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Five Ways to Reduce the Overall Failure Rate of UPS Power Supplies
Release time:
2019-02-20
1. Take Advantage of Peak Power Supply Periods for Charging: For users whose UPS power supplies are consistently subjected to low-voltage mains power or frequent power outages, to prevent premature battery damage caused by prolonged undercharging, it’s essential to fully utilize peak power supply periods—such as late at night—to charge the batteries, ensuring that each battery has sufficient time to recharge after every discharge. Typically, after a battery has been deeply discharged, it takes at least 10 to 12 hours to recharge it back to 90% of its rated capacity. Pay special attention to the selection of the charger. Maintenance-free sealed batteries used in UPS systems must not be charged using thyristor-based “fast chargers.” This is because such chargers can cause the battery to simultaneously experience:
1. Charge during peak power demand periods.
For users whose UPS power supplies are consistently operated under low-voltage mains conditions or frequent power outages, to prevent premature battery damage caused by prolonged insufficient charging, it is essential to take full advantage of peak power supply periods—such as late at night—to charge the batteries, ensuring that each battery has sufficient time to recharge after every discharge. Typically, after a battery has been deeply discharged, it takes at least 10 to 12 hours to recharge it back to 90% of its rated capacity. Pay special attention to the selection of the charger.
Maintenance-free sealed batteries used in UPS systems must not be charged using thyristor-based “fast chargers.” This is because such chargers can subject the battery to a severe charging condition characterized simultaneously by “instantaneous overcurrent charging” and “instantaneous overvoltage charging.” This condition can significantly reduce the battery’s usable capacity and, in severe cases, render the battery completely unusable. When using UPS systems equipped with constant-voltage cutoff charging circuits, be careful not to set the battery voltage protection threshold too low; otherwise, the battery may easily experience overcurrent charging during the initial stages of charging. Of course, it’s best to use a charger that offers both constant-current and constant-voltage charging capabilities for optimal battery performance.
2. Ensure the power supply environment temperature.
The usable capacity of a battery is closely related to the ambient temperature. Generally, battery performance specifications are calibrated under standard room temperature conditions of 20°C. When the temperature drops below 20°C, the battery’s usable capacity will decrease; conversely, when the temperature rises above 20°C, its usable capacity will slightly increase. The degree to which different batteries from different manufacturers and in different models are affected by temperature varies. According to statistics, at -20°C, the usable capacity of a storage battery can reach only about 60% of its rated capacity. It is clear that the impact of temperature cannot be ignored.
Of course, to extend the service life of a battery pack, you not only need to pay attention to proper maintenance and usage practices, but also take full account of the load characteristics (resistive, inductive, or capacitive) and their magnitude when selecting the battery. Avoid operating the battery under excessively light loads for extended periods, as this could result in an abnormally low discharge current that ultimately leads to battery failure.
3. Regular inspections
Regularly check the terminal voltage and internal resistance of each cell in the battery pack. For 12V cells, if the terminal voltage difference among cells exceeds 0.4V or the internal resistance of a cell exceeds 80 mΩ during inspection, each cell should undergo equalization charging to restore its internal resistance and eliminate any voltage imbalance among the cells. During equalization charging, the charging voltage should be set between 13.5 and 13.8V. After proper equalization charging, the internal resistance of most batteries can be restored to below 30 mΩ.
During operation, the aforementioned imbalance caused by the changing characteristics of individual battery cells over time cannot be eliminated solely by the UPS’s internal charging circuit. Therefore, for battery packs that have already exhibited significant imbalance, if offline equalization charging is not performed promptly, the degree of imbalance will only become more severe over time.
4. Re-boost charge
The UPS power supply recharges the battery by utilizing the internal charging circuit for 10 to 12 hours or more before returning to float charge and then operating under load. When a UPS power supply remains in a float-charge state for an extended period without undergoing any discharge cycles, it is essentially in a “stored-in-readiness” condition. If this condition persists for too long, the battery may become unusable and eventually fail due to “overstorage.” The primary manifestation of this failure is an increase in the battery’s internal resistance, which, in severe cases, can rise to several ohms.
It has been found that at a room temperature of 20°C, after being stored for one month, the battery’s usable capacity is approximately 97% of its rated capacity. If left unused for six months, its usable capacity drops to 80% of the rated capacity. Furthermore, if the storage temperature rises, the battery’s usable capacity will decrease even more. Therefore, it is recommended that users deliberately disconnect the mains power input every 20 months or so, allowing the UPS to operate in a mode where energy is supplied from the battery to the inverter. However, this operation should not be prolonged; when the load is around 30% of the rated output, discharging the battery for about 10 minutes is sufficient.
5. Reduce deep discharging
The service life of a battery is closely related to the depth of discharge it experiences. The lighter the load on a UPS power supply, the greater the ratio of its usable capacity to its rated capacity when mains power is interrupted. Under these circumstances, when the UPS power supply automatically shuts down due to low battery voltage, the battery will have been discharged to a relatively deep extent.
How can we actually prevent the battery from being deeply discharged? The solution is quite simple: When the UPS power supply switches from mains power to battery-powered operation—supplying power to the inverter—the vast majority of UPS systems will emit a periodic alarm sound at intervals of roughly 4 seconds, alerting the user that the battery is now providing power. If the alarm becomes more frequent and urgent, it indicates that the battery is already deeply discharged. In such a case, you should immediately take emergency measures and shut down the UPS power supply. Unless absolutely necessary, it’s generally best not to let the UPS continue operating until it automatically shuts down due to excessively low battery voltage.
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