Why UPS Self-Tests Can Give False Confidence: Voltage vs Internal battery Impedance/Resistance

Many UPS systems run an automatic self-test and then display a reassuring runtime estimate (for example, “35 minutes remaining”). The problem is that a self-test often checks basic battery voltage and a light internal load condition — not how the battery string behaves under real, sustained load. That’s how you can end up with a UPS that looks healthy on the screen, but collapses to less than a minute when the critical load transfers to battery.

Voltage is not the same as battery health

Battery voltage can look “normal” even when the battery is near end-of-life. What actually determines whether the battery can deliver current is its internal resistance (often measured as impedance). As batteries age, plates sulfate, connections degrade, and internal resistance rises. Under load, higher internal resistance causes a larger voltage drop — and the UPS reaches its low-voltage cutoff quickly.

Why the UPS runtime estimate can be wrong

Runtime algorithms rely on assumptions: battery model, temperature, recent discharge history, and what the UPS “thinks” the battery capacity is. If the UPS only sees acceptable open-circuit voltage (or a brief test), it may calculate a long runtime. But when the real load hits, weak blocks with high internal resistance sag immediately, dragging down the whole string and causing an early shutdown.

The fix: test internal battery impedance and trend it

An impedance (internal resistance) tester lets you identify failing batteries before they fail under load. The key is not only taking a reading once, but building a baseline and trending results over time. This makes it easy to pinpoint exactly which battery in which string is deteriorating.

Steps to follow when testing internal resistance

1. Number and label each battery (and each string) clearly, so every reading is traceable.

2. Check and torque battery connections to the manufacturer’s specification. Loose or inconsistent torque can distort readings and create hot joints.

3. Take a full set of impedance/internal resistance readings to establish a base (baseline) for each battery in each string.

4. Record results and build graphs/trends over time (monthly/quarterly depending on criticality). Trending is where the real value is.

5. Investigate outliers immediately: a battery that deviates significantly from the string average (or from its own baseline) is a strong candidate for failure under load.

What you gain

• Accurate identification of weak blocks before an outage

• Confidence that runtime estimates match real-world performance

• Clear reporting: you’ll know exactly which battery on which string is failing

Need UPS battery testing in South Africa?

If you’re in South Africa and want proper impedance testing, baseline reports, and clear identification of failing batteries, contact Mtshali Power Electronics & Infrastructure (Pty) Ltd:

Email: service@powerinfrastructure.co.za

Phone: 0311002679