Low Voltage on Battery Mode

Low output voltage reading on a UPS that uses a step approximated sine wave.

Issue

Multimeter does not properly read the voltage of a UPS that uses stepped approximated output sine wave while on battery. Output voltage of a 120 VAC UPS is measured to be between 80-90 VAC. Output voltage of a 230 VAC UPS is measured to be between 170-180 VAC.

Cause

There are generally two types of meters: Average responding and True RMS meters. Average responding meters are more commonly used. True RMS meters tend to be more expensive. If the meter is not labeled "True RMS" it is most likely NOT a True RMS meter.

The issue that arises with a non-True RMS or average responding meter is whether it is measuring the output from a linear load or a nonlinear load. Linear loads include but are not limited to devices such as light bulbs, incandescent lamps and resistive heaters. Nonlinear loads include devices like computers. When measuring the output of nonlinear loads, the average responding meter will typically read LOW. True-RMS meters are most effective when measuring environments with harmonics. When a waveform is distorted from a standard sine wave (the fundamental), an average responding meter may produce readings that are 30-500w. A stepped approximated sine wave appears distorted when compared to a true sine wave; therefore, a reading will produce incorrect results.

When an average responding meter is measuring the stepped approximated output from an APC UPS while operating on battery, this meter will also read low. The wave shape generated is similar to what the meter would see from a nonlinear load, hence the averaging calculation that the meter makes will be miscalculated. A True RMS meter must be used to accurately measure a stepped approximated sine wave.