www.nightvisionforumuk.com

Right tech heads. Here comes the question.....
My thermal is 9 volts. Plus or minus .3 apparently.
It uses 4 aa battery's 2 up and 2 down each 2500mA and when they work they will normally last about an hour and an hour and a quarter.
So I need to make a tester because I think some of them might be heading for the land of fooked. What little I understand about the world of eleckertrickery tells me I need to get a 100ohm resistor and connect it via my multimeter and that should give me my answer.

Does that sound correct?

Fully charge the batteries then fit them to the device and run for 30min. then test Volts and see what the readings are, if you have some that are vastly out from the rest then change but if some are on way out then it would be better to change all 4

Using a resistor as a discharge tester just makes them worse !

Kind regards Splatty

And what happens when the unit won't fire up?

I'm not a sparky so take this with a pinch of salt.

Batteries are best tested under load conditions so I see where you get the resistor idea from. If you have a torch for example that runs on one or two of those cells perhaps you could compare the current(s) between the tail cap contact and the base of the battery when turned on.

Plan B is to put them in this type of charger that shows individual battery voltage and how much charge they are taking:

http://www.ebay.co.uk/itm/261586391328

I'm sure someone will be along soon with the best test solution.

EDIT: this suggests you were on the right track:

https://www.youtube.com/watch?v=HcikzMG7mMU

Do you have a volt meter ?

That I do.
Don't necessarily know how to use it properly though.