Both wet cell and gel batteries are lead-acid batteries and are based on the same chemical principles that have been used for over a century. They are composed of lead and lead oxide plates immersed in an electrolyte that uses a chemical reaction to produce an electric current. The reaction is reversible, and when electric current is applied, the battery is recharged. The principal differences between wet cell and gel batteries are in the electrolyte, rate of charge, gas emission, orientation flexibility and maintenance.
The electrolyte in a wet cell battery is a liquid solution of 65 per cent water and 35 per cent sulphuric acid. The gel battery electrolyte is changed into a gel by the addition of fumed silica or other thickening agents. The presence of a caustic liquid in the wet cell battery is a source of obvious safety issues, and since water loss during normal operation requires the periodic addition of water, the cells cannot be sealed.
Rate of Charge
Gel batteries can provide a higher rate of charge than wet cell batteries. While wet cell batteries need a bank-size-to-load ratio of 4-to-1, gel cell batteries need only a 3-to-1 ratio. The gel battery is also more sensitive to the rate of charge than wet cells during recharging. Recharging the battery at too-high or too-low charge rates can quickly ruin the battery.
Gel batteries are recombinant. That is, the hydrogen and oxygen given off as gases in a wet cell battery recombine in a gel battery to form water. This recombinant chemical process allows the batteries to be sealed so there is no escape of gases from the batteries. Wet cell batteries emit flammable hydrogen and can explode under certain conditions.
Because they are filled with a liquid electrolyte, wet cell batteries have to be oriented with the top up. Gel batteries can be used in any orientation, although there can be some loss of power in nonvertical orientations. In addition, even if the housing is cracked, a gel battery will not leak.
Gel batteries are sometimes called maintenance free because you never have to add water. This is true because hydrogen and oxygen are continuously recombined to form water. Wet cell batteries require the addition of water to replace that lost through normal operation, and checks of the specific gravity to ensure the proper amount of acid may also be necessary.