UPS Common Battery Types, Factors Affecting Battery Life, and the Pros and Cons of Different Battery Types

I: Types of Common Batteries Used in UPS, Factors Affecting Battery Life, and the Pros and Cons of Different Battery Types:

In UPS applications, there are three main types of batteries: open-type liquid lead-acid batteries, maintenance-free batteries, and nickel-cadmium batteries. Currently, most UPS manufacturers use maintenance-free batteries. Below, we will primarily discuss the characteristics of maintenance-free batteries, along with the three types of batteries.

Open-Type Liquid Lead-Acid Battery: These batteries are classified into two categories based on their lifespan: 8-10 years and 15-20 years. Since the sulfuric acid electrolyte generates corrosive gases, these batteries must be installed in a well-ventilated room, away from precision electronic equipment, and the battery room should have corrosion-resistant tiles.
Due to evaporation, open-type batteries require regular measurement of specific gravity, and acid or water may need to be added. These batteries can tolerate high temperatures, high pressures, and deep discharges. The battery room should be smoke-free and equipped with an open-type battery rack.

After charging, open-type batteries cannot be transported, so they must be charged on-site after installation, typically requiring 55-90 hours for initial charging. The normal voltage for each cell is 2V, and the initial charging voltage is 2.6-2.7V.

Maintenance-Free Battery: Also known as valve-regulated sealed lead-acid (VRLA) batteries, these require the following principles during use and maintenance:

a) The allowable operating temperature range for sealed batteries is 15°C to 50°C, but using them within 5°C to 35°C can extend battery life. Below -15°C, the chemical composition of the battery changes, and it cannot be charged. The optimal temperature range for the longest battery life is between 20°C and 25°C. Running the battery in cold temperatures will provide a longer life but lower capacity, while high temperatures will increase capacity but shorten battery life.
b) The relationship between battery life and temperature can be summarized as follows: For every 8.3°C increase above 25°C, battery life will be halved.
c) The designed float voltage for a maintenance-free battery is 2.3V per cell. For a 12V battery, it is 13.8V. CSB recommends 2.25-2.3V per cell. When 120 cells are connected in series, if the temperature exceeds 25°C, the float voltage should be reduced by 3mV for every 1°C increase. Similarly, to avoid insufficient charging voltage, the voltage should be increased by 3mV for every 1°C decrease.
d) After discharge, if the battery is not recharged within 72 hours, sulfate will accumulate on the plates, causing insulation and battery damage.
e) During float or equalization charging, gas produced inside the battery is converted into water at the negative plates, maintaining the battery capacity without the need for added water. However, corrosion of the plates will reduce battery capacity.
f) The lifespan of the battery separator at 30-40°C is only 5-6 months. Batteries that are stored for long periods should be recharged every 6 months. They should be stored in a dry, cool environment. At 20°C, the self-discharge rate of a maintenance-free battery is 3-4% per month and varies with temperature.
g) Maintenance-free batteries are equipped with safety valves that automatically vent excess gas when internal pressure reaches a certain level, and the valve reseals once the pressure normalizes.
h) The cycle life (charge-discharge cycle life) of the battery depends on the discharge rate, depth of discharge, and charging method. The most important factor is the depth of discharge. The shallower the discharge, the longer the cycle life. Maintenance-free batteries have a cycle life of approximately 200 cycles under 100% depth of discharge at 25°C.
i) When the battery reaches the end of its lifespan, it will show signs of capacity loss, internal short-circuiting, deformation of the casing, plate corrosion, and a decrease in open-circuit voltage.
j) IEEE defines the end of a battery’s life as when its capacity drops below 80% of its nominal capacity. There is a nonlinear relationship between nominal capacity and actual backup time, and a 20% loss in capacity can significantly reduce backup time. Some UPS manufacturers define the end of life when capacity drops to 50-60% of the nominal capacity.
k) Mixing batteries with different capacities and brands is strictly prohibited, as it can reduce battery life.
l) If two battery groups are used in parallel, it is essential to ensure that the battery wiring and busbar impedances are the same.
m) Although maintenance-free batteries do not require adding liquid, periodic checks for cracks in the casing or electrolyte leakage are still necessary.
Nickel-Cadmium Battery: These batteries differ from lead-acid batteries as they generate hydrogen and oxygen during electrolysis without producing corrosive gases, allowing them to be installed near electronic equipment. They consume very little water and typically do not require maintenance. Their normal lifespan is 20-25 years, but they are significantly more expensive than the other batteries mentioned, with installation costs about three times those of lead-acid batteries. The voltage for each cell is typically 1.2V, and UPS systems using this type of battery require a higher charging voltage.

II: Advantages and Disadvantages

Open-Type Lead-Acid Battery:

Advantages: Lower investment cost, longer lifespan than maintenance-free batteries, and lower temperature requirements.
Disadvantages: More complex maintenance, requires a dedicated battery room, emits corrosive gases, must be charged for 50-90 hours on-site, and requires specialized maintenance personnel.
Maintenance-Free Battery:

Advantages: No liquid maintenance required, can be transported in a fully charged state, and does not require specialized maintenance personnel.
Disadvantages: Failure to perform restorative charging can damage the battery, temperature-sensitive, shorter lifespan, and more expensive than lead-acid batteries.
Nickel-Cadmium Battery:

Advantages: Lower maintenance requirements, longer lifespan, temperature-insensitive, and no harmful gas emissions.
Disadvantages: The most expensive of the three battery types.
III: Common UPS Batteries

Currently, most computer centers choose maintenance-free batteries due to their easier maintenance. However, the following tasks should still be performed:

Discharge the battery every three to four months to prevent plate oxidation.
Maintain the environment temperature between 20°C and 25°C.
Check the battery connections to ensure they are neither too tight nor too loose.
Replace the battery after three years of use.