A gel battery is a rechargeable valve regulated lead-acid battery with a gelified electrolyte. Unlike a traditional wet-cell lead-acid battery, these batteries do not need to be kept upright (though they cannot be charged inverted). In addition, gel batteries virtually eliminate the electrolyte evaporation, spillage (and subsequent corrosion issues) common to the wet-cell battery, and boast greater resistance to extreme temperatures, shock, and vibration. As a result, they are primarily used in automobiles, boats, aircraft, and other motorized vehicles. These batteries are often colloquially referred to as sealed lead-acid (SLA) batteries due to their non-leaking containers, but they are not completely sealed; the valve regulation system allows for gas to be expelled. Chemically they are the same as wet (non sealed) batteries except that the antimony in the lead plates is replaced by calcium. This preserves the mechanical characteristics but renders the construction far less prone to gassing. The battery type is often referred to as a Lead-Calcium battery.
At high currents, electrolysis of water occurs, expelling Hydrogen and Oxygen gas through the battery's valves. Care must be taken to prevent short circuits and rapid charging. Charging with a constant voltage (called the float charge voltage; 2.26 V per cell for a lead-acid chemistry) can cause a rapid initial current, so therefore it is suggested to begin with a constant current, using constant voltage only for the final portion of the charging. However, the float charge voltage should not be exceeded by much for typical usage, so the switch between the two modes typically occurs when the float voltage is needed to sustain the charging current through the battery's internal resistance (as per Ohm's Law). The easiest way to implement this is to use a constant voltage device with a current limiter.