BYD Seal: 12-volt battery sulfation — the car goes completely dead with a full main battery

This is the fault that catches BYD Seal owners out, and it is worth understanding because it looks far worse than it is. The car becomes completely unresponsive — it cannot be unlocked or started — even though the high-voltage traction battery is perfectly charged. It typically happens after the car has sat for a few days, or while it is plugged in at home, which makes it feel random and alarming. The cause is the ordinary 12-volt lead-acid battery: the factory-fitted one is poor quality and suffers sulfation, made worse because the car's DC-DC converter does not charge it at a high enough voltage. In an electric car the 12-volt battery still runs the locks, the computers and the wake-up circuit, so when it dies the whole car dies with it regardless of how much energy is sitting in the main pack.

• Car completely unresponsive — will not unlock or start — with a charged main battery
• Failure appearing after the car has stood for a few days
• Failure appearing while the car is plugged in at home
• Screens or lights behaving oddly, or the car being slow to wake
• Warning messages about the 12-volt system

Ask the seller two direct questions first, because they are more revealing than anything you can test in one visit: has the car ever failed to wake up or unlock, and has the 12-volt battery been replaced. Owners who have had this remember it vividly, and a replaced 12-volt battery is a good sign rather than a bad one. Ask how long the car typically sits between drives, since standing is what brings it on. Then check what you can: unlock and lock the car several times and confirm it responds instantly each time, and note whether the screens wake promptly or take an unusual moment to come up. Work through the vehicle settings for any 12-volt system message. Look at the 12-volt battery itself where accessible and check whether it is the original or an obvious replacement, and look at its terminals for corrosion. If you can, come back a second time after the car has stood a day or two and see whether it wakes normally. If a workshop inspection is possible: ask for the 12-volt battery to be load-tested and the DC-DC charging voltage checked.

$300 to $1,000 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the car ever failed to unlock or wake up?”
• “Has the 12-volt battery been replaced, and when?”
• “How long does the car typically sit between drives?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.