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Check engine light is on with code p3015 (battery cell 5). I took apart the entire thing and each and every cell tests fine at 8v.
What's the deal? Please help!
I've even tested the 12v and it reads at 12v exactly. The car runs fine, tho sometimes a sluggish start in drive after having gone in reverse. I don't understand why the check engine light is on and the reader is giving me this code if all cells and batteries read perfect.
 

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I have been told by a dealer employed certified Toyota hv battery technician that if you continue to operate your vehicle with a battery code that the battery will continue to get worse meaning that more battery modules will fail. I don't know how accurate that is. Because you have a first generation Prius (up to 2003), if it has a first generation battery then you probably should get it fixed or replaced by a reputable firm. A simple voltage test does not reliably indicate the health of a module.

The follow may be more information that you wanted to know.
IMO:
There are 38 modules in the 2001 hv (traction) battery.
The on-board diagnostics report on the hv module PAIR in question. The OBD2 code of P3015 probably means that the 15th pair from one end is weak, usually determined by the combined internal resistance of the pair. This is not easily measured manually but can be determined by a load test with both a voltmeter and and ammeter. I have read that you may also be able to read the pair resistance with certain OBD2 tools which can fully access the Toyota computer. The module pairs may be numbered from the end opposite the computer (in U.S. standard left-hand drive vehicles, that is the passenger side). This is the way the modules are numbered in the OBD for my Gen2 Prius.

Battery failure seems most often attributable to leaks of Voltage/current caused by some physical breakdown of an internal cell or what is called a cell reversal. A cell reversal is a state of not taking a charge or even taking a negative charge. This is thought to be caused by an extreme discharge which takes the cell below minimum voltage which could happen if the module has a manufacturing defect.

Just FYI, each module (which is half of the pair) has six (6) cells in series. (Individual cells cannot be accessed physically or electrically as long as the module retains its physical integrity. Some technicians have used FLIR infra-red technology to determine which cell is bad, but the bottom line is that the module cannot be repaired and reused reliably.)
 

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I am brand new to Prius having just bought a 2002 with a similar problem (as a project I guess). I can't advise you on the code but just wanted to point out that a 12V battery is only 25% charged when it reads 12V. Fully charged it will read 12.7V at rest. Like you I took the main battery out and all cells tested at 8.16 or 8.17 volts. I bought a cheap bluetooth code reader which didn't work for me so I am off to buy a real OBDii code reader in the morning. I suspect the HV computer module might be bad. The whole pack is reading 310V. A simple voltage reading is not a definitive test of course. I bought a reaktor rc charger balancer expecting to need to rebalance the cells. I'm not sure that is needed now...
 
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