How To Read A Battery Health Report Without Getting Fooled

A battery health percentage on its own tells you almost nothing. It is a single number produced by a piece of software that was not designed to be read by a buyer, and it can be moved several points by nothing more than the temperature of the pack when the test was run.

The Four Numbers That Matter

Every electric car we list carries a full pack report rather than a headline figure. Four readings on that report do the real work, and once you know what they are you can read any report in about two minutes.

• State of health, measured at a stated pack temperature rather than whatever the weather was doing
• Cell balance, expressed as the spread in millivolts between the strongest and weakest module
• Usable capacity in kilowatt hours, not the headline capacity printed in the brochure
• Rapid charge count, which tells you how the car has actually been used rather than how far it has gone

Why Temperature Changes The Answer

A pack tested at five degrees will read several points lower than the same pack tested at twenty five. Neither number is a lie, but only one of them is comparable to the car you looked at last week. If a report does not state the temperature, treat the percentage as decoration.

Cell Balance Is The Early Warning

Capacity fades slowly and predictably. Cell balance does not. A pack with a wide spread between its strongest and weakest module is the one that will develop a fault, and it will usually show in the balance figure long before it shows in the headline percentage.

Anything under thirty millivolts is healthy. Between thirty and sixty is worth a conversation. Above sixty we will tell you plainly, on the listing, before you ask.

What To Do With All This

Ask for the report before the test drive, not after. Check the temperature line first, the balance line second and the percentage last. If the seller cannot produce one, that in itself is the answer.