Battery repair

What those codes are telling you

• Maximum state of charge reduced — often single bad cells, or loose connections between them. The cause is usually hard to find.
• Internal isolation fault, with insulation below 190 kOhm.
• Communication trouble inside the battery management system.
• A high-voltage interlock loop fault, which frequently means water has got in.
• State-of-charge imbalance, limiting maximum cell voltage.
• Cell voltage sensor faults, including high impedance on the sense lines.
• Module voltage outside its permitted range.
• Cell overvoltage, often caused by failed sense wires.

Copper on aluminium

Failed voltage taps — the sense wires — are typical, and the reason is in the construction: copper wires mounted on an aluminium plate. That pairing corrodes, and corrosion costs contact. Isolation faults are just as common and nearly always trace back to moisture that has been getting in slowly for years.

What an overhaul consists of

• The exact state of the pack is established before anything is replaced.
• Failed modules are replaced or repaired.
• Every corroded voltage tap is replaced, and the sound ones are protected against corroding later.
• The pressure equalisation valves are renewed, as they are in a resealing, so moisture does not simply come back.
• Every battery monitoring board is tested individually and repaired where needed. These boards watch their module's cell voltages, temperature and the rest of the safety-relevant parameters.
• The battery management system is checked as a whole before the pack goes back in the car.

Not only the fault that brought it in

A pack opened for one fault gets the faults it has not reported yet fixed at the same time. It is on the bench once; the second visit for a sense wire that was already corroding is avoidable, and avoiding it is the point of opening the pack rather than swapping it.