UPDATE Early 2016. Still working i.e. batteries last 5+ months which is enough. Case closed.
UPDATE Early Nov 2015. Clock on the spot. The new batterypack has worked for 3 months. All is fine.
So with a new Multimeter I of course had to measure how much current (Ampere) the clock consumes. The estimate in previous post – given the assumption that an A23 battery contains 55mAh and runs out in two days (48hours) gave 0.055 / 48 = 0.0011 A = 1.1 mA. The estimate turned out to be quite accurate, in fact the clock consumes 0.720 mA. Not far off!
So with that measurement and given that the new AA-battery pack contains at least 1.8 Ah we should get 1.8 (Ah) / 0.000720 (A) = 2500 hours or 2500/24 = 104 days of power. Not bad. And if the battery contains 2.6 Ah (typical range for AA batteries are 1.8 – 2.6 Ah) then this figure would go up to 150 days.
Still theory – but if true the new battery should last a typical season here. New batteries in the spring – and the clock will run through summer. So far the clock has worked for 6 hours so I have to wait and see..