A UPS or telecom battery string is only as strong as its weakest battery. One failing unit can cut runtime or drop the load entirely, and it usually gives no warning until the moment the batteries are needed.

Why voltage alone can mislead

Float voltage tells you a battery is being charged. It does not tell you whether the battery can deliver current under load. A battery can show a normal voltage while its internal condition has degraded through sulfation, dry-out or corrosion. That only becomes obvious when it has to carry the load during an outage, which is the worst possible time to find out.

What impedance testing measures

Impedance (internal resistance) testing sends a small test signal through each battery and measures its internal resistance. As a battery ages and degrades, its internal resistance rises. Comparing each battery against:

• its own previous readings,
• the other batteries in the same string, and
• the baseline taken when the batteries were new

shows which batteries are weakening long before they fail. As a general guide, a battery whose impedance has risen well above its baseline or the string average should be watched closely or replaced. The exact limits depend on the battery manufacturer's guidance, and are often in the range of 20 to 50 percent above baseline.

Why it is worth doing

• It is done online, with the batteries in service and no shutdown
• It finds weak batteries before they cause a failure during an outage
• You replace only the batteries that need it, instead of guessing or replacing whole strings too early
• It builds a trend history that makes replacement planning predictable

How often to test

Testing at least once or twice a year is common for UPS and telecom batteries, with more frequent testing as batteries age or at critical sites. The first test, ideally soon after installation, becomes the baseline that later results are compared against.

What you should receive

A good test report lists every battery with its voltage and impedance reading, highlights the ones outside the acceptable range, compares the results with previous tests, and gives a clear recommendation for each: monitor, retest, or replace.

How ARCED can help

ARCED performs impedance testing on UPS, telecom and DC battery systems, and can supply and install replacement batteries when they are needed. See our Battery Impedance Testing service and the batteries we supply.