Okay we have Conventional motorcycle batteries, but how do they work? Conventional batteries have a removable filler cap for each cell: 6-volt batteries have three cells and three filler caps; 12-volt batteries have six cells and six filler caps. Years ago, they were called vent caps because each cap has a tiny hole that vents gases into the atmosphere. Newer conventional batteries have a different internal structure that eliminates the vent in each cap and uses a single vent hose.
These caps have several functions. They can be removed so water can be added to replenish what was lost due to charging or evaporation. They also allow you to use a hydrometer to check the condition of the electrolyte in each cell. You probably don't need a lengthy discourse about electrochemical reactions in lead acid batteries. However, a synopsis of what goes on in a battery may help you understand how to maintain a battery when it comes time to buy a replacement.
Batteries produce electricity by means of chemical reactions. The amazing thing about lead acid batteries is that these chemical reactions are reversible. Rather than bore you with chemical equations involving lines, arrows, numbers, and Latin-based symbols, I'll just tell you the basics of where the atoms and molecules are going. Batteries contain lead, lead peroxide, and sulfuric acid mixed with water. When a battery is producing electricity the sulfates from the sulfuric acid react with the lead and lead peroxide to produce lead sulfate and electric current. However, as the battery is busy producing electricity, the acid becomes less concentrated and more watery. This is why discharged batteries are more prone to freezing than fully charged batteries, not that we have much problems with that in Bangkok, but in Chiang Rai it sometimes – in the cold season – it can puzzle the best Thai mechanics. If an excess amount of lead sulfate builds up on the plates, the battery is referred to as being sulfated.
During charging, electricity is put into the battery, and the chemical reaction are reversed. The sulfates are forced off the plates and into the electrolyte where they combine with the water to reform sulfuric acid. The plates revert to their previous states of lead and lead peroxide. During charging, hydrogen and oxygen are released from the electrolyte and vented into the atmosphere. That's what the vent hose is for on a conventional battery. The loss of hydrogen and oxygen from the electrolyte is why water is lost, and distilled water must be added whenever the level of electrolyte drops below the lower line indicated on the battery case.
This water loss is why conventional batteries need to have water added to the electrolyte and why recharging is sometimes needed after water has been added. When checking the electrolyte level in conventional batteries, make sure the motorcycle and battery are upright. Years ago I owned a motorcycle that had the battery on the left side. If often looked like the electrolyte level was at the correct level when it was actually low, the source of the problem was the motorcycle's sidestand.
If the electrolyte is below the lower line on the battery's case, distilled water should be added until the top of the electrolyte is between the upper and lower lines on the case. Water can be added by unscrewing the cap on each cell. The caps are plastic and should unscrew easily.