Used Hybrid and EV Batteries: What 'Tested' Should Mean

Pack, module, cell — three different products

• Pack — the complete assembly with its housing, contactors, busbars, cooling and battery management system. What the vehicle actually has.
• Module — a group of cells in a sub-assembly. Replacing one module inside a pack is common on some platforms and impossible on others, depending on how the pack is built.
• Cell — the individual unit. Sold for rebuilds, rarely for direct repair.

Prices differ by an order of magnitude between these, so the first question on any battery enquiry is which one is actually being sold.

What a capacity test measures

Capacity is charge in and charge out, measured against the original rating. It is expressed as state of health. On a hybrid pack that started at 1.3 kWh, 85% state of health means roughly 1.1 kWh usable — which in practice means the engine cycles more often and fuel consumption rises before anything fails outright.

Capacity alone is not enough. A pack can hold 85% capacity and still be unusable, because one module is weak and drags the string down under load.

The four figures worth asking for

1. State of health, % — capacity against rated capacity, with the test method stated.
2. Module voltage spread, in millivolts — the difference between the strongest and weakest module at rest, and again under load. A healthy pack sits in the tens of millivolts. Several hundred millivolts across modules is a pack on its way out.
3. Internal resistance, in milliohms — predicts heat and voltage sag under acceleration. Rising resistance is the earliest measurable sign of ageing.
4. Temperature spread during charge — a module running hotter than its neighbours is failing, regardless of what its voltage says.

Any one of these alone can be flattering. Together they describe the pack honestly.

What "tested" should look like in writing

A statement, not an adjective:

• Test equipment and method used.
• State of health figure with the rated capacity it was measured against.
• Module spread at rest and under load.
• Donor vehicle, mileage, and why it was dismantled.
• Whether the pack has been stored at a partial state of charge, and at what temperature.
• Photographs of the pack, the terminals and the BMS label.

"Tested and working" with none of the above is not a test. It is a claim.

Storage and transport are part of condition

Lithium packs degrade while sitting. A pack stored for a year at full charge in a hot yard has lost capacity that no bench test will restore. Two packs with identical test figures can have very different futures depending on how they were held.

Transport is a separate problem: used lithium is class 9 dangerous goods, air freight is heavily restricted, state of charge limits apply, and damaged or swollen packs are refused by most carriers. A used pack is frequently sea freight only, with a dangerous goods declaration and a carrier agreed in advance. Plan for that before committing to the part.

When a used pack is the wrong answer

• The BMS is not included or cannot be paired to your vehicle.
• The pack is a different revision with different chemistry, even if the housing looks the same.
• The vehicle has a known failure mode in the cooling circuit that killed the first pack — replacing the battery without fixing the cause repeats the outcome.
• The pack is being bought to "reset" a fault that is actually a cell balancing or software issue.

A used pack is a good answer when it is the right revision, documented, tested with numbers, and the surrounding system has been checked. Otherwise it is an expensive way to move a fault from one component to another.