Why don’t lead acid batteries last forever?

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All rechargeable batteries degrade over time. Lead acid and sealed lead acid batteries are no exception. The question is, what exactly happens that causes lead acid batteries to die? This article assumes you have an understanding of the internal structure and make up of lead acid batteries. If you are not familiar with lead acid batteries, see our article What is a lead acid battery.

Human error

Ironically one of the most common reasons for battery failure is not an actual failure of the battery itself, it is people thinking the battery is dead. Some manufacturers and retailers report that up to 50% of batteries returned under warranty are actually fit and healthy. Another interesting fact is that most people have met someone who replaced their car battery only to find the fault lay elsewhere, such as in the poor performance of an alternator.

This is partly due to our assumption that when our device (be it a laptop, cell phone or car) won’t work, it must be the battery that is to blame. This is often due to the difficulties involved in testing a battery and partly due to the relative low costs of batteries. As a result, there is a temptation to “change the battery and see if that helps” before spending time investigating other causes.  Even technicians often change out batteries first to determine if that fixes the problem, hoping to eliminate wasted time and money looking for issues beyond that.

See Why is my battery not holding its charge? and Why won’t my battery charge? for reasons on how a perfectly healthy battery can appear to be dead.

Electrical shorting

The positive and negative electrodes (plates) in any battery cannot touch each other. If they do, they immediately short out and the cell dies. Note, this does not mean the entire battery suddenly becomes lifeless, it depends how many cells the battery is made from. A 12 volt car battery, for example, is made of six cells. If one cell shorts out, you still have a 10 volt battery which is usually enough to power dashboard lights, but not to turn the starter motor.

Shorting out can occur for a number of reasons

  • Manufacturing defects – badly cut plates can cut through the separator meant to keep electrodes apart, especially if the battery is jolted by a drop or operates in an area with vibration as car batteries do.
  • Active material shedding – in flooded lead acid batteries the active paste applied to the plates gradually falls off as part of the physical wear and tear when chemical reactions taking place. These fall to the bottom of the battery case, but if the build up becomes excessive, this sludge can end up touching both positive and negative plates.
  • Heavy vibration or jolts – this can cause the separator to come loose or split allowing the plates to touch each other.
  • Plate buckling – see below.

Plate buckling

If lead acid batteries are cycled too deeply their plates can deform. Starter batteries are not meant to fall below 70% state of charge and deep cycle units can be at risk if they are regularly discharged to below 50%.

In flooded lead acid batteries this can cause plates to touch each other and lead to an electrical short. In both flooded lead acid and absorbent glass mat batteries the buckling can cause the active paste that is applied to the plates to shed off, reducing the ability of the plates to discharge and recharge.

Acid stratification

Acid stratification occurs in flooded lead acid batteries which are never fully recharged. This is especially common in vehicles which are used for short journeys since there is not enough time to recharge the battery after it was drained to start the engine. It occurs more often in cold weather, because the time needed to recharge is extended as the alternator also delivers power to competing devices such as defroster, heating fans, lights, etc.

Acid stratification has become a more popular reason for battery failure in recent times due to more electrical devices being added to cars and other road transport.

It occurs when the acid in the electrolyte starts to concentrate in the lower half of the unit leading to a build up of sulfates on the bottom parts of the plates. These sulfates in turn reduce the ability of the plates to discharge and recharge. At the same time the more watery electrolyte at the top half accelerates plate corrosion with similar consequences.

Natural sulfation build up

When a lead acid battery discharges, the sulfates in the electrolyte attach themselves to the plates. During recharge, the sulfates move back into the acid, but not completely. Some sulfates crystalize and remain attached to the plates, which means over time, less sulfates are available to be part of the chemical reaction needed for the battery to function.

While sulfation is inevitable, it is accelerated by:

  • storing batteries in hot temperatures accelerating their self discharge rates
  • leaving batteries below a fully charged state for long periods
  • over charging

Some work suggests passing high frequency electronic pulses through the battery can help break up these crystals and let the sulfates move back into the electrolyte. This has lead to a large number of ‘pulse’ chargers appearing on the market. However, solid evidence is inconclusive.

Natural active paste shedding

As discussed above, this can cause electrical shorts. Even when this does not occur, the natural expansion and contraction of materials in a battery that leads to active paste shedding off the plates can at some point leave the plates ineffective and the battery unable to power the device.

Shedding is accelerated in flooded lead acid or absorbent glass mat units if they are used in appliances with heavy vibration, jarring or jolting. This includes items such as motorbikes, jet skis and other power sports vehicles. For these applications, Gel lead acid batteries are recommended, since the silicon gel electrolyte holds the paste in place.

Handling ‘dead’ lead acid batteries

Just because a lead acid battery can no longer power a specific device, does not mean that there is no energy left in the battery. A car battery that won’t start the engine, still has the potential to provide plenty of fireworks should you short the terminals.

As such, even if you believe your battery to be lifeless, be sure to handle, store and ship or dispose of it in the same way as you would a fully charged battery.




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