GEOG 862
GPS and GNSS for Geospatial Professionals

The Power


Battery power

Schematic diagram of a NiCd battery
Nickel Cadmium Battery Schematic

It is fortunate that GPS receivers operate at low power. From 9 to 36 volts DC is generally required. This allows longer observations with fewer, and lighter, batteries than might be otherwise required. It also increases the longevity of the GPS receivers, themselves. About half of the available GPS carrier phase receivers have an internal power supply, and most will operate 5½ hours or longer on a fully charged 6-amp-hour battery. Most code-tracking receivers, those that do not also use the carrier phase observable, could operate for about 15 hours on the same size battery. Since most receivers in the field operate on battery power, batteries and their characteristics are fundamental to GPS/GNSS. A variety of batteries are used, and there are various configurations. For example, some units are powered by rechargeable batteries. Lithium, Nickel Cadmium, and Nickel Metal-Hydride may be the most common categories, but lead-acid car batteries still have an application as well. The obvious drawbacks to lead-acid batteries are size and weight. And there are a few others—the corrosive acid, the need to store them charged, and their low cycle life. Nevertheless, lead-acid batteries are especially hard to beat when high power is required. They are economical and long-lasting.

Nickel Cadmium batteries (NiCd) cost more than lead-acid batteries, but are small and operate well at low temperatures. Their capacity does decline as the temperature drops. Like lead-acid batteries, NiCd batteries are quite toxic. They self-discharge at the rate of about 10% per month, and even though they do require periodic full discharge, these batteries have an excellent cycle life. Nickel Metal-Hydride (NiMH) batteries self-discharge a bit more rapidly than NiCd batteries and have a less robust cycle life, but are not as toxic. Lithium–ion batteries overcome several of the limitations of the others. They have a relatively low self-discharge rate. They do not require periodic discharging and do not have a memory issues as do NiCd batteries. They are light, have a good cycle life and low toxicity. On the other hand, the others tolerate overcharging, and the lithium-ion battery does not. It is best to not charge lithium-ion batteries at temperatures at or below freezing. These batteries require a protection circuit to limit current and voltage, but are widely used in powering electronic devices, including GPS/GNSS receivers.