Gel cell batteries are commonly used in amateur radio emergency communications. Being maintenance-free and spill-proof, they are ideal for portable operation. Properly cared for, a gel cell battery can provide many years of reliable service. Failure to use a charging method specifically suited to a gel cell battery's internal chemistry will dramatically shorten its useful life.

This article will provide some general guidelines and precautions that can be applied to any gel cell. You should, however, check with your battery's manufacturer for information on charging. If the manufacturer has specific recommendations, you should follow them.

Charging Basics

There are several methods for charging gel cell batteries, however, constant voltage charging is the best way. For this type of charging, a fixed, regulated voltage source is used to charge the battery. Special attention must be paid to the exact charging voltage and the initial charging current.

There are two modes for constant voltage charging: fast charging and float charging. Smart chargers, which are available commercially, automatically switch between these two modes. In addition, the most sophisticated chargers will monitor the ambient temperature and adjust the charging parameters accordingly.

Fast Charging (aka Cyclic Charging)

Float Charging (aka Standby Service)

In this mode, the battery is connected to a constant voltage source of 2.25 to 2.30 volts per cell (13.5 to 13.8 volts for a 12-volt battery) at 20� C. Provided that the charging source is regulated at the proper float voltage, the battery will establish its own current level and will be maintained in a fully charged condition. Gel cell batteries can be left in the float charge mode for extended periods of time without damage. This mode is often used to maintain gel cell batteries used in backup power applications.

Precautions

• The charging voltages given in this article assume an ambient temperature of 20� C (68� F). The charging voltages may have to be adjusted when ambient temperature is outside the range of 41� F to 95� F. For temperatures below 20� C, increase the voltage by 2mV/cell/� C for float charging and 6mV/cell/� C for cyclical charging. For temperatures above 20� C, reduce the voltage by the same factor.
• Resist the temptation to use an automotive battery charger on your gel cells. Many inexpensive automotive battery chargers are not properly voltage regulated or current limited.
• Continuous over-charging or under-charging are a gel cell battery's worst enemies.
• Do not store your gel cell batteries in an uncharged condition.
• Avoid exposing the battery to excessive heat. Service life is shortened at operating temperatures above 30� C.
• Never charge a gel cell in an air-tight container or near objects which produce sparks or flames.
• When using a solar panel to charge a gel cell battery, be sure to use a charge controller which properly regulates the charging voltage. Many solar panels are capable of producing as much as 18 volts -- more than enough to damage your battery.

References

Sealed Lead-Acid Batteries - Technical Handbook. Power-Sonic Corporation, San Diego, CA. (Available for download at www.power-sonic.com)

Sealed Lead-Acid Battery Chargers (Spec Sheet). Power-Sonic Corporation, San Diego, CA. (Available for download at www.power-sonic.com) Dated January 2000.

The ARRL Handbook for Radio Amateurs (1995). The American Radio Relay League, Newington, CT. Chapter 11, pp 11.20-11.23.

Disclaimer: This material is provided for general information only. Always follow the manufacturer's guidelines for the proper use, charging, handling, and safety of your gel cell battery.