Figure 7-1 Basic memory back-up circuit including safety devices (diodes and resistor) according to case B of the UL recommendations. Case A is obtained if R is replaced with another diode.
Figure 7-2 Memory back-up circuit employing a controller chip.
Figure 7-3 Typical back-up circuit for personal computers
Figure 7-4 Back-up circuit for a real time clock

7.1 Back-up circuits

Figure 7-1 represents the protection against charging recommended by Underwriters Laboratories for memory back-up circuits (see section 5.2). The diagram reflects case B. Case A is obtained if the resistor R is replaced with another diode.

It is absolutely necessary to provide these safety devices on circuit boards that contain a back-up battery connected in parallel to a power source. The safety devices have to be placed physically as near as possible to the battery. Otherwise unintended shorts between adjacent printed wires or poorly soldered connectors or the like may by-pass the safety devices and allow the power source to pass a reverse (charging) current through the battery.

Figure 7-2 gives an example of a memory back-up circuit employing a controller chip.

Figure 7-3 shows a typical back-up circuit for personal computers. The function of the 10 µF capacitor is to delay the voltage drop on power failure and thus permit the inverter to deliver the stand-by pulse to the microprocessor at the earliest time.

Figure 7-4 is the back-up circuit for a real time clock. D1, D2, D3 are Germanium diodes, D4 and D5 are silicon diodes. The function of D4 and D5 is to reduce the normal supply voltage to the oscillator closer to the level of the back-up battery voltage and thus prevent a larger voltage drop under back-up conditions thus increasing the accuracy of the RTC.

Figure 7-5 shows a back-up circuit for continuous and pulse type loads. D1 and D2 are silicon diodes while D3 and D4 are Germanium diodes or Schottky type MBD701 diodes. This circuit provides back-up to a main power supply with a small continuous load and intermittent higher current pulses. The function of C and D4 is to stabilize the voltage to the continuous load in case the higher current pulse causes the battery voltage to drop momentarily.

Figure 7-5 Back-up circuit for continuous and pulse type loads.
Figure 7-6 Main battery supply circuit for continuous and pulse type loads. D is a germanium or a Schottky type diode.
Figure 7-7 Main battery supply circuit for critical, infrequent pulse loads.

7.2 Capacitor Support

Lithium thionyl chloride batteries can develop internal resistance on prolonged storage or on continuous very low rate operation. This may reduce the amount of capacity available above a certain cut-off voltage. In these situations, a suitable capacitor can increase the available capacity. The following notes are provided to obtain the optimum performance from the batteries. In many cases a lithium thionyl chloride battery with capacitor support will out-perform any other type of battery.

Support of continuous loads
Often the customer's circuitry can be interpreted as consisting of two parts with basically different requirements. One part may consist of the microprocessor and memory. It requires low current and high voltage. The other part may comprise current consumers like transducers, sensors, actuators and the like. It may require pulses of higher current but the voltage is less important.

An advantageous approach to this class of applications is outlined in Figure 7-6. During high current pulses, the microprocessor is powered from the capacitor. The battery does not have to supply voltage and current at the same time.

Support of pulse loads
In cases where the voltage requirement of the current consuming part of the circuit is stringent or when the capacity of the battery shall be used beyond the point of increasing impedance, the situation can be adapted to the battery's possibilities by use of a capacitor that is large enough to back-up all current pulses. Figure 7-7 gives the basic idea of the circuit.

At the first glance this solution may seem to be more expensive than for instance a lithium organic electrolyte battery. However, the useful capacity, the voltage level, and stability of this solution may be so much higher that it becomes less expensive in the long run.

7.3 Battery Packs

The assembly of multi-cell battery packs requires training and experience. Users that are not qualified accordingly should not attempt to assemble battery packs. Sonnenschein Lithium offers a wide range of customized batteries as well as assistance in developing, engineering, and manufacturing additional ones. Please request the Sonnenschein Lithium questionnaire for battery packs. Additional notes on the protection of lithium battery packs are given on its backside. The following instructions and recommendations serve as a guideline to the qualified battery assembler.

Protective Devices
Battery Packs should be designed so as to prevent unintentional shorting of cells, overheating, and excessive charging and discharging currents. Depending on the conditions of use, protective devices may have to be provided in the battery circuit.

Protection Against Charging
If a battery pack can be used in connection with an independent power source, it should be equipped with blocking diodes Ds in each string of parallel connected cells (see fig. 7-8). The leakage current of each diode should not exceed 10 µA.

Protection Against Overdischarge
If a battery pack can be used in such a way that the discharge current exceeds the maximum reverse current values given in the individual cell data sheets then bypass diodes Dp should be provided in parallel with each cell in the pack. The effect of a by-pass diode is that current passes through the diode if a cell is discharged. The cell can thus not be overdischarged.

Protection Against Shorting
Appropriate methods and materials should be applied to prevent the occurrence of internal shorts in a battery pack. The insulation material should withstand mechanical and thermal stress. Often the shrinking sleeve of the cell does not fulfil this requirement and needs reinforcement between adjacent cells.

As a protection against external shorting, internal leads should either be selected so as to withstand the expected maximum short-circuit current or placed in such a way that they cannot induce additional short-circuits e.g. when the insulation melts.

Sonnenschein Lithium cells can be short-circuited without the discharge current rising above a safe value. However, if a battery pack exceeds a certain size, the heat produced during short-circuit cannot be dissipated. In these cases a slow-blow fuse F or a thermal fuse should be provided. This should be done if a current product of 3000 mA (number of cells times maximum continuous cell discharge current) is exceeded.

Selection of Cells
The cells selected for a battery pack should match with respect to type, size and age. Do not mix cells of different technologies or different manufacturers. Marking and type designation should be readable and as required. Cells should be inspected mechanically and electrically before assembly. Inspection criteria are provided upon request.

Assembly Procedure

• Make sure that the component cells are insulated one from the other.
• Position and insulate electrical leads, links and contacts in such a way that short-circuits are prevented.
• Only interconnect the cells by soldering or spot welding to the flat strip (tag) terminations.
• Keep the soldering time as short as possible, below 10 seconds.
  With the larger cell sizes, particularly with the C, D and DD sizes, the preferred cell orientation in a battery pack is upright.
• Do not attempt to solder or spot-weld directly to the cell case. This can result in gross overheating and consequent hazard. Cells are supplied with various terminations for this purpose.
• Use polarized (keyed) battery connectors or at least be sure to properly identify the polarity and protect the ends of battery leads.
• Properly mark the battery pack (type / nominal voltage / date code) and affix appropriate safety
  labelling.
• Do not wear conductive jewellery when assembling battery packs or connecting cells to equipment.
• Always wear eye protection when assembling battery packs or connecting cells to equipment.
• Perform spot welding or soldering behind safety shields.
• Only use suitably insulated tools.
• Use encapsulating, insulating and similar battery pack materials at the minimum required levels, so as to limit the build-up of heat within the pack.
• When molding, make sure not to inhibit proper operation of any safety vents.
• Do not use flammable materials.
• Verify compliance of your battery pack design with the applicable shipping and handling requirements, by performing corresponding shock and vibration tests.

7.4 Lithium Battery Questionnaire

A lithium battery questionnaire is in the Product Data Catalogue. It contains the details that Sonnenschein Lithium engineers need to provide their customers with the most appropriate solution to their application. It forms the basis for calculation of the available battery capacity determining the useful battery life.

Figure 7-8 Typical multi-cell battery pack with protective devices. Additional safety devices according to UL recommendations are needed if a battery pack is used as back-up for another power source.

7.5 Application Proposal and Capacity Calculation

Sonnenschein Lithium Product Data Catalogues and other data sheets contain typical battery capacities for different load and ambient conditions. In most applications, however, these are not constant throughout the battery life. As the effects of conditions changing during battery life cannot be predicted from the data sheets it becomes necessary in most cases to calculate the expected battery using a procedure like the one described below. As a result, an application proposal is
obtained that forms part of the technical quotation.

• Determine the general conditions
  For this purpose, customer name, application field, project name, project size, time schedule etc. should be entered into the lithium battery questionnaire.
• Calculate current consumption
  The current profile is compiled from the basic or quiescent current and the pulse current contributions. These are determined by the amplitude, pulse duration and duty cycle (on/off periods).
• Consider customer requirements
  Requirements with respect to cut-off voltage and operating life should be known as they have influence on the proposal.
• Select battery type
  A battery type will be selected based on the customer's requirements and conditions of use.
• Calculate battery life
  - compile the temperature profile
  - enter the average current corresponding to this temperature
  - enter the battery capacity corresponding to this temperature from the "Available Capacity" diagrams of the data sheets.
  - calculate the availability factor which accounts for the effect of long-term operation as well as the effect of pulse amplitudes
  - calculate the electrical operating life
  - calculate the system life which takes into account those effects that are independent from the electrical processes like e.g. ageing of the isolation system.
  - calculate the resulting battery life which is basically the minimum of the electrical operating life and the system life.
  
• Add remarks
  The proposal may contain additional hints. These may include depassivation procedures, capacitor support and additional remarks depending on the circumstances and the customer's requirements.
• Liability
  Application proposals will usually be concluded by a general liability statement.
  

7.6 Depassivation

Inorganic lithium batteries may under certain circumstances need depassivation before they operate satisfactorily. These depend on the type of battery, storage conditions, current profile, and voltage requirements. The effect is caused by the protective layer which is described in paragraph 3.3. Effects on performance are described in paragraph 2.2.

There exist several depassivation methods, some of which may be carried out even without being noticed as a depassivation procedure.

If batteries are not older than 6 to 12 months before mounting, the temperature cycle and temporary short-circuit during wave soldering may be sufficient for depassivation.

A method that can be applied during manual handling of small numbers of batteries is a short-circuit of several seconds. The method will momentarily break up the protective layer and increase its conductivity by several orders of magnitude. Surprisingly, the same effect may be obtained by shock freezing the battery within the recommended storage temperature range.

A depassivation method that has been suggested for large numbers of batteries in a highly automated assembly line for instrumentation equipped with D-size batteries, is passing a current of 60 mA for 30 s through a resistor of 56Ω. For other battery sizes the current should be adjusted to approximately 2 mA/cm2.The method may be refined by adjusting the current amplitude so that the battery voltage drops to one half of its open circuit value, or by applying a pulse load, or by a combination of both.

If current capability is needed only several weeks or more after installation of the battery it is likely that the quiescent current will depassivate the battery sufficiently so that no depassivation needs to be carried out.