Fall 2002 Issue

Figure 1 . Schematic of the battery helper.
 

This battery helper, advantageous for contesting as a QRP portable or a rover, allows you to operate your 13.8-volt equipment from battery voltages as low as 9.0 volts while drawing 3 amps of current. The project can be completed in just one night.
 

By Charles W. Pearce, Ph.D., K3YWY
 

Portable operation from batteries generally represents a compromise, as amateur radio equipment is usually specified for operation from a 13.8-volt source. A fully charged lead-acid battery generally starts at 12.6 volts. Given even a 15% tolerance on the equipment operating voltage, the battery has only 0.9 volts of headroom before it drops below 11.7 volts. The battery helper fixes this problem by boosting the battery voltage to 13.8 volts, even with a battery voltage as low as 9.0 volts, thereby extending the time of operation from a battery with the advantage of operating the equipment at its design voltage.

Circuit Description

Figure 1 is the schematic for the battery helper. It is a simple circuit, but it is worthwhile to understand the principles of its operation.

The LT1070 is a switching regulator. The basis of its operation is a periodic shorting to ground of the inductor L1 through a switching transistor, internal to the LT1070. This allows energy to be stored in the magnetic field of L1. When the switching transistor is turned off, the energy stored in the inductor will flow through diode D1 into capacitor C1. The rate at which the switching occurs is around 40 kHz. The ideal switching regulator is 100% efficient in transforming voltages, because there are no losses in ideal switches, inductors, or capacitors. Actual circuit elements have some parasitic resistance, and therefore induce loss, but efficiencies of over 90% are possible with this style of regulator. This means very little battery power is wasted in the conversion. This is unlike linear shunt regulators,1 where considerable power is dissipated in the series pass transistor. In our rover station (visit us at <http://www.thegridrangers.org>), we had generated the 13.8 volts by regulating down from the 18 volts we obtained from 12- and 6-volt batteries placed in series. This approach was only about 75% efficient and required an extra battery.

Diode D1 keeps the capacitor from discharging backwards into the circuit. A Schottky diode is preferred for its lower voltage drop and faster switching characteristics, but a silicon diode will work. This diode must be capable of handling not only the average current draw, but also peak currents of up to three times the average current. The resistor network composed of R1, R2, and R3 is a voltage divider that provides a signal to the feedback pin to maintain regulation of the output voltage. This voltage is compared to an internal reference of 1.244 volts. Fuse F1 should not be deleted from the project. Even if you have a polarized plug to prevent the reverse biasing of the circuit, a short circuit on the output will effectively short the battery to ground through inductor L1 and diode D1.
 

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