UPS battery impedance trend, and a replacement date to budget for

Why impedance, and why a trend

As a lead-acid battery ages, its internal resistance rises, slowly at first and then faster near the end of its life. One impedance test tells you where a battery stands today against its reference value. Several tests of the same set tell you how fast it is moving. Both matter: a string at 1.2 times its reference and flat is a different conversation from one at 1.2 times and climbing.

The limits OHMEE uses

Every battery is judged against the reference for its model: warning at 1.25 times the reference, replace at 1.5 times. It is the same rule on the tablet, in the Word report and in the OHMEE report, and the limits are drawn on every chart.

Where the date comes from

The outlook prints a "Budget by" quarter and names the clock that set it. There are three:

• Today's level. Among strings in the service records that were at this level and later crossed the replace line, how soon they crossed. One test is enough for this date.
• The impedance trend. The highest battery per test, drawn on a curve that rises faster each year. A trend needs two tests at least six months apart that are really climbing; without one, the outlook says so instead of guessing.
• Battery age. From the install date or the date code, on a 6-year service life for lead-acid unless the office sets its own.

The earliest of the three sets the budget date, and the report says which one it was, so the office can check it by hand. A single battery that has failed in an otherwise sound string is named as "replace now" on its own, and the rest of the set keeps its date.

Only today's batteries count

A replaced set starts a fresh line; tests of the batteries before never pull the date. A spot test of a few batteries continues the set rather than starting a new one. Readings taken two batteries at a time are halved so they sit on the same trend as single-battery tests.

What the report shows

The Word report and the OHMEE report both print the outlook chart: the measured tests as points on a solid curve, then a dotted curve that carries on from today's test, steepening, across the replace line at the budget quarter. Above the line the chart says what it means: rising risk of failure and thermal runaway. The caption is plain words, for example that resistance is rising faster each year and reaches the replace line around a given quarter.

Lithium-ion units are handled differently: a 15-year design life from the install date or date code, and the state of health from the manufacturer's tool.

What it is not

A budget date is a planning figure, not a guarantee. Batteries can fail early from heat, a bad cell or an outage. The outlook is there so the customer can plan a replacement instead of reacting to one, and it is updated at every visit.