2004 Prius HV battery SOC does not drop below 80%

I have a 2004 Prius, 165,000 miles. The battery needed "reconditioning". So, I took the battery cells and discharged them and charged them 3 times each. 2 amps discharge 2 amps charge. I replaced 3 of the 28 cells (Nimh). Then I balanced them by connecting all the positive terminals together and all the negative terminals together for 2 days resulting in a final voltage of 7.85 volts in each cell. This all took a month. The battery pack is now back in the Prius. The car runs fine. No error codes. But the SOC indicator of the car ranges from 80% to 100% all the time. The SOC never drops below 80%. It used to drop down to about 40% before I "reconditioned" the cells. Any thoughts or help would be appreciated.

 

Thank you for the encouragement. Will the SOC begin to fall into a regular rhythm of 80% to 40% and back to 80%?

 

I am judging from the car energy scale where the top 3 bars are blank or all bars are full.

 

Thanks for the encouragement. Did your Prius's eventually fall into an 80% to 40% and then back to 80% routine?

 

Thanks for the explanation. I have not found that information anywhere else. So, if only the bottom bar is lit the SoC is at 40% and if all the bars are lit the SoC is 80%. Is that correct? The graphic on the MFD cannot show a SoC of 100%. Is that correct?

 

I have done some research and experimentation to resolve some of the Prius SOC questions I have been asking. I got the Hybrid Assistant app for free and bought ($4.99) the Torque Pro app. Both gave the same SOC results. In order to be less stupid with the questions I was asking I looked up the meaning of the acronyms MFD, ECU, and SOC and found Multifunction Display, Electronic Control Unit and State of Charge. The MFD 8 bar graphic was displaying 6 to 8 bars lit all the time with the reconditioned battery. I interpreted this to mean the battery was holding between 80% and 100% of its charge all the time. I now know this to be very wrong. The highest reported SOC with both Hybrid Assistant and Torque Pro with all 8 bars lit was 75%. The lowest SOC reported was 50% with 4 bars lit. On average the SOC varies between 55% and 65%. Going down a long hill (several miles) will push the SOC to 75% and the 8 bar graphic will light all 8 bars. What is not cool is that when you are charging the battery the 8 bar graphic doesn’t follow the rising SOC. That is the number of bars lit does not increase as quickly as the reported SOC goes up. And when you are discharging the battery the number of bars does not decrease as quickly as the SOC goes down. So, you can have 50% SOC with both 5 bars and 6 bars lit. You can also have 7 bars lit with 75% SOC and 65% SOC. Both of the apps I used worked well but the display options of
Torque Pro I liked better. I will make another post of the battery “reconditioning“ methods I used for those interested.

 

If you want to recondition your hybrid HV battery you have to understand that the entire process will take about a month. There are 28 battery cells to be worked on and each will take about a day to discharge and charge 3 times.

The first thing you have to do is remove your HV battery from the car and disassemble it. See the following YouTube video. (6) Hybrid Battery Repair Ultimate Guide (Toyota Prius 2nd Gen 2004-2009) DIY - YouTube

Next you have to number each cell for identification. Then use a volt meter to record the initial voltage of each cell. Cells which are out of range should be replaced. This means if most all the cells have a voltage of 7.71V to 7.76V and one has a voltage of 6.47V that cell should be replaced. That is 16% below average. I got my new cells here Toyota Prius Camry Hybrid Battery Cells 7.7V & UP Nimh HV TESTED 2004-2009 USED | eBay, They were better than any of the cells in my battery pack.

Next you have to discharge and charge each cell 3 times. This will take the better part of a day for each one. Get the following charger for this. SkyRC IMAX B6 V2 60W DC Balance Charger GENUINE RC LiPo Battery RC 688977150128 | eBay The charger will charge at 2amps but will not discharge at 2amps so get a 2.5ohm to 3ohm 20watt resistor and some wire and alligator clips to do the discharging. The resistor will get very, very hot, like 160 degrees F, so don’t lay it on something that will burn. Let it hang in the air by the wires. The charger you bought will discharge at .5 amps and will shut off when done but will take several days to do each one. The resistor will do it in about 1 ½ hours. You have to watch the discharging process and stop it when the cell reaches about 6.4 volts. Below 6.4 volts the voltages drops pretty fast and you don’t want the cell to go below 1 volt or it will be harmed. The cells will expand during this process, not good. Place each cell between two very strong boards held together with clamps (strong clamps) and when the process is done hold the cells clamped until the cell cools off. You need two sets of clamps. If during this process any of the cell discharge in a very short time in relation to all the other cells replace it.

Next you have to load test each cell and replace any that fail. See (6) How To: Load Test Your Prius Battery - YouTube I used a 0.56ohm 30 watt resistor and held it for 30 seconds.

Next you should re-peak each cell. Use the SkyRC charger. The instructions are included in the package. Use a re-peak number of 3. This will take 1 ½ hours each. Again, use the boards.

Next you have to balance all the cells to an identical voltage. See (6) How to balance the toyota hybrid battery - YouTube This takes two days or until the voltage of all the cells hooked together stops changing.

You also have to clean the bus bars and bolts, 28 and 56 respectively.

Finally, you are pretty much done. Reassemble the battery pack and reinstall it in your car. See (6) Hybrid Battery Repair Ultimate Guide (Toyota Prius 2nd Gen 2004-2009) DIY - YouTube Some places say to torque the battery nuts to 48 inch pounds some say 65 inch pounds I used 55 inch pounds. Get this 1/4 in. Drive Click Type Torque Wrench (harborfreight.com) mine cost $20.

All the ECU data that your car remembered about your driving habits and the condition of your battery will have been erased, so you will have to drive a couple of hundred miles for your car to generate new information for it to run as you hoped it would. Good luck!