Excellent idea. I was having issues measuring batteries the other day and thought about making a dedicated battery holder with terminals to easily connect my multimeter. This is so much nicer.

Nice use of flex, and magnets :)  

You could probably even eliminate the button and generate an interrupt on the attachment of the battery, but you might want to keep it just for mode switching or such on different battery types.  For the most part, I suppose the initial voltage would get you 90% of the way there though.

I really like that you have chosen to test both the initial voltage, and then the current over time.  Lots of batteries will recover to nearly full voltage just by equalizing, but then drop like a rock when you try to actually draw power out of them.  Good plan.

Thanks!

I'll take a look at the interrupt idea, that might work

Thanks, but I can't take credit for the load idea. Borrowed that from Andreas Spiess' design

Looks like the reset button is the still the best option. Had a look at the ATtiny datasheet and only pin 7 can be used for a external interrupt from sleep mode,  and it is the clock pin for the I2C display.

For a moment I wondered who would still have floppy disks to test. 😀 I guess a lot of people now haven't seen one.

I figured someone was bound to think that😄