Internal resistance finds failing battery blocks that voltage and the runtime display miss. The technician measures every block in the string and compares each one with the others in the same string. A block far above its neighbours is the weak link, even when its voltage looks normal and the UPS still shows a long runtime.
Signs you can see yourself: a failed self-test or battery alarm, swollen blocks, acid at the vents or a rotten-egg smell. Behind a healthy-looking display, a per-block test is what finds the weak block.
What internal resistance tells you that voltage does not
Voltage alone can mislead: in the reading below, two other blocks of the same string sat at almost the same voltage as the failing one, with much lower resistance. Across our field sheets, the readings looked like this:
| Reading | What we measured | How we read it |
|---|---|---|
| Float voltage, 12 V block | Median 13.52 V; 10th to 90th percentile 12.69 to 13.72 V (824 blocks, 12 sheets) | A whole string under 13 V, or a block near 11 V with swelling, is replace now |
| Internal resistance per block | Median 3.8 mΩ; 10th to 90th percentile 2.4 to 7.2 mΩ (228 blocks, mixed sizes) | Compared block to block inside one string, never across sites or block sizes |
| Remaining capacity | 20 to 50 percent quoted when replacement was recommended | Below about 50 percent: replace within 6 to 12 months; leaks or swelling: immediately |
Resistance depends on the block’s size, so there is no single number that means “bad”. Our report template writes no fixed threshold: a block is judged against the other blocks of its own string.
A real reading: one block in sixteen
These are blocks 25 to 40 of one battery string, from a de-identified GDF field sheet. Block 32 measured 10.1 mΩ at 13.24 V, while the median of all the blocks in this string (the sheet covers more than the 16 shown) was 3.35 mΩ at 13.51 V, about a third of block 32’s reading. The sheet flags only block 32.
| Block | Float voltage (V) | Internal resistance (mΩ) |
|---|---|---|
| 25 | 13.58 | 3.9 |
| 26 | 13.49 | 2.0 |
| 27 | 13.49 | 3.8 |
| 28 | 13.59 | 4.9 |
| 29 | 13.57 | 3.4 |
| 30 | 13.58 | 1.7 |
| 31 | 13.41 | 1.3 |
| 32 (outlier) | 13.24 | 10.1 |
| 33 | 13.44 | 3.2 |
| 34 | 13.44 | 1.7 |
| 35 | 13.45 | 1.6 |
| 36 | 13.62 | 6.7 |
| 37 | 13.64 | 5.5 |
| 38 | 13.69 | 2.9 |
| 39 | 13.57 | 1.9 |
| 40 | 13.55 | 1.8 |
The control is in the same string: blocks 4 and 15 sat at 13.26 V, almost the same voltage as block 32, with 4.8 and 1.8 mΩ. A voltage check would have passed all three. Only the resistance reading picks out block 32.
Why the runtime display can be wrong
The runtime a UPS displays is an estimate. On a degraded bank it can look healthy, which is why our reports never take it as proof: internal resistance and a transfer test are.
- Government office. A 12 kVA online UPS, 17 years old, with a 4-year-old battery bank. The display showed 129 minutes of runtime. Internal resistance failed on 6 of 8 strings, with acid traces at most vents and heat building up in a poorly ventilated room. The batteries were flagged red, replace as soon as possible.
- Transportation hub. On a fleet with 2-year-old batteries and ventilation problems in some rooms, one unit refused its self-test while displaying 1 h 30 of runtime. That figure was judged unreliable.
How the test is done
- Look first: swelling, acid at the vents and corrosion on terminals.
- Measure the string float voltage, then the voltage of every block.
- Measure the internal resistance of every block with an ohmic tester, and note the battery temperature.
- Compare each block with the rest of its string, and with the same block on the last visit.
- With your approval, run a self-test or a battery transfer test and record whether the runtime holds.
The readings are taken with the UPS online, so the load stays protected (see UPS maintenance without downtime). Before anyone works on the batteries themselves, the battery breaker and the DC strings are isolated.
One weak block is enough to matter: Vertiv notes that a single bad cell in a string can put the entire backup power system at risk, and that UPS battery failure is the most common cause of unplanned outages.
Vertiv’s battery maintenance visit records a similar set: total float voltage and charging current, AC ripple, ambient temperature, the temperature and float voltage of every jar, connection resistances and every internal ohmic reading. The inspections listed for IEEE 1188, the recommended practice for VRLA batteries in stationary use, cover the same ground, including cell or unit internal ohmic values and the resistance of the connections.
Ohmic test or capacity test?
| Internal resistance (ohmic) test | Capacity (discharge) test | |
|---|---|---|
| What it shows | Which block is degrading, compared with its string | How much of its rated capacity the battery still delivers |
| Load | Stays online and protected | Planned, with your approval |
| When | At maintenance visits | At installation, then periodically (IEEE schedule below) |
| What follows | Targets the weak blocks and the replacement window | Confirms remaining capacity before a decision |
As summarized by Fluke, IEEE 1188 recommends an acceptance test at the factory or at installation, then periodic discharge tests at an interval no longer than 25 percent of the expected service life or two years, whichever is less, and annual discharge tests once a battery reaches 85 percent of its expected life or has lost more than 10 percent of its capacity. Fluke adds that most manufacturers recommend replacement below 80 percent of rated capacity.
Before you replace on age alone, a capacity test can show how much margin is left: see what a UPS load bank test proves and how to request one.
What a failing string looks like, and what follows
At a fire hall, the string float voltage was low at 206 V. The bank mixed two battery batches bought a year apart: the older blocks were 30 to 50 percent degraded, the newer ones about 10 percent, and the first leak traces had appeared. The full bank was replaced on the same visit with the customer’s approval, and the indicated runtime afterwards was 478 minutes.
The pattern repeats across our reports. Strings of 3 to 4 years fail the resistance test in hot rooms or when ages are mixed. Of 150 written reports, 11 percent record high internal resistance or a failed resistance test, 23 percent acid leaks, corrosion or sulphation, and 5 percent swollen blocks.
| Symptom | Likely cause | What confirms it |
|---|---|---|
| Runtime shorter than expected, or a display estimate you do not trust | Degraded blocks | Per-block internal resistance and a transfer test |
| Failed self-test or battery alarm | Battery at end of life | Internal resistance per block, remaining capacity |
| Swollen or bulging block | Block failing | Visual check: replace now |
| Acid at the vents, corrosion on terminals | Leaking or sulphated blocks | Visual check and resistance per block |
| Rotten-egg (sulphur) smell | Batteries venting | Isolate and ventilate first, then call |
| Hot battery room | Faster ageing | Battery temperature; GDF documents cite the rule that each 8 °C above 25 °C halves VRLA life |
Frequently asked questions
How do I know my UPS batteries need replacing before they fail?
Test them. A per-block internal resistance reading, compared within the string, finds weak blocks that voltage and the runtime display miss. A failed self-test, swelling, acid at the vents or a sulphur smell mean act now.
Is my battery bad if the UPS still shows a long runtime?
It can be. A UPS displayed 129 minutes while 6 of its 8 battery strings failed the internal resistance test. The display is an estimate; resistance readings and a transfer test are the proof.
Why do UPS batteries go bad so quickly?
Vertiv notes that batteries start to deteriorate from the day they go into service, from use and from their environment. Heat and mixed ages shorten their life: in our reports, strings of only 3 to 4 years fail resistance tests in hot rooms or when old and new blocks are mixed.
How long do UPS batteries last?
We tell customers 3 to 5 years for standard VRLA batteries and 8 to 12 years for long-life or telecom-grade blocks. In our reports, the median battery age mentioned is 4 years.
What resistance value means a block is bad?
There is no single value: resistance depends on the block size. A block is judged against the other blocks of the same string, and against its own previous readings.
Get your batteries tested
Our maintenance report includes a battery internal resistance test, a self-test or transfer test, and the readings, with photos and recommendations. To book a battery test or a replacement, see our UPS battery replacement service and preventive maintenance, contact GDF Technologies or call 514-252-8324. For a small single-phase UPS that only needs a cartridge, UPSPLUSBATTERY carries replacement UPS batteries.
Written by Christian Barkley, Director, GDF Technologies.
Sources
- GDF Technologies, field battery sheets and written service reports (150 reports, 2016 to 2026; cases and readings described without client, site, date or UPS model).
- Vertiv, Battery preventive maintenance data sheet (CS-00135). Single bad cell risk, battery failure and outages, measurements recorded on a visit.
- Fluke, Recommended battery tests and testing schedule for battery back-up systems. Summary of the IEEE 1188 inspections and discharge-test schedule; capacity replacement point.
