Battery Testing Fundamentals: Capacity, Voltage, and Internal Resistance — Three Numbers That Tell the Whole Story

Article

May 8, 2020

You cannot judge a battery by a single number. The most common mistake is believing that high voltage means a healthy battery — a battery sitting on the shelf for six months with heavy self-discharge can still show near-full open-circuit voltage while its usable capacity is down by half. Conversely, an old battery with thousands of cycles may show acceptable voltage but sag below the cutoff under load. These two examples demonstrate that single-parameter judgment inevitably leads to misdiagnosis: capacity, voltage, and internal resistance must all be evaluated together.

Capacity, Voltage, and Internal Resistance: Three Dimensions of Battery Health

Capacity tells you how much energy the battery can still store, typically expressed in ampere-hours (Ah) — the total charge deliverable under standard discharge conditions. Capacity fade is the most direct indicator of battery aging. A battery rated at 100Ah that can only deliver 70Ah after three years has a state of health (SOH) of 70%. Capacity testing requires full discharge to the cutoff voltage, which takes considerable time — typically not suitable for quick daily checks, but essential for acceptance testing, annual maintenance surveys, and end-of-life determination.

Internal resistance reflects the impedance inside the battery, measured in milliohms (mΩ). It has three components: polarization resistance (from electrochemical reaction kinetics), ohmic resistance (from electrode material resistivity and contact interfaces), and electrolyte resistance. New batteries typically show internal resistance in the tens of milliohms or lower. As cycle count increases and electrolyte depletes, internal resistance rises slowly. The critical insight is that internal resistance and capacity fade are not linear — resistance changes very little in the first half of life, then accelerates in the latter half. By the time capacity has visibly declined, resistance may have already doubled or more. This makes internal resistance ideal for early warning rather than final judgment.

Voltage is the most intuitive parameter but also the most easily misread. Open-circuit voltage (OCV) reflects only the state of charge (SOC) — how much energy remains — not how much the battery can actually deliver. A 12V lead-acid battery at 12.6V is fully charged; at 12.0V it is about 75% SOC; at 11.4V about 25% SOC; at 10.5V it is nearly depleted. But SOC and SOH are not the same thing — a battery at high SOC with high internal resistance will sag below operating voltage the moment load is applied, and the equipment will shut down regardless. Voltage must always be interpreted together with load, through load testing or capacity testing, never through open-circuit voltage alone.

When to Use Capacity Testing vs. Internal Resistance Testing

Different applications demand different testing approaches, and choosing the wrong method wastes time and can miss real problems. Capacity testing (full discharge) is the gold standard for determining battery health, but it has two prerequisites: it requires fully discharging the battery, which means taking an online system offline; and it takes a long time — a 100Ah battery discharged at 5A needs 20 hours. Capacity testing is appropriate for: new battery acceptance, annual maintenance surveys, end-of-life determination before battery replacement, and deep evaluation during fault analysis.

Internal resistance testing is fast, non-destructive, and can be online. Measurement takes only seconds, requires no removal of the battery from service, and needs no discharge load. This makes it ideal for routine inspections: monthly battery checks at telecom base stations, quarterly internal resistance surveys of UPS battery banks, and periodic screening of data center backup batteries. By comparing internal resistance among individual cells within a battery string, weak cells can be identified — if one cell shows more than 30% higher resistance than the group average, it warrants attention even if its absolute value is still within spec.

An intermediate approach is the load voltage test: applying a short-duration, moderate-current load (typically tens of amps for a few seconds to tens of seconds) and measuring the voltage sag. This is more informative than open-circuit voltage but much faster than full capacity testing, making it suitable for field assessment of whether a battery can survive the momentary current surge when equipment starts up. The PK59A storage battery tester supports this load testing mode — no battery removal and no extended discharge required.

PK59A: Integrating Three Parameters into a Single Handheld Device

Traditional battery testing used three separate devices: a voltmeter for voltage, a discharge instrument for capacity, and an internal resistance meter for resistance. The problems are obvious — multiple devices, complex wiring, uncorrelated data, and results that are hard to synthesize. Field engineers had to write down three separate readings, then enter them into a system back at the office. Any error in that chain — transcription, data entry, or interpretation — meant the data did not align.

The PK59A integrates capacity, voltage, and internal resistance measurement into one handheld device. Its core measurement approach uses a four-wire (Kelvin) connection for internal resistance measurement, eliminating the influence of test lead resistance. A built-in standard discharge circuit enables short-duration discharge verification of capacity when needed. Voltage is sampled through a high-precision ADC. All three parameters are displayed on the same screen, results are automatically correlated, and no manual transcription is needed.

In practice, the PK59A adds value through finer details. It supports multiple battery chemistries — lead-acid, lithium-ion, nickel-metal hydride — all measurable with one instrument, eliminating the need for chemistry-specific tools. It stores test results for comparison against historical data. Its display gives a direct verdict — PASS / WARNING / FAIL — rather than just raw readings, letting field staff assess battery status at a glance. It also features automatic temperature compensation, correcting readings in high- or low-temperature environments so ambient conditions do not distort the results.

Four Common Field Testing Mistakes

In field battery testing practice, several recurring mistakes cause unnecessary losses. Understanding them prevents real problems. The first mistake is measuring voltage without internal resistance. As discussed, voltage reflects only SOC, not actual battery health. Maintenance personnel accustomed to trusting voltage — thinking normal voltage means a good battery — continue operating with batteries whose internal resistance is already very high, until the equipment fails at a critical moment.

The second mistake is ignoring battery string consistency. A battery string is only as good as its weakest cell. If internal resistance varies significantly among cells, the string will sag below cutoff voltage during high-current discharge due to the barrel effect, even if every individual cell is within spec on absolute resistance. When testing a string, always measure both the aggregate and each individual cell, paying close attention to cells whose resistance deviates more than 30% from the group average.

The third mistake is failing to record and analyze data. Many field staff complete a test, read the numbers, and leave — the data never enters the maintenance system. By the next test cycle, there is no way to know what changed in the interim. The correct approach is to record internal resistance and voltage after each test, building a time-series health record for each cell. Once a cell shows an accelerating resistance rise, replacement should be scheduled before it fails.

The fourth mistake is overlooking ambient temperature effects. Battery internal resistance and capacity are both strongly temperature-dependent. For lead-acid batteries, every 10°C rise reduces internal resistance by roughly 30% and increases capacity by about 10%; the reverse happens at low temperature. Without temperature compensation, resistance readings taken in different seasons are not comparable. The PK59A temperature compensation feature exists specifically to solve this problem.

Practical Advice for Maintenance Teams and Purchasers

Comprehensive battery maintenance goes beyond simply measuring voltage. A complete testing workflow should include: acceptance testing of new batteries (confirming initial SOH at 100%), periodic internal resistance inspections during service (quarterly for general use, monthly for telecom base stations), annual capacity spot-checks (fully discharging 20% of cells), and enhanced inspections in areas with frequent power outages.

When selecting testing equipment, price alone should not be the deciding factor. A single integrated device measuring capacity, voltage, and internal resistance is cheaper, more reliable, and far less time-consuming than three single-function devices. The core value of a device like the PK59A is not just measurement accuracy — it is the ability to complete a full battery assessment in five minutes instead of two hours of wiring, discharging, and data transcription.

Purchasers should also pay attention to details easily overlooked in marketing materials: whether the device supports data export for integration with the maintenance management system; whether the battery clips accommodate different terminal types found in the field; whether online (no-remove) testing is supported; and whether warranty and calibration services are adequate — testing instruments themselves require periodic calibration. These details determine whether the device actually gets used or ends up forgotten in a drawer. View the Battery Test Instruments Collection