The temperature of batteries can rise due to external factors as well as the heat generated internally. When batteries discharge, they absorb heat, while during charging, they release heat at a rate of 0.02 watts per Ah per 12V DC.
When installing battery blocks in racks, it is crucial to maintain a minimum 10mm gap between them to prevent overheating and the occurrence of thermal runaway. Thermal runaway refers to a situation where the battery produces oxygen/hydrogen gas or expands. Overheated batteries pose significant health and safety risks and cannot be salvaged.
If a single battery cell deteriorates, it will charge at a faster pace compared to the other cells, leading to overheating as it overcharges to match the charge level of the remaining cells in its group. This creates hot spots. If the cell fails completely, the rest of the pack, now with one less cell, will overcharge again, resulting in further heat buildup and reducing the overall lifespan of the pack.
The frequency of discharge-recharge cycles significantly affects the lifespan of batteries. The amount of energy released during a discharge has a notable impact on battery longevity, primarily due to the deterioration of battery contacts.
Constant discharging can prematurely shorten the battery life. To prevent excessive discharge cycles, it is important to account for fluctuations in UPS activity by compensating for power swells and sags.
Two types of lead-acid batteries exist: open vented and valve regulated lead-acid (VRLA) batteries. VRLA batteries, which have a built-in venting mechanism to address any issues, have replaced the older open vented type and are widely used.
Other battery alternatives include Nickel-Cadmium (Ni-Cad) batteries, which have a wider temperature range of -20°C to +40°C and offer enhanced durability. However, they contain hazardous cadmium and require proper disposal, resulting in higher initial costs and disposal fees. Additionally, there are gel (sealed) batteries, which are not as cost-effective as VRLA batteries and are suitable for use in low temperatures. Their average lifespan, particularly at around 20°C, is approximately four to five years.
Individual batteries typically provide a limited amount of electricity, usually 12V, so they are combined into groups to achieve the required power output. A single battery is referred to as a block and consists of several cells. When these blocks are connected, they form a battery string.
Battery blocks can be connected in series, end to end, to create a series string. The total voltage achieved is the sum of the voltages of all the individual blocks. This overall voltage should match the DC voltage required by the UPS.
A parallel battery string is formed by connecting multiple series strings in parallel. It is crucial to ensure that each series string contains the same number of blocks. This configuration is primarily used to increase the runtime (Ah rating) of the UPS. For example, if three series strings, each delivering 240V with 10Ah, are wired in parallel, the resulting power supply would be 240V with 30Ah.
The use of parallel battery strings offers several important advantages, including enhanced system reliability and reduced risk of a single faulty battery causing a failure in the standby power system.
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