Nimh battery tester

I was having trouble with a timer I use while exercising, it worked but because of the noise from the Nordic Track I wasn't hearing it.  I finally realized that the battery was low.  It is powered (for no good reason other than voltage) by 10 Nimh AA cells in series.  Open circuit when just charged these will be at 14.26 volts.  This gives decent volume to the piezo sounder - even while exercising I hear the clicks.    Taking the battery holder out I noticed voltage was down below 5 volts !  

But because the timer uses very little current I don't routinely check the battery - so I'm thinking I should add some kind of battery test circuit to the timer.  I could probably do this with the timer PIC itself but I've already used all the pins.

I do have the DC Beeper kit:

http://www.wb9kzy.com/dcbeeper.htm

so I could use that although it doesn't load the battery.  Also I can't find the prototype !  I'd hate to build a kit from stock !

I did find this notebook entry for the Sears VOM battery tests:

But I will need to take more data to establish the "red" and "green" areas:

I did find this interesting Nimh discharge curve:

source: https://blog.ampow.com/nimh-battery-voltage-chart/

The decrease of voltage at lower temps is important for setting test thresholds.  In the winter it's rare to see 25C in the house, usually in the unheated basement where the Nordic Track is located it'll be more like 10C.  If 85 mA is used as the load again that would mean a load resistor of 150 ohms ?  (12.5 volts / .085 amps = 147 ohms).  It would have to dissipate about 1 watt (.085 amps x .085 amps * 150 ohms = 1.08 watts) so a 1/4 watt resistor probably NG.

I'll have to think about it, take more data and also try to find that silly DC Beeper board.  The battery should be OK for a while yet.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm