DC Power Supply in Medium Voltage Electrical Distribution System



The DC power supply shall consist of batteries and batteries charger.

Batteries of DC Power Supply - Technical Specifications

The batteries shall be nickel cadmium type with pocket plates type range TP, T.



1. Charging

The cell vents are left closed during all the method of charging detailed.

1.1 Charging According to IE Characteristic

The rising battery voltage is limited to a value of 1.55 - 1.65 V/cell. Until this charging voltage is reached, the charging current should be 1.5 x /5 in order to attain favorable full charging period (cell 100 Ah, /5 = 20 A; 1.5 x /5 = 30 A).

1.2 Charging in accordance with the / Characteristic

By this is meant charging for 7 hours with constant rated charging current. Approximately 140% of the previously extracted capacity in Ah must be recharged. The charging is complete when the voltage at the terminals at constant charging current no longer rises over a period of 30 minutes. The final charging voltage amounts to approximately 1.75 V/cell 1), and is dependent upon the electrolyte temperature it is possible interrupt charging at any time.

1.3 Charging accordance with the W characteristic

The charging period is determined by the initial charging current and the characteristic of the charger. The initial charging current should be limited to /5 to a max. of 1.5 x /5. Through this, the electrolyte temperature and water consumption are maintained within favorable limits. The charging voltage rises to approx. 1.40 - 1.75 V/cell 2).

1.4 Charging period (h) = 1.4 x Rated Capacity (Ah) divided by Average Charging Current (A)

1.5 Equalization Charge

Object: To maintain the capacity and stabilize the voltage situation.

Equalizing charge is an extra charge in accordance with VDE 0510 which is necessary every 6 months, and which must be carried out as follows:

1. Charge with the manual charging mode for approx. 6 hours to a final charging voltage of 1.6 - 1.75 V/cell


2. Charge with the boost charge voltage of 1.55 - 1.6 V/cell of the automatic charging mode for 6 to 12 hours.

1.6 Conservation charging (Parallel Standby Operation)

In parallel standby operation the conservation charging compensates for loss of capacity due to self discharge and maintains the battery in fully charged condition. The conservation charging voltage is 1.4 V/cell ± 0.02 V/cell.

1.7 Float charging

In floating operation, continuous charging compensates for the capacity loss through self discharge, and maintains the battery in full charged condition, and at the same time current peaks or voltage variations are covered. The float charge voltage is at least 1.45 V/cell according to type of load.

2. Electrolyte of the Batteries

The electrolyte is dilute caustic potash of special purity and composition. The requirements of VDE 0510 (regulations for accumulators) must be full filled. The preparation from dry electrolyte (potassium hydroxide) of Caustic Potash ready for filling should be carried out in accordance with instruction 30851.

The density of the electrolyte is no criterion for the state of charge of battery.

Electrolyte No.2 A with a density of 1.19 kg ± 0.02 kg/1 is used for the cell types listed.

2.1 Topping up

Only top up with purified water in accordance with VDE 0510. It is advisable to check newly obtained water for suitability before use. Only top up with electrolyte if electrolyte has been spilt during transit or for some other reason, or for increasing the density to the rated value of 1.19 kg/1. When filling or topping up, avoid spilling any liquid over or between the cells. Carefully siphon off any liquid which may have collected on the cell lids, but do not tip it into the cells.

2.2 New Filling

New filling should only be with VARTA Electrolyte No. 2a, density 1.19 kg/1, if this is not possible, then the supplier must be specially informed as to the purity requirements according to VDE 0510. The timing of the renewal of electrolyte is, in the main, dependent upon the operating conditions. Since an excessive Carbonate content reduces the capacity, the electrolyte must be renewed by a maximum carbonate concentration of 75 g/1 (sec instruction 30850 - exchange of electrolyte). The carbonate content can be checked by the user or by the VARTA service, or by sending a sample of electrolyte to the supplier’s works. The quantity required is approx. 200 ml. The supplier will provide regulation containers on request.

Electrolyte samples should be taken from several battery cells about 1/2 hour after the end of charge, and using a hydrometer. It is pointless to take samples shortly after filling.

Exchange of electrolyte should be made in accordance with the supplier’s operation instructions, which will be made available on request.

3. Temperature

The max. permissible electrolyte temperature is 45 deg. C. If 45 deg. C is exceeded during charging , then the charging current must be reduced, or the charging must be temporarily stopped.

Temperatures down to - 25 deg. C do not harm the battery, they only cause a temporary reduction of capacity during use.

4. General Maintenance DC Power Supply Batteries

The regulation covering the handling and operation of batteries in accordance with VDE 0510, VDE 0100, and VDE 0105 must be observed.

The battery, battery trays, and battery containers should always be kept clean and dry.

Defective vent seals should be renewed promptly. If valves are fitted, these should be cleaned frequently, and defective valves replaced.

Thinly coat blank cell parts from time to time using acid free fat (Vaseline).

It is recommended that separate rooms be used for plant with lead acid and Ni - Cd batteries. If this is not possible then acid fumes must be kept away from the Ni - Cd batteries. Never use lead acid battery tools for Ni - Cd batteries.

Do not lay any electrically conductive objects such as tools or wire on the cell lids, since there is danger of short circuits. Open tank or container lid during charging so that gas from charging can escape.

Do not go near the battery with open flame or fire, since the gas from charging is explosive.

Batteries Charger of the DC Power Supply

The batteries chargers shall be with transistors as actuator in output circuit.

The batteries charger shall have the following features:

  1. Rated for continues short circuit.
  2. Close control accuracy.
  3. Short control response time.
  4. Low residual ripple.
  5. EMI protection group K or N.
  6. High efficiency.
  7. Not affected by irregularities in power supply (such as from emergency generators).
  8. Cannot be overloaded, even in manual mode.
  9. Simple adjustment.
  10. Modular, plug in type.
  11. Electronic controls.

Technical Data of the DC Power Supply

1. Electrical data

(Value for NiCd battery chargers in brackets).

Line voltage: 220 V ± 10%.

Line frequency: 50 Hz ± 3%.

Line current, approx. max.: 10.5 (11.5) A per 1 KW of nominal charging power.

Cost max. approx.: 0.75 at max. charging power.

Nominal charging power: See rating plate.

Max. charging power: At 2.4 (1.6) V per cell.

Adjustment range: U-characteristic: 1.8 to 2.8 (1.2 to 1.85) V per cell, I - characteristic: 5 to 100 % of rated charging current.

Accuracy of control: U - characteristic: ± 0.1%, starting at no load condition, I - characteristic: ± 0.1% upto short circuit. This means that a fully discharged battery can be charged at full rated power starting at 0V per cell.

: The charger is rated for continuous short circuit operation.

2. Charging Cycle

Normal mode: IU characteristics as per DIN 41773. If the charger is also equipped (see optional equipment on page 6): IUoU characteristic or IoU characteristic.

Manual mode: IU characteristic. Both curves of the characteristic, I and U can be switched separately to the manual mode and can be adjusted by means of potentiometers, independent one of the other. The control accuracy (and thus the continuous short circuit rating) is maintained in full.

3. Batteries Charger Design

The design of the charger meets the requirements of the latest edition of the applicable VDE codes.

4. Mechanical Construction

Sheet steel, upright, free standing cabinet as per DIN 41490 with front door and transport eyes.

Arc of door swing


180 deg.

For units with lower capacities


Sheet steel, wall mount cabinet.

Safety category


IP 20.



Textured enamel.



Frame:RAL 7003.

Panels:RAL 7032.

5. Operating conditions

The charger must be installed in a closed room, protected against corrosive gases and excessive dust.

Maximum allowable operating values:

Ambient temperature: 35 deg. C.

Relative humidity: 75%.

Altitude: 1000 m above sea level.

Installation of the DC Power Supply

The charger must be installed in a dry room in such a way that the required cooling air can flow unimpeded into and out of the equipment.

The room temperature must not exceed the specified maximum.

1. Line power supply

The line voltage and grounding conductors are to be connected to the terminals, marked accordingly.

Conductor size must be sufficient for the supply current as specified on the rating plate.

2. Battery connection

Connect positive clamp to the positive battery terminal, receive clamp to the negative terminal. Selection of the charging cables must consider not only the maximum charging current but also the voltage drop (see “rule of thumb” below).

When using the charger in stationery systems, the following points must be observed:

3. Circuitry design:

1. Disturbance resulting from mutual influence of the system components, one on the other, must be kept as small as possible.

2. It must be possible to maintain the requirements of safety regulations at all times, even during repair work, and particularly in uninterrupted Dc power supply operations.

3. operation without a battery must be prevented by positive and active measures in the system, as unstable operating conditions can arise when line voltage is present without the characteristic of the battery (such as damping of return energy) being present in the system.

Maintenance of the DC Power Supply

The battery charger requires no great amount of attention. Accumulated dust should be blown out occasionally.