Transponders
Identification transponder systems, mobile data carriers, road pricing systems, sub-sea transponders

Automotive Electronics
Mileage or kilometre counters, onboard computers, electronic monitoring, navigational equipment, airbag sensors and gas generators, belt straighteners, car radios, container temperature loggers, community traffic control systems, community vehicle locating systems, locating equipment for vehicles, traffic volume control, traffic chart recorders, taximeters, tyre pressure control systems

High Temperature Systems
Measurement-while-drilling (MWD), machine control, various industrial and automotive applications

2.1 General
This brochure deals with Sonnenschein Lithium Batteries. They belong to the thionyl chloride 3.6 Volt system and are manufactured in three basic series that differ by the process details of manufacture and are optimized according to the target application characterized by the following keywords.

Voltage Stability
It is a general feature of thionyl chloride batteries that voltage remains stable throughout their lives. The discharge curve typically has a rectangular shape, as can be seen from figure 2-1. A slight decline of the voltage that may occur during medium current discharge is due to an increase of internal resistance. Whenever discharge is interrupted voltage will return to its original value. This makes it possible to use virtually 100% of the battery's available capacity at a level well above 3 Volts. Please refer to paragraph 2.8 for more information on this subject.

Voltage Delay
When a battery is taken from the shelf and put on load for the first time, the cell voltage will drop from open circuit voltage (OCV) to an operating voltage that is a function of the discharge current. At low currents, the voltage level will stabilize instantly, see curve A in figure 2-2. However, at higher current values, there may be a transition period, during which the initial voltage drops below
the plateau voltage before recovering. During his period, voltage may stay above the application cut-off voltage which is typically between 2.5 V and 3.0 V. Curve B in figure 2-2 represents this case. If current increases even more, voltage may drop below cut-off for a short time. The time before it recovers to the application cut-off is referred to as the delay time and the lowest value of voltage reached is called the transient minimum voltage (TMV), see curve C in fig. 2-2. The voltage delay phenomenon is due to passivation. It is related to the protective layer that forms on the anode surface and is described in more detail in chapter 3. Once a battery has been depassivated which means voltage has reached the normal plateau of operation it will not passivate again unless there is a subsequent long period on open circuit.

The degree of passivation is a function of storage time, current temperature during storage, and mechanical aspects. Thus, passivation will usually grow with storage time and temperature. Depassivation can be effected by current flow as well as mechanical shocks, vibration, and temperature cycling. As a rule of thumb a current of 2 µA/cm2 of lithium anode surface area will prevent passivation and allow for immediate voltage response above typical application cut-off values. The same can be achieved by daily pulses corresponding to equivalent or slightly smaller average values.

	Figure 2-1 Discharge curves of ½AA size cells, type SL-350, at +25 °C. Grey curve: 180Ω (30 hours) Blue curve: 180 kΩ (4 years) The circles indicate voltage recovery to 3.67 Volts (dashed line) whenever discharge is interrupted.
	Figure 2-2 Transient voltage curves A low current: no voltage delay B medium current: voltage stays above cut-off C high current: voltage drops momentarily below cut-off

SL-700 series
In general, the description in the previous paragraph holds for lithium thionyl chloride batteries of all three basic series. The SL-700 series, however, offers the advantage of an improved TMV and voltage delay time after storage. This is effected by a denser and more compact morphology of the protective layer on the lithium anode surface. Figure 2-3 for example shows the transient voltage curves of one year old SL-350 and SL-750 batteries on a load of 330 ?. While the voltage of SL-350 drops to 1.8 Volts, SL-750 stays above 3 Volts right from the start.

This advantage of the SL-700 series lasts for a maximum period of a few years on storage. It is impaired by storage at increased temperature levels and by continuous small current operation. As a result, the SL-300 series is usually preferred for long-term applications above 3 years of storage and operating life.

Figure 2-4, as an example, shows the development of TMV with storage time. The curves were obtained for ½AA size cells of the SL-300 and SL-700 series.

With respect to voltage delay, the SL-500 series behaves like SL-300.

End of life indication
Towards the end of life on long-term, continuous discharge, the initial resistance of the batteries will
increase. As a result, voltage on load and particularly during current pulses, will gradually decline. This feature can be used for an end of life indication typically 3% before the operating life time comes to an end. The indication voltage is a function of the discharge current, the application cut-off voltage, the temperature range, and the required warning time. Both the accuracy of end of
life indication and the length of the warning time can be increased by using current pulses and by confining indica350tion to a narrow temperature range (fig. 2-5). Application support for the design of an effective end of life indication is offered by Sonnenschein Lithium engineers on a per case basis.

	Figure 2-3 Discharge of ½AA cells on 330Ω after one year of storage at +25 °C. Blue curve: SL-750 Grey curve: SL-350
	Figure 2-4 Typical behaviour of voltage delay over storage time for two basic series. Discharge at 25 °C using the 100 hour rate (2 mA/cm²) Data obtained with ½AA size on 330Ω
	Figure 2-5 Principles of End of Life Indication. Solid blue curve: Discharge on continuous load at +25°C. End of life indication will occur approximately 3% before cut-off (based on total operation life). Dashed blue curve: If test pulses are employed indication can be extended to approximately 15% of the total operation life if the cut-off voltage refers to the continuous load level and 5% if it refers to the pulse load level. Grey curve: A seasonal temperature cycle can distort the discharge curve. End of life indication may occur at the grey circle for the first time leading to an early battery exchange. As a correction, indication can be suspended during temperature excursions. Alternatively, the limits or test pulse amplitude may be adjusted accordingly.

2.3 Discharge Current and Capacity

The available capacity generally depends on the discharge current or discharge time as indicated in figure 2-6. In the nominal range of discharge current or discharge time, the available capacity achieves its maximum value. At lower discharge currents, the selfdischarge becomes significant because of the longer discharge time, and the available capacity is reduced accordingly. At higher discharge currents, effects caused by the speed of ion transport progressively reduce the
discharge efficiency. The internal resistance increases and the available capacity is reduced. When opening a cell that was discharged with such a high current, it can be found that reaction products, that are deposited uniformly over the pore volume of the cathode during low and moderate current discharge, have now occupied and blocked the first few layers of cathode pores. It can thus be concluded that one reason for lower capacity at high current discharge is the reduction of accessible
cathode pore volume.

In the literature, the current at which a battery delivers 76 % of its saturation capacity is often referred to as its standard current. The battery will be overloaded if current is increased beyond this point.

2.4 Current Pulses

A typical pulse discharge pattern consists of a low continuous current drain with periodic or random short pulses at a higher current level. Generally, the duty cycle or ratio between on and off time ranges from 1:10 to 1:10 000 (fig. 2-7). The available capacity becomes now also a function of the duty cycle. For large duty cycles (1:10), it is close to the available capacity corresponding to the peak current. For small duty cycles (1:10 000), available capacity increases and tends to reach the value corresponding to the average current. Figure 2-8 gives an example.

	Figure 2-6 Dependence of capacity on current. Self discharge increases with operation life. Overload occurs if current exceeds the standard current corresponding to 76 % of the saturation capacity.
	Figure 2-7 Schematic pulse discharge pattern. Duty cycle means the ratio between ON- and OFF-time.
	Figure 2-8 Effect of pulse discharge on available capacity to 2 Volts at 25°C grey curves: constant duty cycle blue curves: constant average current as % of nominal current Data obtained with SL-780 batteries

2.5 Storage Life and Operating Life

While it has been found that it is practically impossible to apply standard methods of accelerated ageing to lithium thionyl chloride batteries in order to obtain reliable predictions of future performance, three methods can be used to collect data on long-term behaviour. These include actual long-term discharge, the extrapolation method, and the microcalorimeter method. The extrapolation method implies long-term storage or discharge combined with periodic determination of residual capacity. In the microcalorimeter method the heat output of cells on storage or on load is used to attempt a prediction of the loss that is due to self-discharge.

Actual discharge
Actual long-term discharge is the most accurate and reliable method. Unfortunately it is very time consuming. However, over the last decade, an extensive data basis has been collected by Sonnenschein Lithium to allow the prediction of expected storage and operating life times for a range of environmental conditions and required life times that covers all major application fields.
Figure 2-9 gives an example.

Two additional methods
The other two methods may be applied if results are needed quicker and it is not possible to refer to existing data obtained from actual long-term discharge. Both methods can accelerate the test duration to approximately 10 % to 30 % of the actual storage or operating time required for the application. When using the extrapolation method, it is important to carefully select the discharge
parameters for the residual discharge. Current capability and anode passivation may change over the years and lead to erroneous results if discharge is too fast or takes place at a temperature that deviates from the optimum. Figure 2-10 gives an example for the extrapolation method.

The microcalorimeter method is fairly expensive and sophisticated. It yields the heat output corresponding to the present status of a specimen. If this is extrapolated over the future operating time, an estimation of the integrated energy loss can be obtained. The test object, however, usually slightly changes its properties with time. As a consequence, careful calibration of the instrument and observation of the battery's heat output over several months are stringent prerequisites for meaningful predictions. It is also essential to observe a statistically relevant sample size. If substantial deviations of the data are found within the sample, this usually reflects the sensitivity of the method to various kinds of error possibilities rather than the battery performance itself. It should be noted here that results from the actual long-term discharge method usually do not deviate by more than ±5 % within the sample while standard deviations of ±50 % are typical for microcalorimeter studies conducted with normal carefulness.

Results
It is a conformable result of these methods that batteries of the SL-300 and SL-500 series have a capacity loss on storage of less than 0.5% per year while it is 2% for batteries of the SL-700 series. The self-discharge rate on operation as indicated above, is a function of the discharge current. Its value is 3 to 4% per year for an opera ting life of ten years.

	Figure 2-9 Data basis for discharge of ½AA cells of type SL-350 at +25 °C. This diagram comprises a total of 85 discharge curves on constant load from 180Ω (left) to 390 kΩ (right). The load resistors were 180Ω, 560Ω, 1.8 kΩ, 5.6 kΩ, 18 kΩ, 39 kΩ,82 kΩ, 180 kΩ, and 390 kΩ respectively. Batteries were taken from the shelf after one year of storage at room temperature. Depassivation takes place during the first per cent of the discharge.
	Figure 2-10 Extrapolation method for operation life on continuous load without pulses.

2.6 Orientation

Depending on mechanical cell design and system properties, there is a certain dependence of available capacity on cell orientation during discharge. The effect is caused by the tendency of the electrolyte to move towards the void and inactive space of the battery if the orientation deviates from the preferred direction. The capillary effect of the cathode and separator pores acts against this tendency. As a result, the orientation effect is smaller for thin cathodes than it is for thick ones and
is not even observable when discharge currents are very low or when batteries are moved during discharge.

The general capacity availability as a function of orientation can be summarized as follows:

• Throughout the nominal discharge current range, available capacity is practically unaffected if batteries are discharged upright or horizontally.
• At the low discharge current end or at infrequent, short, high current discharge pulses, capacities are practically unaffected if discharged upright or horizontally.
• At the high discharge current end, available capacity of the small and flat cells (AA, 2/3AA, 1/2AA, 1/6D, 1/10D, BEL) is virtually unaffected by orientation.
• At the high current end, available capacity of big cells (C, D, DD) is affected if the batteries are discharged upside down. Therefore this orientation should be avoided if possible.
• Available capacity of all cell sizes is not affected by orientation if they are moved occasionally during discharge.

2.7 Temperature Dependence

The nominal operating temperature of most basic series of Sonnenschein Lithium Batteries ranges from -40 °C to +85°C. Flat cells tend to expand somewhat at the high temperature end. They are limited to +75°C for this reason. The SL-500 series is designed so as to with stand temperatures up to 130°C. High temperature batteries for use up to 150°C are available upon request. At the low end of the temperature range, an extension to -55 °C and even below is possible although storage down to -55 °C and operation down to -40 °C covers virtually all practical target applications. The freezing point of thionyl chloride at -105°C may be regarded as a limiting factor.

Generally, temperature has an influence on the ion mobility in the electrolyte and on the morphology of the protective layer. Thus, current capability increases with temperature but the effect is compensated to a certain extent by the increase of passivation during storage and self-discharge during operation.

Figure 2-11 shows the dependence of available capacity of SL-360 batteries on temperature. The nominal capacity of 2.3 Ah is marked by a black dot. It is found at room temperature using the nominal current which corresponds to the 1000 hour rate. The figure shows the range of capacities found for discharge down to an end voltage of 2.0 Volts. Three current levels are represented in the figure. At each current level, available capacity increases with temperature starting from the low temperature end to a maximum somewhere between 0°C and +40 °C. The maximum for medium currents is found at 25 °C. Its position moves to higher temperatures with increasing current. If temperature is increased beyond the maximum, self-discharge will determine the result.

A look at the three curves reveals an interesting feature: At the high temperature end, the medium current yields the highest capacity while both higher and lower currents lead to a lower efficiency. This can be explained by the competition between self-discharge and passivation, which counteract each other. A passivated anode providing medium current will suffer little from selfdischarge. If the current is increased, however, passivation does not take place and the self-discharge due to the increased temperature reduces available capacity. At low currents, passivation is high and self-discharge per unit time is very low. But if the current is decreased further, the operating times involved are so long that the integrated loss due to self-discharge increases again and results in a reduced available capacity.

While the preceding discussion may explain some of the more basic features of the thionyl chloride system, it does not necessarily stress the extraordinary and powerful long-term and high temperature performance of these batteries. Figure 2-12 may help to demonstrate this excellence. It shows the results of a discharge test of ten batteries of type SL-550 (½AA) at 150 °C. On a load of 560 kΩ corresponding to an average current of 6 µA, the batteries operated for more than 5 years yielding 65 % of their nominal capacity.

	Figure 2-11 Temperature dependence of available capacity for three different current levels. Size AA, type SL-360
	Figure 2-12 Long-term discharge of ½AA size cells, type SL-550 at +150 °C for more than 5 years on a continuous load of 560 kΩ corresponding to a current of 6 µA

2.8 Environmental Conditions

Due to its reliable design, the Sonnenschein Lithium Battery is serviceable under extreme environmental conditions.

Altitude and Pressure
The sealing method and general properties of the battery allow storage and operation at any altitude from the earth's surface to deep space without degradation. In the opposite direction, pressure can be increased up to 20 atmospheres or more. Static force of up to 200 N on the positive terminal is allowable.

Vibration
The batteries can be subjected to normal vibration conditions during transport and operation. As a consequence, they can be used as a power source in any kind of transport system. Some types have even been proposed as a power source for electronic devices in car wheels.

Magnetic Properties
The can and cover are made from carefully nickel plated cold rolled steel and have the normal magnetic susceptibility of this material.

Humidity
As the cell voltage of lithium batteries exceeds the voltage needed for electrolysis of water molecules, they have to be protected from liquid water and condensation. A film of water across the battery terminals may not only lead to corrosion but also to external discharge. The Sonnenschein Lithium Battery will, however, not be affected by damp heat or humidity without condensation.

2.9 Internal Resistance

The internal resistance of a battery is derived by calculation from the voltage behaviour during pulse loads. Assuming that the same value is obtained if amplitude, duration, and frequency of pulses are changed, internal resistance can be used to predict the voltage response of the battery under arbitrary pulse loads. Unfortunately, it turns out that internal resistance of inorganic lithium batteries depends on numerous factors which include storage time, temperature, history, level of background current, depth of discharge and a few others. This makes it difficult to predict the battery's behaviour from one or even a few internal resistance values.

It is, however, important to develop a general understanding of the evolution of internal resistance with operating time in order to optimize the useful battery life. Figure 2-13 shows the discharge curve (1) of a Sonnenschein Lithium Battery on a continuous load corresponding to approximately 10 µA/cm², superimposed with 6 pulses per hour of 10 mA/cm². The operating life is approximately 9 months. For 97 % of the battery's life, the voltage Ug on the basic load remains above 3.6 Volts.

Internal resistance is represented by curve (3). It was calculated from the voltage drop on application of the pulse load using the equation.

With c = continuous discharge p = pulse load

Curve (2) shows the voltage Up during pulses.

When discharge commences, the internal resistance drops from its initial value - corresponding to anode passivation - to a stationary plateau value.

It is only after 70 % of the battery's life that the internal resistance rises again, indicating that the battery approaches its end of life. If the application requires pulses, battery voltage may drop below the required limit at this point. Making use of the fact that the electromotive force of the battery remains above 3.6 Volts until complete exhaustion, it is possible, however, with the aid of a suitable capacitor to extend operating life beyond this point if the required pulses are not too long. For additional details please refer to chapter 7.

Figure 2-13 Schematic diagram showing the evolution of internal resistance during cell discharge at room temperature. The continuous current of approximately 10 µA/cm² is superimposed with 6 pulses per hour of 10 mA/cm² for 0.5 s. In order to make the diagram independent of battery size, the internal resistance on the secondary ordinate was multiplied by the electrode surface area.