Failure Modes Battery

 

Failure modes battery

 

Lead-acid (flooded) failure modes

Ø  Positive grid corrosion

Ø  Sediment (shedding) build-up

Ø  Top lead corrosion

Ø  Plate sulphation

Ø  Hard shorts (paste lumps)

 

Each battery type has many failure modes, some of which are more prevalent than others. In flooded lead-acid batteries, the predominant failure modes are listed above. Some of them manifest themselves with use such as sediment build-up due to excessive cycling. Others occur naturally such as positive grid growth (oxidation). It is just a matter of time before the battery fails. Maintenance and environmental conditions can increase or decrease the risks of premature battery failure.

Positive grid corrosion is the expected failure mode of flooded lead-acid batteries. The grids are lead alloys (lead calcium, lead-antimony, lead-antimony-selenium) that convert to lead oxide over time. Since the lead oxide is a bigger crystal than lead metal alloy, the plate grows. The growth rate has been well characterized and is taken into account when designing batteries. In many battery data sheets, there is a specification for clearance at the bottom of the jar to allow for plate growth in accordance with its rated lifetime, for example, 20 years.

At the designed end-of-life, the plates will have grown sufficiently to pop the tops off of the batteries. But excessive cycling, temperature and over-charging can also increase the speed of positive grid corrosion. Impedance will increase over time corresponding to the increase in electrical resistance of the grids to carry the current. Impedance will also increase as capacity decreases as depicted in the graph in figure 2.

Sediment build-up (shedding) is a function of the amount of cycling a battery endures. This is more often seen in UPS batteries but can be seen elsewhere. Shedding is the sloughing off of active material from the plates, converting to white lead sulphate. Sediment build-up is the second reason battery manufacturers have space at the bottom of the jars to allow for a certain amount of sediment before it buildup to the point of shorting across the bottom of the plates rendering the battery useless. The float voltage will drop and the amount of the voltage drop depends upon how hard the short is. Shedding, in reasonable amounts, is normal.

Some battery designs have wrapped plates such that the sediment is held against the plate and is not allowed to drop to the bottom. Therefore, sediment does not build-up in wrapped plate designs. The most common application of wrapped plates is UPS batteries.

Corrosion of the top lead, which is the connection between the plates and the posts is hard to detect even with a visual inspection since it occurs near the top of the battery and is hidden by the cover. The battery will surely fail due to the high current draw when the AC mains drop off. The heat build-up when discharging will most likely melt the crack open and then the entire string drops off-line, resulting in a catastrophic failure.

Plate sulphation is an electrical path problem. A thorough visual inspection can sometimes find traces of plate sulphation. Sulphation is the process of converting active plate material to inactive white lead sulphate. Sulphation is due to low charger voltage settings or incomplete recharge after an outage. Sulphates form when the voltage is not set high enough. Sulphation will lead to higher impedance and a lower capacity.

Lead-acid (VRLA) failure modes

 

ü  Dry-out (Loss-of-Compression)

ü  Plate Sulphation (see above)

ü  Soft and Hard Shorts

ü  Post leakage

ü  Thermal run-away

ü  Positive grid corrosion (see above)

 

Dry-out is a phenomenon that occurs due to excessive heat (lack of proper ventilation), over charging, which can cause elevated internal temperatures, high ambient (room) temperatures, etc. At elevated internal temperatures, the sealed cells will vent through the PRV. When sufficient electrolyte is vented, the glass matte no longer is in contact with the plates, thus increasing the internal impedance and reducing battery capacity. In some cases, the PRV can be removed and distilled water added (but only in worst case scenarios and by an authorized service company since removing the PRV may void the warranty). This failure mode is easily detected by impedance and is one of the more common failure modes of VRLA batteries.

Soft (a.k.a. dendritic shorts) and hard shorts occur for a number of reasons. Hard sorts are typically caused by paste lumps pushing through the matte and shorting out to the adjacent (opposite polarity) plate. Soft shorts, on the other hand, are caused by deep discharges. When the specific gravity of the acid gets too low, the lead will dissolve into it. Since the liquid (and the dissolved lead) are immobilized by the glass matte, when the battery is recharged, the lead comes out of solution forming threads of thin lead metal, known as dendrites inside the matter. In some cases, the lead dendrites short through the matte to the other plate. The float voltage may drop slightly but impedance can find this failure mode easily but is a decrease in impedance, not the typical increase as in dry-out. See figure 2, Abnormal Cell.

Thermal run-away occurs when a battery’s internal components melt-down in a self-sustaining reaction. Normally, this phenomenon can be predicted by as much as four months or in as little as two weeks. The impedance will increase in advance of thermal run-away as does float current. Thermal run-away is relatively easy to avoid, simply by using temperature- compensated chargers and properly ventilating the battery room/cabinet. Temperature-compensated chargers reduce the charge current as the temperature increases. Remember that heating is a function of the square of the current. Even though thermal run-away may be avoided by temperature-compensation chargers, the underlying cause is still present.

 

Nickel-Cadmium failure modes

NiCd batteries seem to be more robust than lead-acid. They are more expensive to purchase but the cost of ownership is similar to lead-acid, especially if maintenance costs are used in the cost equation. Also, the risks of catastrophic failure are considerably lower than for VRLAs. The failure modes of NiCd are much more limited than leadacid. Some of the more important modes are:

o   Gradual loss of capacity

o   Carbonation

o   Floating effects

o   Cycling

o   Iron poisoning of positive plates

Gradual loss of capacity occurs from the normal aging process. It is irreversible but is not catastrophic, not unlike grid growth in lead-acid.

Carbonation is gradual and is reversible. Carbonation is caused by the absorption of carbon dioxide from the air into the potassium hydroxide electrolyte which is why it is a gradual process. Without proper maintenance, carbonation can cause the load to not be supported, which can be catastrophic to supported equipment. It can be reversed by exchanging the electrolyte.

Floating effects are the gradual loss of capacity due to long periods on float without being cycled. This can also cause a catastrophic failure of the supported load. However, through routine maintenance, this can be avoided. Floating effects are reversible by deep-cycling the battery once or twice.

NiCd batteries, with their thicker plates, are not well-suited for cycling applications. Shorter duration batteries generally have thinner plates to discharge faster due to a higher surface area. Thinner plates means more plates for a given jar size and capacity, and more surface area. Thicker plates (in the same jar size) have less surface area.

Iron poisoning is caused by corroding plates and is irreversible.

Figure 2 Changes in impedance as a result of battery capacity