You are looking at a used BYD with a few years of daily driving behind it. The asking price looks sensible, the bodywork is tidy, and the seller says the battery still feels like new. What you need is not a friendly assurance but a written record of what the car's own battery data showed on the day you inspected it.
This checklist is not a pass-or-fail test and it will not certify anything. It is a way to collect battery data in a repeatable order, note the conditions the readings were taken under, and leave with something you can compare later.
What battery data can and cannot tell a used EV buyer
On a used electric car, the traction battery is the most expensive single component and the one you can influence least after purchase. That is why buyers reach for numbers: a state of health figure, a cell voltage spread, a charging history. Those numbers are useful, but they describe conditions at a moment in time rather than the remaining life of the pack.
A state of health value is usually an estimate produced by the vehicle's battery management system from charge throughput, internal resistance estimates and observed behaviour. It is influenced by temperature, recent driving, recent charging and the state of charge at the time. Two cars of similar age and mileage can therefore show different figures without either being faulty.
The practical rule is that one reading is a data point, not a verdict. What matters is whether the readings are consistent, whether they were taken under comparable conditions, and whether anything suggests a question worth putting to a qualified technician.
Prepare the vehicle and the working conditions first
Most confusing battery data comes from an inspection that was rushed. Park the car on level ground in a well-ventilated area, away from traffic, with the parking brake applied and the car switched off. Note the odometer reading and the indicated state of charge before you touch a tool, then record the ambient temperature and whether the car had recently been driven or charged. Both change what the readings will look like.
Read the vehicle handbook before you start, and follow the instructions supplied with whichever diagnostic device you intend to use. If any high-voltage or battery warning message is displayed on the dashboard, or if the car reports a charging-related fault, stop and let a qualified technician look at it. Traction battery packs operate at high voltage, and nothing in a buyer's inspection is worth opening, probing or dismantling high-voltage components. This checklist is limited to reading data the vehicle already exposes and writing down what you see.
The used BYD battery data checklist
Work through the following in order, writing down each item as you go: later readings only make sense against the conditions you recorded first.
1. State of health, with its context
Start with the state of health figure if the vehicle or the tool exposes one. Then record the conditions beside it: state of charge, ambient temperature, whether the car had recently been charged, and whether it had recently been driven. A state of health figure without that context cannot be compared with anything, including other cars you are looking at.
2. Cell-level battery data
Cell-level readings show how individual groups inside the pack are behaving relative to each other. Record the highest and lowest values you can see and the difference between them, together with the temperature and state of charge at the time. Cell readings drift apart or together depending on those conditions, so a spread that looks alarming at one state of charge may look ordinary at another. If the difference is large, note it precisely and treat it as a question for a qualified technician rather than a conclusion.
3. Charge and discharge statistics
Look for cumulative counters, session counts and any record of high-power charging events. Read these as a description of how the car has been used, not as evidence of damage. A car that has been rapid-charged often is not automatically a bad purchase, and a car with few rapid charges is not automatically a good one. What the statistics give you is a basis for asking the seller a specific question instead of a vague one.
4. Stored fault codes and live data
Read the stored diagnostic trouble codes for the battery and high-voltage systems and record every code exactly, including any described as historic or intermittent. A stored code is a clue about something the car logged at some point; it is not proof that a fault is present now. Clearing codes tells you nothing useful about a used car and destroys the record you came to collect. Where the tool supports live data, note which parameters were available and what values appeared at a steady state, such as while parked with the ignition on and no load applied.
Making the record worth keeping
A used EV inspection is most valuable when the same things are recorded in the same way each time. Use a single page with fixed fields: date, ambient temperature, state of charge, odometer, whether the car had been driven, the state of health figure, the recorded cell values, the codes found, any warning messages displayed, and the tool or app used. Add photographs of the values on screen if you can, because a photograph of a reading is far harder to dispute later than a sentence written from memory.
Keep the raw values and the conditions side by side. If you later look at a second car, or at the same car again, the conditions tell you whether the two sets of numbers belong in the same conversation. This is the same discipline that makes a workshop's own records useful, and it is worth reading more about what a buyer can learn from used EV battery data before you start comparing cars, and about why temperature and state of charge matter when reading battery data.
Finally, separate what you observed from what you concluded. A note saying that the state of health reading was taken at a known state of charge after an overnight rest is an observation. A note saying the battery is fine is a conclusion, and it is not something a single inspection can support.
Where a battery scanner fits, and where it does not
Some of the data described above can be read through a vehicle-specific battery scanner rather than through the car's own display. For owners and buyers working with selected BYD electric vehicles, the TopTekk TTB030 Power Battery Scanner is designed for that job: it reads battery state of health, cell-level battery data, charge and discharge statistics, stored diagnostic trouble codes and live data, and connects through the OBD2 port over Bluetooth. The BYD models listed by the store are the Atto 3, Dolphin, Seal and Sealion 6, and the device is a one-time purchase with no monthly subscription and a one-year warranty.
Two qualifications matter as much as the capabilities. First, it is not a universal tool: it is intended for selected BYD EVs, so you must verify your exact vehicle against the published compatibility information before ordering. Second, a scanner reads data; it does not certify a battery, predict its remaining life or replace an inspection by a qualified technician.
Common mistakes when reading used EV battery data
Comparing readings taken under different conditions. A figure captured at a high state of charge on a warm afternoon is not comparable with one captured at a low state of charge on a cold morning.
Treating one state of health number as a purchase decision. The number is an estimate generated under specific conditions. Use it alongside cell data, stored codes, usage history and a physical inspection.
Reading a stored code as a confirmed failure. Stored codes describe what the vehicle logged, sometimes long ago. Have any code that concerns the battery or high-voltage system assessed properly.
Clearing codes to tidy up before a sale or a purchase. Clearing removes the evidence a buyer or a technician would want to see, and it does not fix anything.
Assuming coverage extends to every model year. Vehicle coverage is specific. Check the published compatibility list for your exact car before relying on any tool.
Frequently asked questions
Is a used 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 single low reading is a reason to look more closely and to have a qualified technician assess the car, not a reason to reject it on the spot.
Can I check a used BYD battery without any tools?
You can record a great deal without tools by noting the indicated state of charge, the odometer, any warning messages and how the car behaves on a test drive. Reading state of health, cell-level data or stored codes requires a suitable device or a workshop visit.
Will reading battery data guarantee the battery is healthy?
No. Battery data shows what the vehicle reported under the conditions of your inspection. It cannot certify the pack, predict future degradation or substitute for a professional assessment of a car you are considering buying.