Short answer: before you buy a pack, do three things — confirm the car still enters EV mode, read the block voltages with a hybrid-capable scan tool, and check the battery cooling fan and the 12 V battery. A pack that holds EV mode and shows an even block spread is usually not the fault, whatever code is stored.
What the battery ECU is actually measuring
The ECU does not measure “battery health” in the abstract. It measures the voltage of each block under load and compares the spread. When one block sags while the rest hold, the ECU stops trusting the pack — that is what triggers P0A80 or P0A7F, not a single bad cell somewhere in the middle.
So the question worth asking is not “is the battery bad?” but “are the blocks still even?”
Test 1 — does the car still drive on electric power?
Warm the car up, find a quiet road and lift off the throttle at about 30 mph. A healthy pack takes over and the engine drops out. If EV mode never engages, or engages for a second and drops out, capacity has fallen far enough that the ECU is refusing to use the pack.
This is the single most useful check because it needs no tools and it tells you how far the pack has fallen.
Test 2 — read the block voltages
A hybrid-capable scan tool will show block voltages live. What you are looking for:
- Even spread at rest — the blocks should sit close together.
- Even spread under load — this is the one that matters. A pack can look fine parked and fall apart under acceleration.
- One or two outliers — if a couple of blocks drop away from the rest, those modules are done and the pack will follow.
Record the values before and after a road test. A screenshot of the freeze-frame data at the moment the code set is worth more than any description of symptoms.
Test 3 — the two free checks people skip
These cost nothing and fix a surprising number of “failed” packs:
- The HV battery cooling fan and filter. Toyota issued a service bulletin because dust and debris in the cooling fan set P0A80 and P0A7F on packs that were otherwise healthy. On a dusty car or one with high mileage, inspect this before anything else.
- The 12 V battery. A weak 12 V supply produces spurious module and communication codes that look like high-voltage faults. Load-test it first.
What the numbers mean
| What you see | What it usually means | What to do |
|---|---|---|
| EV mode works, blocks even, code stored | Stored history or an unrelated fault | Clear the code and monitor |
| EV mode drops out, blocks even | Capacity loss across the whole pack | Plan a replacement, not urgent |
| One or two blocks clearly lower | Module failure — the classic P0A80 pattern | Replace the pack (module-level repair rarely lasts) |
| Blocks even but the fan is blocked | Cooling fault, not a battery fault | Clean the fan, clear the code, retest |
| Voltage sag under load with even blocks at rest | High-resistance joints | Check busbar torque and terminals before buying a pack |
When a replacement is the honest answer
If the pack no longer holds EV mode, and the block spread widens under load, and the cooling fan is clean — the pack is finished. That is normal: a Ni-MH pack in daily use reaches end of life at 20–30% capacity loss, and no amount of driving style brings it back.
A replacement pack is a direct swap: 14 to 40 modules depending on the model, 100.8 V to 288 V, and no coding afterwards. Current prices by pack size are here.
Frequently asked questions
Can a dealer test my battery for free?
Some will read the codes for a diagnostic fee. Ask for the block voltage data, not just the code — the code tells you what the ECU concluded, the block data tells you why.
My scanner only shows P0A80. Is that enough?
It tells you the ECU has condemned the pack. It does not tell you whether the cause is a weak module or a blocked cooling fan, which is why the fan check comes first.
Will a battery reconditioning service fix this?
Reconditioning can temporarily even out a pack that has drifted. It does not restore cells that have lost capacity, so the effect fades — often within months.
