Vehicle battery problems often aren’t just about low charge. Most of the time, something inside the battery is wearing out or failing. That’s why knowing how battery testing works helps you spot problems faster, especially when you already know your vehicles and how they’re used.
This guide explains the main types of battery testing, what each one does, how it works and why it matters in the workshop.
The most common types of automotive battery testing
These are the most common battery tests you’ll use, and what each one tells you:
Conductance testing (measures internal resistance)
Conductance testing sends a small AC signal at a single frequency through the battery and measures how it reacts. It’s really checking how hard it is for electricity to flow inside. This is called internal resistance. If resistance is high, the battery isn’t working as well.
Conductance is the opposite of internal resistance. In simple terms, the test checks how easily electricity can flow through the battery. Many testers show the results as internal resistance because it’s easier to understand the battery’s condition that way.
That’s because sulphation – a natural byproduct of using a battery – starts to form on the plates. This material blocks some of the chemical reactions that generate charge. That makes it harder for the battery to deliver power.
A high resistance often means more sulphation. But it’s not the only cause.
There’s also grid corrosion. The lead grids that hold the active material on the plates slowly corrode. This is most likely in high temperatures or when the battery has been overcharged. That damage reduces conductivity and increases internal resistance.
Then there’s active material shedding. During regular cycling, some of the material on the plates breaks loose and settles at the bottom of the battery. It’s even worse if your vehicle regularly experiences heavy vibrations. That leaves less surface area to support chemical reactions, pushing resistance even higher.
And that’s not even to mention electrolyte dry-out, acid stratification, plate growth, separator wear or the effects of temperature. Yep, there’s a lot at play here!
Conductance testing is great for checking cold cranking amps (CCA) in lead-acid batteries. It sends a small current at a single frequency (around 90 Hz) through the battery. At that rate, electrical noise (capacitive and inductive) levels out, so the meter gets a clean read on internal resistance.
Important to know: The battery’s temperature could skew your results here. However, most advanced modern testers will compensate for this, so it shouldn’t make too much difference. If in doubt, wait a little while for the battery to cool.
Load testing (simulates operational conditions)
Load testing puts the battery under load, just like starting the engine or running the heater and lights. The tester applies a controlled load and checks how well the voltage holds up.
A sharp voltage drop indicates internal issues such as weak plates, failing cells or reduced capacity. That means it can’t supply the power needed under heavy load.
Because load testing replicates actual operating stress, it reveals problems that other tests might miss. For instance, a battery might look fine at rest but fail when the driver turns the key. It’s extremely valuable for confirming whether a battery is fit for service or needs to be replaced.
However, load testing is more invasive than methods like conductance testing. If you do it too often or do it wrong, it can speed up battery wear. For that reason, it’s best used selectively, usually as a follow-up test when other readings suggest a possible problem.
You’ll use a special tester, like the CPX or DPA, that applies a fixed load for about 10–15 seconds. While it runs, you (or the machine) watch the voltage. If it drops below the limit, the battery is weak or failing.
Impedance testing (measures for early signs of degradation)
Impedance testing works by sending a small alternating current (AC) signal through the battery. It’s the same principle as conductance testing but across a range of frequencies. You’re measuring the change in impedance across different frequencies. This gives a far more detailed picture of the battery’s internal resistance and capacitance.
The result is a graph or dataset called an ‘impedance spectrum’, which shows how the battery reacts across different frequencies. This can help catch wear before it becomes a real problem. It is one of the best tests for early warning signs. It can spot problems long before the battery’s performance drops. That gives you time to plan a replacement and avoid breakdowns.
Important to know: impedance testing is less common in basic workshop battery testers. Conductance testing is the automotive benchmark and pretty much always used for warranty.
Voltage measurement (assess SoC)
Voltage testing is one of the simplest ways to roughly estimate a battery’s state of charge (SoC). It involves checking the open-circuit voltage (OCV – the voltage when the battery isn’t under load). You then estimate the state of charge using standardised voltage-to-SoC curves developed for different battery types.
However, you’ll quickly realise that this alone doesn’t tell the whole story. For example, cold temperatures result in a lower voltage reading without the battery losing capacity. On the flip side, an older battery might show a normal voltage but not be able to deliver enough current under load.
You can use a basic multimeter or a battery tester for this. It’s quick and doesn’t stress the battery, making it a good first step in battery checks. However, check the temperature and battery type, and to factor these into your results. Also, remember that this isn’t an in-depth test.
How to interpret your battery tester results
Testers can show all kinds of numbers, graphs or charts. What they mean depends on the type of test and the model you’re using. To make sense of them, you need to understand the test and how the battery is used.
Some results are always a warning sign:
- High internal resistance means the battery probably needs reconditioning or replacing.
- Low voltage means the battery isn’t holding enough charge right now, but it’s not necessarily permanently damaged.
Reconditioning can help bring a weak battery back to better health. It usually involves controlled charging and discharging cycles designed to break down some of the harmful build-up inside the battery, like sulphation on the plates. Some more advanced chargers will have reconditioning modes.
However, this is where record-keeping comes into play. Although your vehicles are mass-produced and the models are probably used by hundreds of other fleets, you use them differently. For example, do your drivers make regular stops, such as for deliveries or to pick up people? Do they turn the engine off? What’s their average speed? Do they drive an HGV or coach on motorways for eight hours at a time? How many electronic components do they usually use or need?
All these things affect how hard your batteries work every day. Keep good records of each battery’s performance on each vehicle. That way, when you test batteries, you can compare current results to your historical data. This helps you decide the best time to maintain or replace batteries. Our ROBIS system does all this and more.
Calibrate your battery tester to maintain reliability
Like all electronic devices, testers rely on internal components (resistors, sensors, circuit boards etc.) that wear over time. These parts can drift slightly out of spec, even with normal use. When that happens, the tester might still work and might appear completely fine, but the results it gives you start to shift. And in battery testing, even a slight shift can mean a big difference in diagnosis.
That’s why regular calibration matters. Calibration adjusts the tester back to its original accuracy range using certified standards. Most manufacturers suggest doing this once a year, but it depends on how often you use it and where you keep it.
Calibration is a quick and straightforward process. You can either send the tester to our approved service centre or use a calibration tool if your device supports in-house adjustments. Some testers include calibration reminders as part of their software.
Don’t skip calibration. It might seem small, but a tester that isn’t calibrated can give wrong readings. This might make a good battery look bad or a failing battery seem okay. That leads to wrong decisions and could even risk your warranty.
Remember, your battery tester is there to help you stay on top of battery issues across your fleet. It makes sense to set aside a bit of budget to keep it performing at its best.
Otherwise, it won’t give you the reliable results you need, and that defeats the whole point of having it.
Get in touch with Rotronics for more about battery testers
Knowing how battery testing works helps fleet managers and technicians choose the right tools. It also makes it easier to understand what the results are really saying about battery condition. That means better decisions, more accurate diagnosis and more reliable vehicles.
At Rotronics, we offer battery solutions designed for fleet workshops. Our testers and chargers provide accurate, detailed data to help you maintain batteries and extend their life.
Want to discover more? Get in touch with our friendly team here at Rotronics. We’d be delighted to learn about your fleet or garage and to walk you through the best options for your specific needs, with no pressure to buy anything you don’t need. Reach out today, or explore our full range of battery testers here and see how we help keep fleets moving.
