still putting out full voltage not powering my scales.

I assume that they put out a consistent voltage but the amps drop? Is this they reason that button batteries have become so common place?

 

[ajs]

With primary lithium cells the discharge generally manifests itself as increased internal resistance. It isn't a question as to whether it can supply volts or amps as they are fundamentally interconnected in any given circuit. Rather as the current drawn from the battery increases the voltage it supplies decreases.

A modern DMM as a voltmeter has essentially infinite resistance for common and garden purposes, from an electrical perspective it is effectively not there at all, so you read the full voltage. In the device current is being drawn which reduces the voltage. The same voltage is still being generated as it is governed by chemistry, but it's being lost inside the cell.

 

[TheUnicorn]

? so under load the battery is not putting out 3v but as the multimeter is not putting the battery under load reads it at 3v ? have I understood that right

 

[MorrisWoodman12]

@ajs is exactly right. If you really want to measure the condition of a battery it should be measured under load, preferably a similar one to the product it is used in. this will expose any deterioration due to internal effects like an increase in the internal resistance.

EDIT: our posts crossed. Yes you have understood it correctly.

 

[TheUnicorn]

so as I couldn't get a probe to the underside of the battery, in situ in the scales, in practical terms there is no way of testing the battery, just replace it and see if the device works?

It was a bit annoying last night not knowing if the machine or the battery was dead.

could I remove the battery holder from some 3 volt fairy lights and try to power them with a button battery? trying to think of a solution that would test any size (dimensionally, not voltage) of battery

 

[Chubber]

My inexpensive multimeter has a '9v battery' and '1.5v battery' test position. I believe they put an appropriate resistance in series with the battery to determine whether or not it can deliver the power it is designed to.

Am I thinking along the right lines, please?

 

[John Brown]

A new 2032 should measure closer to 3v6 when open circuit, IIRC, so 3v0 is not so good.

 

[toolsntat]

so as I couldn't get a probe to the underside of the battery, in situ in the scales, in practical terms there is no way of testing the battery, just replace it and see if the device works?

It was a bit annoying last night not knowing if the machine or the battery was dead.

could I remove the battery holder from some 3 volt fairy lights and try to power them with a button battery? trying to think of a solution that would test any size (dimensionally, not voltage) of battery

You could possibly slip a bit of tin foil under the blind side with a bit of tape to insulate from the live battery rim.
Cheers, Andy

 

[TheUnicorn]

A new 2032 should measure closer to 3v6 when open circuit, IIRC, so 3v0 is not so good.

'open circuit' meaning not under load? so I can still use the meter to test, just need to look for the actual (full) voltage as oppossed to the nominal voltage of 3v?

 

[Spectric]

Just re visit ohms law, this is why you can get electrocuted by touching an open circuit neutral and an issue when dealing with PME electrical supplies..

 

[Amplidyne]

My inexpensive multimeter has a '9v battery' and '1.5v battery' test position. I believe they put an appropriate resistance in series with the battery to determine whether or not it can deliver the power it is designed to.

Am I thinking along the right lines, please?

Possibly /probably. Try reading the voltages with the battery tester setting and with just DC volts. If the battery tester reads slightly lower then odds on it has a loading resistor built in. The resistor is in parallel with the battery to put a dummy load on it.