You have found a used MG EV that suits your budget and your daily drive. The seller is helpful, and offers to show you the state of health figure on the car's screen: a reassuring number that appears to settle the question of whether the battery is sound. Before you treat that figure as an answer, it is worth knowing what it represents, how the car produced it, and what a fuller battery data session would add.
This article is a practical checklist for reading that data with care: collecting a repeatable set of readings, noting the conditions they were taken under, and understanding the limits of any single number. It is not a pass-or-fail inspection, and nothing here certifies a battery.
What a state of health figure actually represents
State of health is usually an estimate produced by the battery management system rather than a direct measurement of remaining life. The system builds that estimate from information it has gathered over time, including charge throughput, estimates of internal resistance, and how the pack has behaved under load at different temperatures.
Because it is an estimate, it moves. A car that was fast-charged repeatedly in hot weather, or one that has spent long periods sitting at a high state of charge, can present a different figure from a similar car with a gentler history. Two used MG EVs of comparable age and mileage can therefore show different values without either of them being faulty.
The practical consequence is straightforward: treat one figure as a single data point rather than a verdict on the car. What matters more is whether the readings you collect are consistent with each other, whether they were taken under comparable conditions, and whether anything suggests a question worth putting to a qualified technician.
Charge and discharge history: the story behind the figures
Many battery scanners can display charge and discharge statistics, and these are often more informative than a single health percentage. They describe habits: how often the car was charged, whether charging sessions were usually completed or interrupted, and how much energy has passed through the pack over its working life.
Read them as history rather than as a score. A car used mainly for short urban journeys and topped up overnight will look different from one used for long motorway runs and rapid-charged mid-route. Neither pattern is automatically better, but the pattern helps you understand the context of everything else you collect.
Cars built in different model years, or sold in different markets, may present these statistics differently or not present them at all. If a category is missing, note the absence rather than filling the gap with an assumption.
Cell-level readings: the spread matters more than any one cell
Under the pack-level figures sit the individual cells. Cell-level data can be useful because a pack is limited by its weakest group, and a noticeable spread between the highest and lowest cell can be a reason to look closer.
It is equally important to know what cell data cannot tell you. A spread measured at one moment reflects the temperature, the state of charge and the load at that moment. Comparing a cold, partly charged pack with a warm, fully charged one produces an unhelpful comparison. Our guide to temperature and state of charge when reading EV battery data explains why context changes the picture, and our article on cell imbalance in EV battery data looks at how to interpret a spread without overreading it.
If a reading looks unusual, the sensible next step is to repeat the measurement under known conditions and then seek a professional assessment. Cell-level data is information for diagnosis, not a diagnosis by itself.
Stored fault codes and live data add context
A stored code is a record that the vehicle logged a condition, not an explanation of why it happened. Combined with live data — values that update while the car is running — codes help a technician understand what the system was seeing at the time.
Two habits make these records far more useful. First, save them before anything is cleared, because the conditions attached to a fault, such as the state of charge or the temperature when it was stored, are lost once the code is erased. Second, record the codes exactly as they appear, including any pending or historical status, rather than summarising them from memory.
Some stored entries relate to events resolved long ago, so treat history as a clue about the car's past rather than proof of a current fault.
Conditions that change what you see
Battery data is sensitive to the state the car is in, so a reading is only as useful as the notes that accompany it. Before you record anything, capture at least the following:
- the indicated state of charge at the start and the end of the session;
- the odometer reading;
- ambient temperature, and whether the car had just been driven or had been parked for hours;
- whether the car was recently charged, and by which method;
- any warning message shown on the instrument cluster or centre screen.
These notes turn a collection of numbers into a record that can be compared later. Without them, a reading taken on a cold morning and one taken after a long drive are indistinguishable, and any apparent change between the two is impossible to interpret.
What to record during the inspection
Set aside a proper session rather than a quick glance. Park on level ground in a safe, well-ventilated place, apply the parking brake and switch the car off as described in the handbook. Then work through the data in a fixed order, so that a second session can be compared with the first:
- identity of the car — model year, odometer and a reference you can match to the listing;
- date, time and place of the session;
- state of charge and ambient temperature;
- pack-level figures, including state of health and any energy or range estimates the car displays;
- cell-level readings, if available, including the highest and lowest values;
- stored codes, copied exactly, together with their status;
- anything you observed that the data does not capture, such as a warning message that appeared briefly or a change in how the car drove.
Keep the original files or screenshots alongside your notes. A photograph of the screen is far easier to trust later than a transcription, and a record of the conditions is what makes the figures meaningful to somebody else.
Where the data stops and a technician begins
Reading data is not the same as diagnosing a fault, and it is important not to let a tidy-looking report create false confidence. Battery data describes what the vehicle reported under the conditions of your session. It cannot certify that a pack is healthy, predict how it will age, or confirm that a car is a sound purchase.
Some situations sit clearly outside what a data session should attempt. Anything involving the high-voltage system, a damaged pack, impact damage underneath the car, or a warning that asks you to stop driving belongs with a qualified technician following the manufacturer's procedure. Do not attempt to open, remove or work on high-voltage components.
If the car you are considering shows something you cannot explain, the sensible route is a professional inspection before you commit, together with a written record of what was checked. Our battery data checklist for a used BYD EV follows the same approach for a different range, and is a useful comparison if you are weighing up more than one car.
Checking compatibility before you rely on a tool
Some of the data described above is visible in the car itself, but state of health, cell-level readings and stored codes generally need a suitable device or a workshop visit. If you plan to inspect a car yourself, check tool compatibility before you buy, because coverage in this area tends to be specific to particular vehicle ranges rather than universal.
The TopTekk TTB031 Power Battery Scanner for MG EV is a power battery scanner made for MG EV owners. It reads battery state of health, cell-level battery data, charge and discharge statistics, diagnostic trouble codes and live data, and it connects through the OBD2 port via Bluetooth. It is a one-time purchase with no monthly subscription and carries a one-year warranty.
Two boundaries are worth stating plainly. It is designed for the MG EV range, so check your exact vehicle against the published compatibility information before ordering rather than assuming coverage. And like any battery data tool, it reports what the vehicle provides: it does not certify a battery and it does not replace a qualified technician's assessment.
Frequently asked questions
Is a used MG EV with a low state of health reading automatically a bad buy?
No. State of health is an estimate influenced by temperature, state of charge and recent use. A lower reading is a reason to look more closely and to arrange a professional assessment, not a reason to reject the car on the spot.
Can I inspect a used MG EV battery without any tools?
Up to a point, yes. You can record the indicated state of charge, the odometer, any warning messages, charging behaviour and how the car drives. Reading state of health, cell-level data or stored codes needs a compatible device or a workshop visit.
Does reading battery data prove the battery is healthy?
No. Battery data describes what the vehicle reported during your session. It cannot certify the pack, predict future degradation or guarantee that the car is a sound purchase.