Causes of overheating
High battery temperatures can be caused by both the ambient temperature of the environment in which they are stored, and heat generated by the batteries themselves. When discharging, batteries absorb heat. However, as batteries charge, they give off heat at the rate of around 0.02 Watts per Ah per 12V DC. It’s therefore, important to ensure that the cooling system maintains optimum battery temperatures.
Battery blocks installed in racks should have a minimum 10mm gap between each block for ventilation. This is especially important to avoid thermal runaway should a block go open circuit – a battery condition which carries the danger of possible oxygen/hydrogen gas production and battery swelling. Batteries with this condition cannot be recovered and must be replaced as they have become a serious health and safety hazard.
As an individual battery cell weakens, so it will charge at a faster rate than the others. It will heat as it overcharges until the other cells in its group reach their correct charge. This creates hot spots and, should the cell fail completely, it may cause the rest of the pack, now one cell short, to overcharge, again creating further heat and shortening their life.
A major contributory factor to reduced battery life is the number of battery discharge-recharge cycles. The amount of energy that a battery delivers during a discharge has a direct impact on the reduction of battery life. The main reason for this is the deterioration of the battery contacts.
Constant discharging of batteries will cause them to reach their end of life prematurely. Short UPS activity due to compensating for swells and sags should be considered as battery discharge cycles.
Different battery alternatives
There are two types of lead-acid batteries: open vented and valve regulated. The newer valve regulated lead acid (VRLA) batteries (which vent in the event of a problem) have now largely replaced the older open vented type. Other types of battery are: •Nickel-Cadmium (Ni-Cad) batteries. These are much more tolerant of both higher and lower ambient temperatures, typically between -20ºC to +40°C, and have an extended operational life. However, disadvantages include a high initial purchase price and a high disposal cost because of policies governing the disposal of cadmium. •Gel (sealed) batteries. Being more expensive than VRLA batteries, these are designed to operate at very low temperatures and, being able to operate in temperatures as low as -30ºC, they have a manufacturer’s average life expectancy (at around 20ºC) of four to five years.
Battery configuration – series and parallel strings
Because individual batteries only deliver a relatively small amount of electricity, usually 12V, they are grouped to deliver the required amount of power. A single battery is referred to as a block. This block will be made up of a number of cells. When the blocks are connected together, they form a battery string.
A single string of battery blocks can be connected in series end to end. The overall voltage achieved is the total of all the individual blocks added together. This should match the DC voltage required by the UPS.
The capacity of the string remains the same as each individual battery block. If for example twenty 12V, 10Ah battery blocks are connected together in series this way; the result would be 240V with a 10Ah capacity.
A parallel battery string is a number of series strings wired in parallel. Each of the series strings must contain the same quantity of blocks. The primary reason for this configuration is to increase the Ah rating, the runtime of the UPS. Therefore, if using the previous example of the series string delivering 240V with 10Ah, if three such strings are then wired in parallel, the result would be a power supply of 240V with 30Ah.
Major benefits of parallel battery strings are improved system resilience and protection against the risk of a single faulty battery being responsible for the failure of your standby power system.
Testing and replacement cycles
Unfortunately, UPS batteries don’t last indefinitely. Battery life expectancy is dependent on a number of factors such as frequency of use, duration of use, heat and humidity etc. As a result, all manufacturers recommend that batteries are periodically tested to ensure they are still functioning correctly. A UPS Health Check will include a reading of your overall battery capacity.
A battery with a five-year design life should certainly be checked after three years and we advise replacing after four years. For batteries designed to BS-EN 6290 pt IV (with a ten-year design life), we advise that replacements should occur every eight years.
Because batteries are such critical elements of your standby power system, it is absolutely imperative that their operational capacity is known. You will want to know if your batteries are fully charged, how much deterioration there has been since installation, and what is their present life expectancy. This is known as an impedance test, and we would recommend this should be undertaken annually. During an impedance test, the engineer will test individual battery cells on their life expectancy and durability. He will take meter readings from the cell terminals of each cell block to identify any faulty cells. Where batteries are parallel connected, a string will be isolated and tested while the UPS remains operational. The impedance test will provide a detailed assessment of your batteries and, crucially, their life expectancy.
Every UPS deployed to protect the continuity of critical IT systems should be regularly maintained.
A standard UPS maintenance visit will include checking the batteries to provide an overview of their health, checking ambient temperatures and giving advice. However, we recommend that standard maintenance contracts be upgraded to include annual impedance testing.
If you look after your batteries, they’ll last. If not, they’re likely to fail much earlier than you expect – possibly with disastrous consequences.
The single most important fact in preserving battery life is to maintain the battery temperature between 19º and 22ºC. Regular maintenance and annual impedance testing will give you peace-of-mind and provide a clear picture of the state of your batteries.