Even today’s most advanced power backup systems rely on the relatively low-tech lead acid battery. While they are limited by low energy-to-weight and energy-to-volume ratios (i.e., they are big and heavy), they are capable of high-surge currents, so they are ideal to start the generator, as well as providing runtime during a utility failure.
However, this means that battery breakdown in a UPS system could be as critical as any grid malfunction. So it is important that batteries are maintained in optimum condition according to their application. Replacing a battery before it has reached its end of service creates the risk of further battery faults and unnecessary costs, as a result of manufacturing errors or preventable issues. For the system to be reliable, batteries must be monitored and managed.
A battery provides electrical charge as the result of an electrochemical reaction. However, performance begins to degrade from the moment dioxide paste is factory-applied to their lead grids. Unlike many components in a mechanical system, there may be no outward indication of impending battery failure (except in the most extreme cases such as thermal runaway).
In addition, while each cell of every battery has its own unique rate of deterioration, a variety of factors contribute to actual rates of decline and length of battery service life. According to Battery Council International (2010), preventable issues and manufacturing errors combine for a total of 69% of battery failures.
Operating temperature has the most impact on premature battery failure. Higher temperatures within the battery cells cause its chemical reactions to speed up. This increases current draw, water loss, and the interior rate of corrosion on the positive grid material.
Grid corrosion can lead to short circuits within the battery due to the compact design of modern batteries. Because normal chemical reactions within the battery cause corrosion (shedding lead from the plates) within the grid; these reactions can be decelerated but not stopped. Typically a battery that fails because of grid corrosion has been in service longer than its expected lifespan.
Sulphation can occur when a battery does not receive a complete charge, and is common where the battery is used in stop/start applications. Lead dioxide disintegrates on the negative electrode, reducing active surface area and causing capacity loss. It also reduces the battery’s consequent ability to receive a charge, causing a longer charging cycle as internal resistance is increased.
Short circuits can be caused as paste on the positive electrode becomes porous, causing a loss of contact between the positive material and the grid. Little by little, during discharge the plates can shed paste. If (or when) this shed material makes contact with the plates, the cell will short-circuit.
Dry out – or water loss – is cause where overcharging increases the acid concentration in the electrolyte. This also increases self- discharge and sulfation rates. As the battery gases, it loses water, leading to eventual dry- out, capacity loss, and ultimately separator (insulator) breakdown. In today’s sealed batteries, water loss leads to dry-out and decline in capacity.
Thermal runaway occurs when the temperature inside the battery is high enough that it is unable to be dissipated from the battery casing, causing a temperature increase around the exterior of the battery. This, in turn, increases the temperature within the battery ultimately leading to case meltdown and exposed battery grid.
Top mossing is a result of inaccuracy or carelessness during the manufacturing process. Separators and plates are poorly aligned, causing plate areas to become exposed. This exposure allows a crystalline “moss‟ to form, leading to self-discharge or “soft short.”
Lead acid batteries remain a cost effective solution in back-up applications. The technology is well understood as well as being ubiquitous. In addition, there is a good eco-system of specialist companies, together with dependable remote management software, to help monitor and maintain lead acid batteries, as well as advise on replacement. With the correct management, they can provide years of reliable service.
For more information, “Verifying the Health of Emergency or Back-up Batteries” is a white paper available as a free download from the Schneider Electric website.