What is the Methods of Working The Battery?
The secondary cell batteries can be utilized in many ways. However presently float working is adopted.
In this system the battery is always kept connected across the load in the float system in a fully charged state. But the operating conditions are so maintained that the floated battery does not discharge into the load as long as the mains power supply is available. There is absolutely no break in the DC power supply when the mains supply fails as the emergency battery takes over the load instantaneously.
The charge/discharge system was the standard system for manual exchange and telegraph power plant operation for a number of years but has been superseded by float systems for all the installations from late 60’s onwards.
The float system can be worked on a fully automatic basis or on a semiautomatic basis with a minimum of maintenance attention. Float systems are economical since practically all the DC power requirements are taken directly from the public AC mains supply through converters. With the battery connected in parallel almost full battery capacity is available to carry the equipment load under mains failure conditions. It is usually to provide a battery capacity equivalent to six times the busy hour load in float systems.
The operating procedure for floating batteries depends to a great extent on the floating voltage. The floating voltage cannot be higher than 2.35 volts per cell since beyond this limit, overcharging and vigorous gassing of the battery occurs. The lower limit for the floating voltage of a lead acid cell is 2 volts, since below this voltage, the battery will discharge. Within the permissible range of floating voltage from 2.0 to 2.35 volts per cell, three distinct types of full float schemes have been evolved. They are as follows:-
- Batteries floated between 2.02 to 2.07 volt per cell or divided battery float system.
- Batteries floated between 2.15 to 2.20 volt per cell or parallel battery float system.
- Batteries floated between 2.24 to 2.30 volt per cell or end cell switching system.
Requirements of float scheme
- Voltage should be maintained between prescribed limits.
- The capacity of the battery should be adequate to meet emergency.
- It should last longer.
- The energy losses should be minimum.
Details of float schemes
In automatic telephone exchanges the permissible range of voltage is from 46 to 52 V. Instead of referring these float schemes by the floating voltage per cell, they are often named after the total number of cells in each set of the floated battery. From this standpoint these float scheme are usually referred as the 25 cell, 24 cell and 23 cell schemes.
The 24 cell float scheme or parallel battery float scheme has now been prescribed as the standard method of supplying power to telephone exchanges. It is also adopted for 60V plus and minus supply in telegraph offices and telex exchanges.
In this scheme the battery voltage is maintained within very close limits (51.5 ± 0.5). The required battery capacity, (ordinarily for 6 to 8 busy hours ) is provided equally in two sets of batteries to facilitate maintenance attention to any one battery set. Both the battery sets are floated in parallel simultaneously. Ref. Fig.6. as the floating voltage of 2.15 V per cell is within the trickle charging range, a steady trickle charge current flows and recoups all capacity losses due to local action, partial discharge on peak loads etc., incurred during the floating periods.
Thus the floated battery is maintained at its full capacity, which is available as reserve for mains failure conditions. Refresher charges at three monthly intervals and yearly reconditioning cycle are given to the battery to ensure that the battery is kept in a healthy condition. Under mains failure conditions the battery voltage drops down to 48 volts for 24 cells and the lower operating voltage of exchanges is limited to 46 volts thus the end point for the discharge of the cells has to be limited to 1.92 volt per cell. Thus only about 52% of the rated capacity of the cell is available for use. Therefore higher capacity batteries are required.