Lead Acid Batteries: Some keys to long service life

Lead Acid Batteries

Some keys to long service life
By Joe Escobar

Some mechanics think that maintaining lead acid batteries is a magic craft that relatively few have mastered. They just can't figure out why batteries aren't lasting as long as they should. They are quick to blame the battery manufacturer claiming they were sold a defective battery. That may be the case in a very slim number of cases, but the fact of the matter is other factors come in to play that can seriously decrease the service life of your lead acid battery. Knowing the factors and how to counteract them can give you the longer service life you have been looking for.

In order to understand the factors that lead to shorter service life in your lead acid battery, it is important to have a basic knowledge of battery theory and construction. But first, lets go over some important safety precautions.

Safety precautions
There are several safety precautions that should be observed when working with lead acid batteries.

Wear splash proof goggles and protective clothing. Use care not to spill or splash the electrolyte. It is an acid and can cause severe burns to the skin.

When lifting a battery, you should wear some form of back support and avoid bending or twisting in awkward positions. Also avoid putting excess pressure on the end walls of the battery, since this could cause electrolyte to escape through the vents. When possible, use a battery lifting strap or lift it with your hands placed at opposite corners.

If electrolyte is spilled or splashed on your clothing or body, it should be neutralized immediately with a solution of baking soda and water and then rinsed with clean water.

Getting electrolyte in your eyes can be extremely dangerous. If this happens, force the eye open and flood it with cool, clean water for at least five minutes. Contact a doctor immediately. Do not add any eye drops or other medication unless advised to do so by the doctor.

If the electrolyte is taken internally, drink large quantities of water or milk followed with milk of magnesia, beaten egg, or vegetable oil. Call a doctor or poison control immediately.

If preparing electrolyte from concentrated sulfuric acid, always pour the acid into the water. Never pour water into an acid because a violent chemical reaction will result.

Keep all open flames, sparks, burning cigarettes, and other ignition sources away from batteries at all times.

Avoid the use of uninsulated tools. If uninsulated tools are used, severe arcing may result with possible harm to personnel and damage to the tools and cells within the battery. All jewelry including watches, bracelets, and rings should be removed as they may fuse themselves to the connectors and cause severe burns.

Finally, separate areas should be used for lead acid and NiCad battery maintenance to prevent electrolyte contamination in the batteries.

Basic theory
Lead acid batteries are named after the two major components of the battery - lead and acid. More specifically, the battery is made up of positive plates of lead peroxide (PbO2), negative plates of pure spongy lead (Pb), and a liquid electrolyte between the plates of sulfuric acid (H2SO4) and water (H2O). The sulfuric acid and water are mixed in a ratio so that a specific gravity of 1.285 to 1.295 is achieved for a fully charged battery.

Electrochemical reaction
All batteries use an electrochemical reaction to deliver current. The first battery created by Alessandro Volta in 1800 was constructed of a stack of alternating layers of zinc, blotting paper soaked in salt water, and silver. This arrangement was known as a voltaic pile and is the basis of battery construction today. The electrochemical reaction that takes place in a lead acid battery can be written in a formula as follows:

PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O

Note: If the battery is discharging, the formula is read from left to right. During charge, the formula is read from right to left.

When the battery is delivering current, the sulfuric acid in the electrolyte breaks down into positively charged hydrogen ions (H2) and negatively charged sulfate ions (SO4). Ions are atoms or molecules that are either positively or negatively charged. An ion that is positively charged has a deficiency of electrons while an ion that is negatively charged has an excess of electrons. The negatively charged sulfate ions (SO4) combine with the lead in the plates (Pb) and form lead sulfate (PbSO4). At the same time, they give up their negative charge creating an excess of electrons on the negative plate.

The hydrogen (H2) ions go to the positive plate and combine with the oxygen of the lead peroxide (PbO2) forming water (H2O). During this process, they take electrons from the positive plate. The lead (Pb) of the lead peroxide (PbO2) combines with some of the sulfate ions (SO4) to form lead sulfate (PbSO4) on the positive plate.

As a result of the electrochemical reaction described above, the positive plate ends up with a deficiency of electrons while the negative plate has an excess of electrons.

When the battery posts are connected by an external conductor, the electrons from the negative plate flow to the positive plate. This flow will continue until both plates are converted to lead sulfate and no further chemical action is possible. Then the battery is discharged.

When a battery is charging, current is passed through the battery in the reverse direction. A current is applied to the battery with the positive pole connected to the positive plate and the negative pole connected to the negative plate. If the electromotive force (EMF) of the source is greater than the EMF of the battery, the current will flow in the reverse direction.

When the current flow is reversed, the sulfate ions (SO4) are driven back into the solution where they combine with the hydrogen (H2) ions of the water forming sulfuric acid (H2SO4). The plates are then in their original composition of spongy lead (Pb) on the negative plate and lead peroxide (PbO2) on the positive plate.

Knowing that the sulfuric acid in the electrolyte is used up as the battery is discharged and that it is returned to the electrolyte when it is charged, measuring the specific gravity of the electrolyte will give an indication of the state of charge of the battery.

Service life killers

Overcharging and undercharging kill lead acid battery service life. It may seem ironic or even ridiculous, but both of these conditions will adversely affect your battery in the long run.

Overcharging is arguably the greater of these two evils. Overcharging the battery, either in the form of too high a regulator setting or as a result of too much bench charging can damage the battery. Excessive gassing can occur causing a reduction in electrolyte levels. If left uncorrected, reduced electrolyte levels can lead to rapid sulfation of the plates. Regularly checking electrolyte levels is a must, and if they are low they should be filled with distilled water.

I mentioned earlier rapid sulfation of the plates is an adverse effect of low electrolyte levels. Battery sulfation is the leading cause of premature disposal and battery replacement. It is the most destructive process that affects the life of your lead acid battery.
But what is sulfation? Well, if you go back to basic battery theory, when a battery discharges, lead sulfate forms on the plates. During recharge, this soft spongy material is converted back into the battery's electrolyte solution. However, if the battery is left in a discharged state for any appreciable length of time, the lead sulfate will grow into a hard, white crystalline formation which is known as sulfation. This condition closes the pores in the active material and destroys the plates. It is the prime cause of a battery's loss of recharging capacity. That is why undercharging, or more specifically, leaving a battery undercharged for too long, will have a detrimental effect on the battery. Not only will sulfation dramatically reduce the battery's ability to store energy, it effectively "kills" more batteries than any other malady.

A sulfated battery may be damaged beyond repair when it is placed on a charge, because instead of changing back into spongy lead or lead dioxide some of the lead sulfate is dislodged from the plates in small particles and drops to the bottom as sediment. This material is lost from active use forever.

During normal operation, battery cells shed a small amount of active material. However, this process is accelerated in a sulfated battery, and its life is greatly reduced. In extreme circumstances, sulfation on the plates can become so drastic that the thickness of the plates increases to the point that they touch each other and short out.

If an aircraft flies frequently, then sulfation should not be a major concern unless there are problems with the charging system of the aircraft. However, if the aircraft is rarely flown or if it is only flown for short hops, the battery may not be getting properly charged. It takes from 60 to 90 minutes to charge a battery during flight. Flights that are significantly shorter than that will not allow the battery to fully re-charge. If ground based charging is not used to keep the battery at full charge in this situation, sulfation will be a problem.

A cure for sulfation?
There is a fairly new technology that claims to be able to reduce the effects of sulfation in lead acid batteries. Magna Labs markets a product called Aero Bat Pac that uses Pulse Technology as a de-sulfator/maintainer. The basic theory of the system is that the crystallized sulfation formations are electronically dissolved back into the electrolyte solution.

In the end, the easiest way to keep sulfate problems at bay is to keep a close eye on the battery and keep it maintained and fully charged according to the manufacturer's instructions.

If there are any issues you are unsure of or if you are not getting the most out of the battery in your aircraft, contact the manufacturer for assistance. Their experts can help determine the cause of any problems and help get you on the road to battery longevity. Even though it may seem ironic for them to want their batteries to last longer (i.e. you not giving them money by buying their batteries as frequently), they truly want your batteries to last longer. After all, all things being equal, value will make a sale every time. And value in batteries equates to longevity.

Additional ReSources
Concorde Batteries
(626) 813-1234

Hawker Energy Products
(660) 429-2165

Magna Labs

Teledyne Continental Motors
Battery Products Division
(800) 456-0070