posted 04-18-2007 09:38 AM ET (US)
Inside of a valve-regulated lead-acid (VRLA) battery you have the same chemistry as in a flooded cell lead-acid battery. If the voltage in a cell exceeds about 2.39 volts, the chemical reaction drives hydrogen and oxygen out of solution and into a gas. This is what occurs when the cell is "over-charged" and is called out gassing. This produces a loss of electrolyte in the batteryIn a flooded cell battery which is vented to the atmosphere, the hydrogen and oxygen gases can vent from the battery. In a VRLA battery they are retained in the battery, where a catalytic converter tries to change them back into water. As long as there is not too much gas produced, the battery remains sealed and no electrolyte is lost. The VRLA battery cannot be completely sealed or the pressure could build up to a dangerous level, so a valve regulates the pressure and allows out-gassing if the pressure is too high. When out gassing occurs there is a loss of electrolyte in the VRLA battery. A flooded cell battery can be refilled with water and recover from loss of electrolyte due to over charging. A VRLA cannot.
The exact voltage when gassing will occur depends on temperature. For a 12-volt battery it is nominally
2.39 X 12 = 14.34 Volts
The charging system should be designed to not exceed this voltage.
Unfortunately, most older outboard motors have a permanent magnet alternator, and the voltage output of the alternator will increase with engine speed. Unless there is a voltage regulator circuit, the charging voltage will rise and often will exceed 14.34 volts. Therefore, if you use a VRLA battery with an older outboard motor that has an unregulated or poorly regulated battery charging circuit, there is a good chance you will subject a VRLA battery to chronic over charging. Depending on the design of the VRLA battery and its regulating valve, this could result in loss of electrolyte. Loss of electrolyte, of course, reduces the capacity of the VRLA battery and eventually results in failure.
Because they are openly vented, a flooded cell battery is subject to some loss of electrolyte, and thus it require occasional refilling with distilled water and recharging afterwards. A flooded cell battery may also produce some gas vapors which are corrosive, and consideration should be given to the effect this may have on its surroundings. For this reason, most flooded cell batteries are mounted in battery boxes which can contain any corrosive vapor or spilled electrolyte.