When this movement first appeared in the early 1960s, various literature showed a variety of ways to power the movement. In the left photograph below, one demonstration model had what appeared to be two battery compartments on the left hand side of the watch; in fact, only one was connected to the movement with the other solely for storage of a spare battery; it also had a display back. In the right photograph, a Landeron accumulator version of the Landeron 4750 based watch is being recharged using a conventional Ever Ready 1.5v U10 battery.

The first set of photographs below have kindly been supplied by David Read who owns these rarities. They show three Avia using the accumulator version of the Landeron 4750 movement:

I’d been searching for an example of these rare accumulator watches for many years (about 10 years I think!) but in 2012, two came my way, rather like London buses! Both look to be ESA demonstrator models: one with “ELECTRIC” on the dial and the other with “SWISSELECTRIC”. And both have their Landeron calibre numbers on the dial. The “A” on the dial probably indicates this is the accumulator version of the watch. There are more photographs of the “SWISSELECTRIC” watch here.

These watches date from the early 1960s and although many watch makers used these Landeron movements, nearly all are powered by conventional WD-4 or WD-5 batteries. The accumulator version of these Landeron-based electric watches did not last very long and, as a result, this version of the watch are hard to find.

Originally, the Lechanché 80 mA re-chargeable cell needed charging every six month but after 50 years, many of these cells only hold a charge for a few weeks. Sadly, the Lechanché 80 mA re-chargeable cell is missing from the “ELECTRIC” version of this watch but the one in the “SWISSELECTRIC” holds a good charge for at least 6 weeks. It is sometimes possible to revitalise these 50 year old Lechanché cells and David Read has kindly provided the following guide for me, and I reproduce it here with his permission:

The restoration of old rechargeable cells (accumulators) is problematical and often not possible. However, it is worth trying because if the cell has not leaked, corroded or been abused by having been left connected to a load when completely flat, it might be possible to improve its ability to hold a charge. There seems to be little written about this, so the following notes in connection with the Avia accumulator watch are my own.

1) A brand new rechargeable cell has a very low internal resistance. This means that there will be negligible voltage drop across the cell itself. This in turn means that an EMF of 1.5 will provide sufficient ‘surplus’ voltage to effectively charge the accumulator by using the charging attachment supplied with the Avia. It will also be safe because as the terminal voltage of the cell rises to about 1.3 when fully charged, the ‘surplus’ voltage to do the charging will fall to 0.2 V and charging will self-terminate.

2) However, as rechargeable cells age, or are left unused for many years, they can develop a significant internal resistance. In this condition the simple charging circuit will include the resistance of the aged cell to the flow of charging current and a voltage drop will, therefore, occur across the cell itself. This voltage drop can be more than the difference between 0.9 volts discharged and 1.3 fully charged and will quickly approach (or even exceed) that of the single 1.5 volt cell being used to charge it. As a result, there will be little or no ‘surplus’ charging voltage available and insufficient current will flow through the charging circuit.

The capacity of the cell used in the Landeron 4750 is normally shown as 80 mA, so if one chooses a charging current of 10 milliamps and a 12 volt charging voltage, one will need a resistance of 1200 Ohms to control the current to 0.01A. (12 V divided by 0.01A equals 1200 Ohms). It is best to make the resistance variable so that the amount of current can be precisely adjusted to suit the degree of internal resistance that has developed in the cell and at the same time be controlled to 1/10 of the nominal capacity of the accumulator.

Now as the charging current causes a voltage drop across the cell being charged, it will, nevertheless, be such a small proportion of the charging voltage that the current will continue flow through the cell. It will not be a true constant current supply but its variation will be small. It is vital to remove the cell from the watch when it is being charged from a constant current source such as described above because if the voltage drop across a cell is more than 2 volts the movement will be in danger of being seriously overdriven.

4) The procedure outlined above will ensure that a charging current goes through cell, and sometimes an improvement in its ability to hold a charge and drive the watch will take place. However, the extent to which it is possible to recover a cell that has developed significant internal resistance will depend on whether or not the internal resistance of the cell is reduced by the charging process. This can be observed by measuring the voltage drop across the cell whilst on charge and noting if the voltage drop reduces. Ideally, it should reduce to 1.4 which is the terminal voltage of a freshly charged cell and will, therefore indicate that the internal resistance is negligible. However this will not be case with an old cell that has been left completely discharged for some years. For these difficult cases, one can try rapid short pulses of high current to try and break down the internal resistance.

5) Rechargeable cell (accumulator) chemistry is complex and when in poor condition, or past the point where a good recovery can be made, some remarkably high voltage drops can occur on charge, even when only 8 to 10 mA is flowing. I have known some very high voltage drops across rechargeable cells whilst on charge. This means that if a cell is being charged in the watch, a dangerous amount of overdriving the balance will occur. Efforts to recondition a long unused or apparently dead cell should therefore be done with the cell out of the watch. You can see from this that is important to have the charging current supplied in series with an ammeter and also to have a high impedance voltmeter across the cell in order to keep an eye on what is going on with respect to the voltage drop across the cell.

6) The two attached images illustrate the simple quasi-constant current circuit described.

In order to charge the cell out of the watch I have made a simple connection to it by means of a cloths peg with two brass nails in its jaws. You should be able to see this in the images. In order to control the current to an appropriate figure, the circuit contains a wire wound variable resistance of at least 1000 ohms. One of the images shows a known good cell being charged and the other image an old cell from an Avia that will only drive a watch for a few hours.

In the first image you will see that I am charging a coin cell in good condition. The cell is too thick for use in a watch but a perfectly good example to use for this illustration. Note that the charging current is 8 milliamps and the voltage across the cell is 1.3 volts. In other words there is negligible resistance in the cell and its voltage simply reflects that it is fully charged. This is how it would look with a new Léclanché for the Avia.

In the second image you will see the stainless steel case back and cell of a gold plated Avia between the jaws of the cloths peg. Note that with a charging current of 8 milliamps the voltage drop is 7 volts. Dangerously high if the cell was being charged in the watch case. However, over an extended charging period this dropped to 5 volts showing that some improvement was taking place. Even so, the internal resistance is very high and it is unlikely that this cell will improve much.

 

My Avia in the 18 carat gold case has a cell which will drive the watch for a week but no more. All in all one has to be lucky with these early cells because whilst some improvement can often be brought about by what I have outlined above, it is rare to get one in which the problem of high internal resistance can be cured completely.

C’est la vie!
David Read
29-9-2012