Let’s look once more at the charging process. During the discharge the active mass on the plates has turned into lead sulphate. When charging the lead sulphate of the positive plate turned into lead dioxide whilst the lead sulphate on the negative plate turns into spongy lead. At the end of charge oxygen is released at the positive plate and hydrogen at the negative plate. When both gases can rise to the surface and leave the electrolyte, there will be water loss and the battery will need to be refilled with water.
Due to the difference in charge acceptance between the positive and negative plate gas will be released at the positive plate slightly earlier than at the negative action. By the time that oxygen is released on the positive plate already a fair amount of spongy lead has been formed on the negative plate. So if we could bring about that the oxygen does not rise to the surface, but instead travel to the negative plate, it will react with this spongy lead and form lead oxide. Subsequently the lead oxide will react with the electrolyte and turn into lead sulphate.
Lead oxide turning into lead sulphate is, as we know, the result of discharge. So we may conclude that by bringing the oxygen from the positive plate into contact with the negative plate, just before the latter reaches its gas voltage, a self-discharge will occur that is equivalent to the charge. This means no gas voltage – and therefore no water loss. For whom all this is too much, it should suffice to say that when oxygen can reach the negative plate it will ultimately recombinie into water – hence the name recombinant battery.
Recombinant batteries need for proper functioning some overpressure and are therefore sealed. A self-closing safety valve opens when the pressure exceeds preset level (> 0.18 bar) and will close as soon as the balance is restored (< 0.15 bar). This is why these batteries are often referred to as VRLA (Valve Regulated Lead Acid), the gas that is released in the case of overpressure will mainly consist of oxygen, but it contains some hydrogen. Because of the sealed construction and the pressure within, this water loss cannot be replaced and constitutes therefore an irreversible shortening of service life.
The most important features of a VRLA battery are:
Totally maintenance free.
Low gas emission under normal conditions, in a ventilated surrounding never exceeding the critical concentration of 4% which makes oxyhydrogen explosive.
No free acid in the event of damage.
Can be side-mounted.