UPS battery maintenance and IEEE 1188 practice

The baseline is the measurement that matters

An internal resistance or impedance reading means little on its own. What matters is how far each block has moved from the reading it gave when the string was new. EnerSys sets the threshold for further evaluation at a 50% change from baseline ^[2]. A string installed without a baseline reading loses that reference, so the first visit after any replacement should record one for every block.

Readings at every visit

IEEE 1188, as EnerSys quotes it, recommends an internal ohmic measurement of each cell or unit at quarterly intervals ^[1]. Each visit should also record:

• float voltage for the string and each block
• the temperature of the room and of the blocks
• the condition of terminals and connectors, with torque checked where the manufacturer specifies it
• any swelling, leakage or corrosion

Capacity testing

Ohmic readings show a trend; only a capacity test shows how much runtime the string will really give. EnerSys points to the IEEE 1188 test procedure ^[6] and says to always complete a full discharge test, because partial discharges can give a false picture of the battery’s state of health ^[3]. A capacity test takes the string out of its protective role for the duration, so it is planned with the load on a protected path or during a maintenance window.

When to replace

CSB advises replacing a battery system when it reaches 80% of its rated capacity, or before ^[5]. Ohmic trends tell you which strings are heading there; a capacity test confirms it. Plan the replacement as the string approaches that point rather than after it fails during an outage.

New batteries waiting to be installed

Batteries lose charge on the shelf. EnerSys allows up to six months from shipment to the first charge for batteries stored at 25 °C or cooler ^[7], and CSB puts its VRLA self-discharge at about 3% per month at 25 °C ^[8]. Check the manufacture date on replacement blocks, and do not build a string from stock of mixed age.