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Archive for March 12th, 2009

Clock-radio Backup Battery vs Current Drain

Clock-radio battery hack

Clock-radio battery hack

The little red Battery Sentinel LED on our old Realistic (a.k.a. Tandy a.k.a. Radio Shack) clock radio was on this morning, which means that, once again, the backup battery needs attention.

It’s supposed to use an ordinary 9V battery, but it ate two or three of those a year. Given the absurd cost of 9V batteries relative to AA cells, that stopped making sense pretty quickly.

Most devices with backup batteries draw essentially zero power from them during normal operation. This gadget draws 6 µA.

An alkaline 9V battery has a capacity of about 500 mAh, maybe more with a low-drain load like this. That should last for a few years:

500e-3 / 6e-6 = 83k hours = 500 weeks = 10 years

Alas, the clock battery monitor is really fussy and triggers the LED when the voltage drops under about 8.5 V.

[Update: the clock does a “battery test” every day, which probably accounts for the short battery life. I haven’t measured that current… or the duration of the test.]

Fortunately, the clock case has a recessed bottom that fits a standard AA cell holder like a glove. I wired up 1-1/2 4-cell holders (yes, I should have used 7 cells, but I wasn’t sure what the upper voltage limit might be) to a standard 9V battery snap connector and screwed the assembly to the case.

Now all I must put up with are the weak AA cells I got from batteries.com; the most recent order was a disappointment.

Memo to Self: That snap connector has red = negative / black = positive!

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