Aug 2015. Clock Ampere Surprise (1/2)

I was overly optimistic about my separate power feed to the quartz converted clock. I assumed a 12V battery (type “A23”) would keep the clock going for a year or two. That is not the case. After installation the battery was drained in days. I thought it was a bad battery so I bought a new one. But after a couple of days clock was dead again. I’ve mailed to “speedycables” in the UK (that did the conversion) to get some specs but havent received any answers yet so time to google Ohm’s law and things about electric power. Question is, how much power can a Quartz clock consume?

Here’s an estimate. 

Wikipedia says a typical 12V A23 battery has something like 55 mAh. That translates into that the battery is capable of delivering 0.055A (Ampere) for 1 hour. I wish I had timed it but didnt, but say that the battery was drained in two days – i.e. 48 hours. Given Amp-Hour logic we get I = 0.055 (Ah) / 48 (hours) = 0.001146 Ampere. Not much. But if true, a lot for a tiny battery like the A23.

To put it into perspective a dashboard light bulb consumes 3 Watts. Given the electric power law we have P = U * I, or I (in Ampere) = P (in watts) / U (in Volts) = 3 / 12 = 0.25 A. Hence a light bulb takes 0.25 Ampere whereas the clock takes something like 0.001146 Ampere. More than expected but ok, could be reasonable. Or, a A23 battery would keep a 3W bulb lit for 0.22 hours. Sound very low so I am not sure now. But given that the clock runs for a couple of days and a typical instrument bulb would last only for 10 minutes may be reasonable. Now I get a bit confused but regardless, I know that this is a…

Bad Solution:

Skärmavbild 2015-07-05 kl. 20.56.52

A broader perspective may be the car battery itself. A normal battery would have 60Ah or more. 1 Ampere is A LOT. It doesnt run a starter engine but can do a lot of things for 60 hours. As an example it would run 4 fully lit 3W 12V lamps for 60 hours or 2.5 days. A headlight can be a lot more, say 50 Watts and that would mean (I = P / U) that it consumes 50/12 = 4 Ampere. So a battery should give full headlight for 60/4 = 15 hours. That’s why we should leave the lights on over night 🙂

So what to do?

Ok, I need more energy than what a A23 battery loads. So after some searching I found a battery holder serie-coupling 8 AA (or “R6”) batteries. A normal 1.5V Alkaline AA battery will deliver (wikipedia gain) 1800 – 2600 mAh. That is equal to 1.8 to 2.6 Ah. So lets take the lowest value to continue, 1.8Ah. A battery holder serie coupling eight 1.5 Volt batteries will give 8 * 1.5V = 12 Volts. If each battery houses 1.8 Ah in 1.5V I assume that 8 serie coupled giving 12V houses the same, i.e. 1.8Ah.

Given that and the Ampere-consumption found above the mathematics should be simple. The short cut would be to compare 1.8 Ah (AA Alkaline battery) to 0.055Ah (A23 battery). 1.8 / 0.055 = 32 times longer life. And if the A23 battery lasted for 48 hours the 8-AA-pack should last 32 * 48 h = 1 570 hours = 65 days (instead of 2 days). Same comes out if you go by the formulas and current (Ampere) consumption. This is conservative estimates I think, if the clock worked three days instead of two, and if my new AA batteries carries 2.6Ah rather than 1.8 the life span will go from 65 days to 141 days.

Well this is just the theory, but this battery pack will be installed shortly. Read next post (2/2) for more info.

EDIT: this pack lasts 6 months+


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