Introduction: How to Run a Battery Electric Clock on Dead Batteries
Few electronic projects are as simple as this one. It will appeal to those who get a kick out of extracting the last vestiges of energy from flat batteries.
Step 1: Overview
Most households will have several examples of a clock containing the sort of movement shown in the picture.
This sort of clock runs on a 1.5Volt cell and will function until the battery voltage drops to around 1 Volt and an alkaline AA cell will last several years. I have found experimentally that this sort of clock movement is extremely voltage tolerant and will happily run at 2 Volts. This fact enables us to run such a clock on two dead alkaline AA cells in series that would otherwise be thrown out or sent for recycling.
You will need hook up wire, masking tape, kitchen towel, a seal-again plastic bag, solder, a soldering iron powerful enough to tin the ends of the batteries and some half inch wooden dowelling. You will also need a couple of dead alkaline batteries, ideally discharged to below a Volt but above, say, 0.7 Volt. It does help to have the means of accurately measuring the battery voltage and you may find that batteries removed from some applications still read 1.2 or 1.3 Volts and can be transferred to clock duty in their own right. Note that zinc/carbon cells are not recommended as these may be more likely to leak.
A possible source of 'dead' batteries is the recycling bin provided at some shops and we have a well organised one at our local library. Perhaps, with permission, such bins could be raided for supplies and you may find that the proportion of batteries still reading above 1.2 Volt will allow you to run your house clocks for free and bypass the need for this project.
Step 2: Battery Clock Power Requirements
It may be instructive to show just what the power requirements of this sort of clock movement actually are. To this end I put a 10 Ohm resistor in series with the battery and monitored the current with an oscilloscope.
The first picture is a screen dump of the current flowing through the 10 Ohm resistor. It shows very narrow pulses once per second with virtually zero consumption in between. The second picture shows the narrow pulse expanded and shows a pulse magnitude of roughly 4 milliamps with a pulse width of 32 milliseconds. Averaged out over a second this equates to a continuous current of around 130 micro amps which I find incredibly low--these clock movements are very efficient indeed and, I think, a credit to the designer.
Step 3: Assembly
The end connections of the batteries should be gently abraded to assist the soldering process. The two batteries are taped together with masking tape, the positive connection of one next to the negative connection of the other and the 470uF electrolytic capacitor then taped towards on end of the assembly as shown in the second picture above.
The negative connection of one battery is then connected to the positive connection of the other at the lower end of the assembly using cored solder and a short length of hook up wire as shown in the second picture above. By this means the batteries have now been connected in series. At the other end of the assembly the electrolytic capacitor is connected across the positive and negative connections and two lengths of hook up wire are connected, one to each connection of the electrolytic capacitor. These form the power feed to the clock so cut the lengths accordingly. See the very simple circuit diagram above. Be sure to observe polarity when connecting the electrolytic capacitor.
The batteries may be considered as long past their best and the function of the electrolytic capacitor is to provide a reservoir of power during the short pulse every second.
The third picture shows that the assembly in picture two has given us just over two volts.
Step 4: Bringing It All Together
Before we install our battery assembly in the clock we have to take some precautions. Dead batteries tend to leak and create a corrosive mess and render battery connections useless although modern alkaline cells are better in this respect.
We have no battery connections here in the normal sense but leakage into a much loved family timepiece is unthinkable. We must hope that our assembly does not leak but plan for the fact that it probably will. We start by wrapping the batteries in kitchen towel and taping up tight with masking tape to form a compact package. This is then placed in a plastic seal-again bag which is sealed with just the two wires poking out.
The plastic bag is then taped to the clock with masking tape as in the picture. Observing polarity, the wire ends suitable bared are wedged against the clock battery connections using a length of wooden dowel sawn to be the same length as an AA cell. A small covering of aluminium foil over the dowel ends can aid the making of electrical contact.
This construction may seem very crude but it is effective and it does mean that the clock can be restored to its original condition in a matter of seconds when the batteries are truly dead.
Step 5: Some Afterthoughts
The obvious question is "How long will these batteries last"?
The photo shows the clock/battery combination after nearly two months of operation. The voltage has dropped by just 0.1 Volt and projecting forward might indicate that a year or two might be achieved so long as there is no catastrophic leakage.
Further, in separate Joule Thief experiments I am finding that alkaline AA cells discharged to around 0.7 Volt and subjected to a current drain of only a milliamp or so seem to maintain this voltage or even recover slightly and our assembly may stabilise somewhat at around 1.5 Volts . Since our average current drain here is less than a milliamp we may be pleasantly surprised.
When these cells are finally recycled they will be well and truly 'dead'.
Finally, although this project may seem eccentric to some, there are many who consider that we throw away a lot of energy when discarding batteries and get a kick in salvaging some of this.
I have embedded a YouTube video that covers most of the points in this presentation.