How to Handle the Limited DC System Voltage Window

Designing a DC system requires consideration of the ability to work within a certain DC voltage window. The maximum voltage provides the high-end of a voltage range that is available to charge the battery, whereas, the minimum voltage gives the lowest voltage within that range that is acceptable to the system so that the battery can be discharged.

It is not unusual for this voltage window to be very narrow, where specifications can be as close as +/- 10% from the nominal DC voltage.

This limited system voltage window can be a challenge to take into consideration when designing a DC system, especially when using it with Ni-Cd back up batteries that have a wide voltage window. What is needed is a boost or equalize charge to reach the maximum voltage that is effective for the attached batteries.

This paper will present a different system designed to cope with high voltage on the load output during boost or equalize charge with a Ni-Cd battery.

CHARGE AND DISCHARGE OF A NI-Cd BATTERY CONSISTING OF POCKET PLATE ELECTRODES

Constant voltage charge

The common method to charge a battery in stationary applications is carried out by a constant voltage system. The recommended solution is to use a two-rate type that is able to provide a constant voltage charge and a lower floating voltage or single rate floating voltage. This two-level charger has a high voltage stage to charge the battery properly after a discharge followed by a lower voltage float level charge.

This results in a quick charge of the battery and relatively low water consumption, due to the low-level float charge.

Two level charge

Boost charge: 1.55 – 1.70 V/cell

Floating: 1.40 – 1.42 V/cell

A high voltage will increase the speed and efficiency of recharging the battery.

In reality, a single level charger can often be found. This is surely a compromise between a voltage high enough to charge the battery and low enough to have adequate water consumption. Download the full paper here.