multimeter - what can they do ?

[kahunabean]

Can a multimeter (which the trainspotter in me has always wanted to own) measure the mAH of a battery ? I have a batch of rechargeables - some are past it, others are fine. I realise (now !) that mixing up the good and bad ones in a batch of 4, say, means that the device relying on that batch of batteries doesn't work for more than a uselessly short time. So I need to (easily) differentiate between the goodies and the baddies. Without the help of Dirty Harry (!) I can think of primitive ways of testing them, but i want to go hi-tech....

[Squirrel]

I don't know much about the internal working of a battery, but I don't think you can tell exactly how much charge is left in them. On standard AA sized cells (the common 1.5V ones), there is a label on the side that says "1.5V, 0.5Ah", so you could time how long you have left from 0.5Ah, I suppose. The problem, as you know already, is that the multimeter can't "know" what kind of cell it is testing e.g. if it's a small 1.5V 0.5Ah cell or a large 1.5V 50Ah one. Also, I think that the current diminishes as the battery wears down, so a graph of time against current could help.

[tomr]

Not on its own.
Ah ratings of batteries work as the name suggests, a 1Ah battery will supply 1A for one hour, or 0.5A for 2 hours etc. (mAh are milliAmpere hours A/1000)
To measure the capacity of a battery would require a test set which provided a load and measured the discharge current (or battery voltage) over a period of time. This is the sort of test that is applied to a car battery - a load is applied and the voltage is monitored across the load. If it decays too quickly the battery is shot.
Make sure that you calculate your load values correctly so as not to have overly hot resistors.

[vaa]

the question interests me 2:
Problem: verifying the MaH claim of a cheap 7,4V 800MaH NiMh rechargeable camera battery.

How would such a test go, so that it is relevant for practical purposes (digital-camera relevant type of load)?