UPS Uninterruptible Power Supply Principle and Lead-acid Battery Maintenance（part 1)

I. UPS Uninterruptible Power Supply : The principle is actually not complicated.

UPS stands for Uninterruptible Power Supply. Its core mission can be summarized in one sentence: when the mains power is normal, it supplies power to the equipment and charges the battery; when the mains power fails, the battery immediately takes over, and the equipment remains unaware of the interruption.

Based on their operating methods, UPS systems are divided into three types, which I will rank for you:

Backup type: The device is normally powered directly from the mains; it switches to battery inverter only when the voltage is unstable or there is a power outage. The switching time is approximately 2-10 milliseconds. This is sufficient for most home computers, but not for precision instruments. Suitable for individuals or offices with limited budgets.

Interactive online type: Features built-in voltage regulation; uses a transformer to adjust for mains power fluctuations, eliminating the need for frequent battery switching. More stable than standby type and cheaper than online type . Commonly used in small servers and network cabinets.

Online type: This is true “uninterrupted” operation. Regardless of the quality of the mains power, the load is always powered by the inverter. The incoming mains power is first rectified into DC, part of which charges the battery, and the rest is inverted into pure AC output. Switching time is zero, and the waveform is perfect. This type is essential for medical equipment, industrial PLCs, and data centers.

Regardless of the type of UPS, the final hurdle is the battery. Currently, over 90% of UPS systems still use valve-regulated sealed lead-acid (VRLA) batteries . Therefore, to understand UPS systems, you must first understand lead-acid batteries.

II. Lead-acid batteries: How do they store electricity, and why are they prone to failure?

Lead-acid batteries have a history of over 150 years, but their principle is extremely classic: the mutual conversion of chemical energy and electrical energy.

Discharge: Lead (Pb) at the negative electrode and lead dioxide (PbO₂) at the positive electrode react with the electrolyte (dilute sulfuric acid H₂SO₄) to become lead sulfate (PbSO₄), simultaneously generating current. The sulfuric acid concentration decreases, and the water content increases.

Charging: When an external direct current is applied, the lead sulfate is converted back into lead, lead dioxide, and sulfuric acid. As the sulfuric acid concentration rises, the battery is “fully recharged”.

Advantages: Inexpensive, reliable, and has strong high-current discharge capability.
Disadvantages: Low energy density, short cycle life (typically 300-500 deep charge-discharge cycles), and highly susceptible to overcharging and over-discharging.

Many users complain that “UPS batteries fail after two years,” but this isn’t because the batteries are bad, but rather because maintenance hasn’t kept up. The following points are the most crucial maintenance guidelines for lead-acid batteries.