There is no one ‘best’ battery technology for data centre operators. Each type of battery has its own advantages and disadvantages and operators should choose the most suitable battery product according to their requirements.
I. Lead-acid batteries in data centre
Lead-acid batteries remain popular today because they have a proven track record of reliability. They are the most economical choice for large-scale applications, offering excellent performance and efficiency, low internal impedance, high tolerance to improper handling and high procurement costs.
The electrolyte used in lead-acid batteries consists of water and sulphuric acid, and an electrode plate consisting of sponge lead (negative electrode) and lead oxide (anode). The main type of lead acid battery cell is the valve regulated lead acid (VRLA) battery, also known as a ‘sealed’ or ‘maintenance free’ battery.
VRLA batteries are sealed, but have a valve that allows the internal accumulation of gases to be vented to the atmosphere. They do not normally require direct maintenance and do not need to be filled with water, as the hydrogen released during the charging process recombines with oxygen to form water internally. There are two main types of Valve Regulated Lead Acid (VRLA) batteries on the market, the difference being in the electrolyte mixture: glass microfibre spacer (AGM) batteries have the electrolyte held in a highly porous microfibre glass spacer; whereas gel batteries have an electrolyte gel consisting of a mixture of sulphuric acid and silica.
The AGM type of sealed valve-regulated lead-acid (VRLA) battery is commonly used in UPS power supplies because of its lower internal resistance, higher specific power and efficiency, lower self-discharge rate and lower procurement cost. Glass microfibre spacer (AGM) batteries are faster to charge and can provide high currents for short periods of time.
Liquid-rich lead-acid batteries have the electrode plates submerged in an acidic electrolyte. As there is no seal and therefore hydrogen gas produced during operation is vented directly into the environment, their ventilation system must be more robust than that of sealed valve regulated lead-acid batteries (VRLA). In most cases, the battery bank is housed in a dedicated room. Liquid-rich lead-acid batteries must be kept in an upright position for operation and require manual topping up of the water level.
They have a longer service life and higher reliability than sealed valve regulated lead-acid batteries (VRLA). The battery compartment for lead-acid batteries must be kept at a reasonably constant temperature (20-25°C) to avoid shortening the service life and even causing damage.
2. Lithium ion batteries in data centre
In a lithium ion battery, the “cathode” is usually a metal oxide and the anode is usually porous carbon graphite. Both are immersed in a liquid electrolyte made from lithium salts and organic solvents.
There is a wide range of lithium-ion batteries, which can be simplified into six types: lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium manganese cobalt oxide (NMC), lithium iron phosphate (LFP), nickel cobalt aluminium oxide (NCA), and lithium titanium oxide (LTO). The choice between these cells depends on several factors and precise comparisons are not possible as many aspects such as mechanics, cell size and active material mix play an important role in performance.
Lithium-ion batteries are becoming an increasingly attractive alternative to lead-acid batteries in data centre environments, where power availability is definitely a top priority and lithium-ion batteries offer a higher level of reliability than lead-acid solutions. Not only is each individual battery itself safer and more stable, but each battery module has an electronic controller that continuously checks the battery for any signs of performance changes.
The temperature, current, voltage and charge status of each battery is monitored at cabinet level, giving a clear picture of current battery status and predicting future run times and performance. Lithium-ion batteries can be charged faster than lead-acid batteries, provide more discharge/recharge cycles than lead-acid batteries, and offer higher power density and efficiency, especially at high discharge rates. This eliminates overuse of the battery while reducing the amount of space required for battery installation. Although the initial purchase price of lead-acid batteries is lower, lithium-ion batteries last at least twice as long as lead-acid batteries of the same specification, thus reducing the overall investment cost. Labour costs associated with battery removal and replacement are also reduced. Lithium-ion batteries generate less waste heat, resulting in lower cooling costs and a reduced carbon footprint.
3. Nickel-cadmium batteries in data centre
Nickel-cadmium battery electrodes consist of nickel hydroxide (positive plate) and cadmium hydroxide (negative plate). NiCd batteries have a long operating life (up to 20 years) and can cope with extreme temperatures (-20°C to 40°C). They also have a high cycle life and are well tolerated to deep discharges. Other benefits relate to the low internal resistance, which provides high power density and fast charging capability. NiCd batteries offer long storage times and a high degree of protection against mishandling.
However, NiCd batteries are much more costly than conventional sealed valve-regulated lead acid (VRLA) batteries. In addition, the battery disposal/recycling process is costly as both nickel and cadmium are toxic. NiCd batteries also require maintenance in the form of water addition, especially in high-cycle applications, or at high charge rates with certain charging methods.
