The battery is one of the most important parts of an uninterrupted power supply (UPS) system, and is directly related to the reliability of the entire UPS system. Even the most advanced UPS system is unable to provide uninterrupted power if the battery fails, so it is not advisable to take any risks by using an inferior battery to save costs. Doing this will ultimately affect the reliability of the UPS system and can cause even greater losses.
The battery has the shortest mean time between failure (MTBF) in the entire UPS system. If the battery pack is correctly used and well maintained, its service life can be extended. However, if the battery pack is not correctly used or maintained, it will shorten the battery life. This article presents some basic principles of a UPS battery and the precautions to take when using it.
Popular battery chemistries
There are several types of batteries that are often used for power storage, but considering the load conditions, operating environment, service life and costs, valve regulated lead acid (VRLA) batteries are the most commonly used batteries for UPS systems. The main feature of lead-acid batteries is that oxygen is generated on the positive plate during charging, and is reduced to water on the negative plate by a chemical reaction.
Compared to traditional lead-acid batteries, a VRLA battery does not need to be refilled with water or have its electrolyte levels adjusted, and is therefore referred to as being ‘maintenance-free’. That does not mean, however, that no maintenance is required for VRLA batteries – in fact, all batteries need to be properly used and maintained.
The ambient temperature has a significant impact on the battery. If the ambient temperature is too high, more gas will be generated during the battery charging process and may cause thermal runaway. If the ambient temperature is too low, it will cause poor charging efficiency, resulting in the battery not being fully charged and eventually affecting the battery life.
It is therefore recommended that the battery be installed in an ambient temperature of around 20°C to 25°C. As the battery performance will be affected by the ambient temperature, it should not be used at an ambient temperature of below 5°C or above 35°C, as doing so will reduce the battery capacity and greatly shorten its life.
Temperature effects must be considered when the ambient temperatures are below 5°C or above 35°C, as the charge settings can be adjusted for temperature compensation. The temperature coefficient for cycle service is -5 mV/°C per cell and for standby use (trickle charge or float charge) is -3,3 mV/°C per cell.
Depth of discharge
The depth of discharge is also an important factor affecting battery life. The deeper the discharge, the fewer the number of cycles the battery can provide. Therefore, avoiding deep discharge will be a good way to protect the battery. Most UPS systems have protection built in to shut the UPS down when the battery is discharged to about 10,5 V. However, if the UPS is under light load or no-load discharge mode, it might still cause deep discharge of the battery.
During transportation and shelf storage, the battery will inevitably lose some of its power, which is called self-discharge. Therefore, before installing and using the battery, the battery voltage should be checked to determine the remaining power. If the voltage is too low, supplementary charging is required. For batteries that are not being used or are being stored for an extended period of time, they should be recharged every three months.
A quick way of determining the remaining battery power is by measuring the open circuit voltage of the battery. Using a 12 V VRLA battery as an example, if the open circuit voltage is above
12,5 V, the battery may still have more than 80% power. However, if the voltage is lower than 12,5 V, the battery should be recharged. If it is lower than 12 V, the battery’s power delivery may be less than 20% and it needs to be recharged immediately. If the voltage cannot be recovered after it has been charged several times, it means the battery is unusable.
A UPS is a continual power system that provides emergency power to a load when the main input power fails. The battery is in standby mode under normal conditions but plays the role of a power bank to provide the necessary power when the mains power is off. This ensures continuity of power supply.
To prolong the service life of the battery, UPS chargers are often designed with a constant-voltage current limiting mode. This means that when the battery is fully charged, the equipment will switch into floating mode, and each floating charge voltage is set to about 13,6 V. If the charging voltage is too high, the battery will be overcharged. Conversely, if the charging voltage is too low, the battery will not be fully charged.
An abnormal charging voltage may be caused by an error in the battery configuration or due to a charger failure. Therefore, when installing the batteries, be sure to pay attention to the correctness of the specifications and quantity of the batteries. Do not mix batteries of different specifications, brands and batch numbers, and do not use a poor-quality charger. The heat dissipation issue also needs to be noted.
Advanced battery management systems
Many high-end UPS systems now use ABM (advanced battery management) three-stage intelligent battery management solutions, which divide the charging process into three phases: initial charging, float charging and resting:
1. Constant voltage equalisation charging of the battery to 90% capacity.
2. Float charging mode to fully charge the battery to 100%, and then stop charging.
3. Natural discharge, in which the battery discharges via its own leakage current until the low-voltage limit is reached, and then repeats the above three stages.
This method changes the traditional charging design in such a way that the battery is not always kept in a floating state, thereby prolonging the life of the battery.
It is important to monitor the following conditions of the battery pack or individual battery during use: the terminal voltage and floating charge current of the battery pack; the voltage of each battery cell; and the ground resistance and insulation of the battery pack and the DC bus.
Do not increase or decrease the load on any single battery cells in the battery pack. It will result in an unbalanced battery capacity, uneven charging and reduced battery life. The battery should be installed in a clean, cool, ventilated, dry place and away from heaters or other sources of radiant heat. The battery should be placed upright and not tilted, and the terminal connections between each battery should be firm.
Faulty procedures or inadequate charging, over-discharge, over-charge and insufficient charging time will result in the failure of the battery to recover normal capacity, reduced capacity, or shortened battery service life. It is necessary to perform periodic maintenance for assurance of the optimum battery reliability. It is recommended that these inspections should be performed at least every three months.
In general, periodic maintenance will include visual inspection of the battery, ambient temperature checking, capacity test, voltage measurement, float voltage inspection, high-rate load test, resistance and that the connections are properly secured.
These maintenance tasks are designed to determine the gradual decrease of capacity of the system and to detect any abnormal error or individual battery condition that may impact on system reliability. It is also suggested to discharge/recharge the batteries periodically to keep them active, and to do so at least every three months.