6.6. How do I select the battery cabinet when using long run UPS?
Three parameters need to be considered when selecting battery: voltage, charging current and backup time.
The voltage is the total voltage of the battery cabinet, which is summed by each battery pack when they are connected in series. For example, a battery cabinet contains 16 pcs of 12V battery, and all of them connect in series, the nominal voltage of this battery cabinet is 192Vdc. It would match the UPS which should connect 16 pcs of battery, battery voltage 192Vdc or charging voltage 218.4. When we talk about a 12V battery, it means the nominal voltage of this battery is 12V. For most 12V lead-acid batteries, the charging voltage is around 13.65~13.7V. Therefore, the charging voltage of 16 pcs of battery connected in series is 218.4~219.2V. This value should be able to be found on the datasheet of UPS.
The 2nd parameter is charging current, which should meet the requirement or recommendation of the battery. Most battery datasheets show “Maximum Charge Current”, usually it’s around 0.3C. For normal operation, charging current is 0.1C as the best practice. It’s never less than 0.05C. C rate is the rate of the charging/discharging current over battery capacity. 1C means one hour charge, that is to charge an empty battery to full in one hour. So, 0.1C means 9 hours to charge to full, that’s pretty common design. Most UPS have at least 1A charging current, and connect to a 9Ah battery or smaller to make sure the battery can be charged to full no longer than 9 hours. In very extreme cases, users use 0.05C to charge the battery, which is verified not to damage or downgrade their batteries by some battery manufacturers. However, it is not guaranteed for all batteries. Even if it’s workable from a technical point of view, people do not use this extreme case either because it might cause another concern that mains could be off twice in a short period, for examples, 10 hours. Then the backup time would be only half as the plan. This should be considered too.
The last parameter is backup time, which is the key to select a suitable battery cabinet. Because lead-acid battery has very different capacities at different discharging current. The capacity of the same battery could be several times difference in different discharging current. It can not be calculated without the battery discharge characteristics table.