ell employ systems which do not converters, provides the primary source of energy. need end cell switching techniques, and which instead use a single battery and a voltage regulator device (Pine, 1979). For the future, the use of switched mode power systems in asso- 24.3.3.3 Main exchange plant ciation with sealed lead acid cell batteries are likely to be used for the small exchange, with a small engine-generator set provided for For larger exchanges a double battery float system (Spurgin, 1970; long term standby power. Pine, 1971) can be used comprising two battery strings of 25 cells. Where a reserve of more than a few hours is required for small The arrangement is shown in Figure 24.11. With this system, the exchanges, and it is provided by a standby engine generating plant, battery is maintained at -51.5 volts, thus floating each cell at 2.06 the accommodation needed, and the environmental problems due to volts per cell. The designed end of discharged voltage is 1.83 volts noise for instance, tend to become substantial. This problem is per cell, that is -46 volts across the exchange load. Cells floated at becoming ever more prevalent as electronic equipment decreases in this voltage will lose of the order of 1% of their capacity each day, Rectifier No. Voltage Monitor Mains/Standby Supply Charge Switch Rectifier No.2 Coupling Switch 25 Cells Figure 24.11 Main exchange plant. (Batteries are normally coupled. They are uncoupled during charging, one supplying the load whilst the other is being charged.) D.C. systems 24/11 and hence it is usual to uncouple and recharge each string of cells in grow as the provision of the communications equipment load turn every couple of weeks. This is done under manual control, and grows. Further, this sort of system does not require the use of booster rectifier capacity has to be provided for the recharging common control systems, the failure of which could inhibit the safe process. functioning of the whole plant. The basic arrangement is set out in This basic form of design ca