Battery Watering: Questions and Answers

Conventional lead acid batteries contain a liquid “electrolyte” which is a mixture of sulfuric acid and water.

Each cell in a battery resides within its own vented container (known as a “jar”), ensuring that the active material contained in the battery plates is continuously bathed in electrolyte while small amounts of oxygen and hydrogen gas (produced mainly during charging) are freely released. These gases represent the breakdown products of water, and this indicates that water is being “used up” to some degree and must eventually be replaced. The bigger portion of water lost in a battery that is run at elevated temperatures is lost by simple evaporation out of the vented cap on the cell, but all things considered, a lead acid battery in good condition does not need much maintenance attention at all. Mostly, it amounts to periodic checking of the electrolyte level and water additions weekly, or less often depending on individual duty cycles. An aftermarket watering system, installed on the battery in place of the stock vent caps simplifies this process even further.

We get questions on battery watering often at Arcon, and some of the questions follow:

What kind of acid do I add to my battery?
There is a simple answer here: No kind of acid at all. Under normal conditions a battery loses only water (in the form of vapor, hydrogen and oxygen), and only water should be replaced. Addition of acid would increase the sulfuric acid concentration of the electrolyte, potentially damaging the battery. If the cells have lost acid due to a tip over, or as a result of repeated over-filling and flush-out, some acid may have to be replaced, but this is a job for a trained battery technician. Sulfuric acid of a high concentration is used for replacement, but strong acid can only be added to a battery cell under controlled conditions- otherwise, it will not mix properly and cell damage can result. Safety is an issue here as well.

Why does my battery overflow occasionally?
It shouldn’t, but it will if it is overfilled. Keep in mind that the electrolyte level rises as the battery is charged. It is at its lowest level in the cell when the battery is discharged. It is common to add water to a battery before it is charged, but care must be taken to ensure that the fill level is appropriate considering this rise in electrolyte height. Batteries differ in the extent of this change in level, so it is difficult to make a blanket recommendation here. If the battery regularly overflows despite reasonable attention to proper level, a two-step process may be followed at those times a battery requires water:

  1. Before charging the battery, add just enough water so that a low liquid level is visible.
  2. After charging is complete, add enough water to bring the level up to the bottom of the vent opening barrel (usually about ¾ inch below the top of the cell).

The above steps will prevent overflow. Step one ensures that the electrolyte level has not gone too low and exposed the plates to air for the charge cycle. Step two brings the level up to full height at a time (battery fully charged, electrolyte at highest point) when accidental overfilling would not be a problem.

The most efficient watering approach? Use a hose-end filler nozzle that shuts off at a preset level, or install a single-point watering system. A single-point watering system is a simple plumbing arrangement on top of the battery that replaces the ordinary vent caps with valve-equipped caps connected to a single fill point by means of plastic tubing. A single-point watering system ensures accurate fill level and saves time- the operator does not need to have access to the top of the battery. An evaluation we have done has revealed that some systems fill much faster and are configured much better than others, so pay attention to the details and seek advise.

Is distilled water necessary for battery watering?
No. In most of the country, tap water is of sufficient quality to use for battery watering. Battery manufacturers can give you further guidance in areas that are known to have extremely hard water.

A factory representative can tell you what level of processing your problem water might need to make it suitable for your battery. Water at the purity level (and cost) of distilled water is unnecessary.

After I remove the caps to water the battery, should I leave them off for charging? Caps should always be securely in place. Not only does this prevent foreign material from entering the battery, it also stops splashing and loss of acid mist during battery charging. Aftermarket caps are available with special condensation and drain-back features that promise to recover some of the water vapor that is normally lost during battery operation- potentially reducing battery watering requirements.

Remember that water added to a battery is less dense than the dilute acid already in the cell. Water will sit on top of the electrolyte until it is mixed in by the bubbling, gassing behavior of the cell during a full charge cycle. This fact is important if you take hydrometer readings of the electrolyte; the reading will not be accurate after watering until after the following charge. Also note that batteries exposed to extremely low temperatures for long periods can be damaged by freezing unless they are fully charged, especially after any water additions.

For more information, contact Arcon Equipment Inc. (440) 232-1422.