Prius Hybrid Battery Recondition Guide

I am writing this to document my entire process of rebuilding the hybrid battery from my 2011 Prius. All of this information is out there, but I found it difficult to find one complete source. At this point, you end up having to look at outdated forum posts and blogs. I hope that this will be a more complete documentation/guide of the process.

DISCLAIMER: HIGH VOLTAGE BAD. HIGH VOLTAGE KILL YOU. HIGH VOLTAGE NO FEEL GOOD

DISCLAIMER: THIS IS THE PROCESS I USED. IT WORKED FOR ME; HOWEVER, I DO NOT GUARANTEE ANY OF WHAT IS BELOW TO BE TRUE OR CORRECT.

Here is an outline of what I did:

1. I bought a 2011 Prius with nonfunctioning Hybrid system
2. I bought a Mini VCI cable downloaded Techstream
3. I ran a few tests on techstream to get an idea of the situation
4. I removed the hybrid battery
5. I took a part the hybrid battery
6. I cycled the modules
7. I charged the modules
8. I load tested the modules
9. I paired the modules to form blocks
10. I put the battery back together
11. I installed the battery

At this point, the hybrid system should normally be fixed. But I chose to ignore an issue I should have looked into while the battery was out.

1. I replaced the Hybrid Battery Voltage Sensor and battery wiring harness

1. Bought the Prius

a. I was in need of a better commuter car and like a project, so I had been looking for a Prius with a bad battery for a while. Ever since I read a Jalopnik article about buying Priuses super cheap and fixing them for super cheap, I have always wanted to do this.

b. The Prius I bought was a 2011. It had a bad hybrid battery. From the mismatched white paint and not-so-perfect panel gaps, it’s obviously been in a few fender-benders, but it has 150k on the clock which is almost brand new in my mind.

c. The previous owner said it sat for a couple months, and when they went to drive it, the hybrid system was gone. This may or may not have been the case; I’ll go into this later. There was also a slight smell. I thought it was mildew. It wasn’t.

2. Buying MVCI Cable and Getting Techstream to work

a. I bought a MVCI cable with a Techstream CD off of amazon. I’m purposely not linking it because they all pretty much work, and I’m sure as soon as I link it, the amazon listing will go away.

b. The cable I got came with a Techstream v15. I couldn’t get really get this to work. From forums, I found a v12 download that works, but you just have to set it to Europe. I was able to do this on a Windows 10/64 bit laptop without trying anything special.

c. For the MVCI driver, I used the Autokent Driver Installer that came on the CD with my cable. To run the installer, turn off all of your malware/virus protection and run the installer as an admin. Once it has installed, run the program as an admin as well. It says to download the 1.4.3 driver, but if you do this, I found you will not be able to utilize the customize feature on Techstream. Instead, I downloaded the 2.0.1 driver, and it works fine.

d. As far as getting Techstream to work, there is an exact process you need to do. The MVCI cable is essentially just an interpreter that acts as a medium between techstream and the car. The cheaper cables (all cables that aren’t >$500) only reset their serial connection when they are turned on. This means the order you turn things on and plug things in is super important. The process goes as follows:

1. Shut down laptop
2. Plug MVCI cable into laptop
3. Turn on Laptop (This initializes the serial connection with the cable)
4. Turn on techstream
5. Plug Cable into car
6. Turn car on
7. Connect to car

3. Running Tests on Techstream

a. Once I got the car hooked up to techstream, I tried to do a general health check on the car. This took a while and Techstream crashed.

b. After this, I just did a health check on the hybrid system. The health Check showed a P0AA6, P0B47, P3020, C1259, C1310, B1504, U0155.

1. P0AA6 means there is a short between one/ or some of the modules.
2. P0B47 means that one of the battery voltage circuits, c, is lower in voltage
3. P3020 means block 10 becomes weak
4. C1259 and C1310 have to do with traction control and I assumed this would go away once the battery was fixed. They did.
5. B1504 and U0155 I kind of just ignored. I assumed they would go away once the battery was fixed. They did.

The P0AA6 code was the first sign of my Voltage Sensor/harness needing to be replaced, but I got so caught up in fixing the battery, I did not take the time to logically think out the codes. Normally a P0AA6 is really bad and means a module is leaking electrolyte causing a short. I chose blissful ignorance rather than thinking about this too much.


c. I also decided to record the battery voltages while driving. To do this I selected the Hybrid Battery pertinent data group and recorded a snapshot while for a minute or so. I then selected the battery block voltages (you can do only 8 t a time). I graphed them, then overlapped them.


e. The fact that Blocks 1 and 2 go to zero a lot means they are shorting out. Yet I still chose to hope that fixing the battery would fix this. It did not, but more on that later.

f. Also at this point, I was feeling confident. I ran some battery test on Techstream and promptly fried my 12 volt auxiliary battery. Apparently one of the many warnings I clicked through said it was supposed to be on a charger or what not. So just a WARNING DON’T BE STUPID ON TECHSTREAM, IT CAN REALLY MESS YOUR CAR UP


4. Removing the Hybrid Battery

a. To remove and take a part the battery, I followed this video. Pretty straightforward, I don’t think I need to elaborate.

b.

5. Disassembling the Battery

a. REMOVE HIGH VOLTAGE BATTERY DISCONNECT

b. I used the above video to take a part the battery

c. Disconnect 12 V auxiliary battery before removing battery and REMOVE HIGH VOLTAGE BATTERY DISCONNECT

d. Take lots of pictures.

e. CHECK THE VOLTAGE SENSOR HARNESS PLUG. Mine was burnt up. If yours is burnt up you’ll need a new harness and maybe voltage sensor. More on this later.

f. As I was disassembling it, it became clear someone else had worked on it before. The modules were numbered. One of the nuts holding on a bus bar was completely loose. This sort of scared me, but also I was in so deep at this point I didn’t really care.

g. Once I removed the modules in the compression holder from the rest of the battery and took off all the wires/buss bars, I began charging/discharging the modules.

h. REMOVE HIGH VOLTAGE BATTERY DISCONNECT



CHARGING THE BATTERY
6. Background

a. The Prius hybrid battery is made up of 14 blocks wired in series. In each block, there are two modules in series.

b. The blocks are numbered 1-14. Block 1 is the block that has the negative terminal of the 14 blocks. In gen 3 Priuses, block 1 is closest to the ECU, in gen 2, block 1 is the furthest from the ECU.

c. The purpose of cycling the modules by discharging/charging them is to normalize their capacities as well as raise their working capacities. In each module, there are 6 battery cells in series. As the modules work over time, the 6 cells can charge/discharge in slightly different amounts. This means some of the cells will have higher voltages than these others. When cells in series are charged, they stop charging as soon as one of the cells is fully charged. This means if one cell is greatly out of balance with the others, it will drastically decrease the capacity of the module. By cycling the modules, you are balancing out the cells. When the modules are charging, the cells that get fully charged first get rid of excess charge as heat. Each cycle brings the cells closer together. Because heat is a byproduct of the cycle, it is important to manage heat well as you charge the batteries.

d. As the modules charge, they can swell. YOU DO NOT WANT THEM TO SWELL. ONLY EVER CHARGE THE MODULES WHEN THEY ARE BEING CLAMPED TOGETHER. YOU WILL RUIN A MODULE IF IT IS NOT PROPERLY SECURED WHILE CHARGING. I charged the modules in the holder-thing that they are in inside the car. This worked OK. I saw some minor swelling but that seems to be normal. The swelling went down after the cells cooled down. You want to have a fan blowing over the pack as it charges. Also, never charge cells that are close together for the sake of heat management.

e. I first went through and numbered/measured the voltage of each module as they were when I took it out of the car. You need a multimeter that can read at least 2 decimal places. I made a spreadsheet and put these measurements in it. Right away I noticed that the two modules that makeup block one were around 5.5V. The rest of the modules ranged from 7.0V-7.8V. This spread would normally be pretty bad, but the Prius had sat for a couple of months before I bought it, so I didn’t worry about it too much.

f. Warning: Any time a number, voltage, or current is given in this section it is what I used. It seems like every forum thread recommends different charge/discharge rates. I found a lot of support for the numbers I used, but they definitely are on the higher end of the spectrum. If you think about it, it probably is safer to use small charge/discharge rates. But I was on a time crunch and chose to be ignorant of the more responsible way to do it. I believe my process worked fine, but only time will tell. If I were to do it again, I’ll use the same numbers.

g. Before this point, I also had cleaned the copper bus bars. I explain this below.

7. Cycling the Modules

a. To restore the modules, you have to cycle them through charge/discharge cycles. Most people recommend 3 cycles. I chose to cycle the batteries 3 times, charging at 5A and discharging at 2.5A to 6.0V. I set a current cutoff at 8500mAh.

b. I ordered a SKYRC Ultimate Duo 400W 20A AC/DC Balance Charger/Discharger.

c. SKYRC Ultimate Duo 400W 20A AC/DC Balance Charger/Discharger/Power Supply (US Plug)

d. I could not find any reviews of this charger on Prius forums, but I could not find up-to-date reviews of any of the chargers the old posts recommended. All the chargers are probably made in the exact same factory and seem to update every two years or so, so I assumed this one should work.

e. There was a slight learning curve to this one. First, do not even try to use the computer program for the charger. It does not work well at all.

f. My process for cycling the batteries is as follows:

i. Navigate to battery memory, Make a new battery that is NIMH, 6S, discharge to 1V/cell, charge cutoff at 8500mAh. Charge cutoff has to be this high because any lower and the batteries will not have enough time to hit a deltaV and stop charging. Once these are entered save the settings. Charge capacity may be under general settings. Also make sure the DeltaV setting is as low as possible.

ii. Once saved, go back to the battery selection under the memory, and hold enter on the battery profile made above. This should take you to the charging menu. Make sure the Discharge current is set to 2.5A. Make sure the discharge voltage is 6V.

iii. Navigate to the C/D screen. You want to make sure you are charging then discharging (C/D). This charger only shows you the info from the last action it did. If you discharge then charge, then it will only show how much it charged it. You want to keep track of how many mAh are discharged as this is the true reflection of the module’s working capacity. Make sure it is set to cycle the modules 3 times. With this system, it will cycle the modules 3 times, then when it is done it will show the final discharge capacity. It would be nice if it showed all three discharge/charge capacities, but oh well.

iv. If the charger flashes something like “OVER CAPACITY LIMIT” or something like that, it means you need to raise your charge capacity cut off.

v. Record the final discharge voltage in a spread sheet

vi. With this system, you will have to go back and charge each module once you have cycled each one as it discharges them last, but that is no issue.

g. I wired alligator clips to the connectors for the charger to make it easier to do the process.

h. Start charging at 1 and 15 or something like that. Because I knew 1 and 2 were bad, I skipped them. This is just to make sure you heat does not become a problem from charging modules close to each other.

i. After cycling all the modules, most of my modules were around 5500 mAh. There were 6 around 4500 mAh, and the two I had already decided were bad. I found a good deal for 5 used modules on eBay. Make sure you buy from a reputable dealer that shows the module voltages. You do not want to buy a module that is lower than 7.6V. Try and buy the newest possible modules you can so 2010-2015.

j. I replaced the 5 worst modules with the ones I bought. I cycled these modules as well.

k. For the modules that were still lower than 5500 mAh, I kept cycling them till they got closer to the average. Once I was done, the lowest module was at 5400 mAh working capacity and I was good with that.


8. Final Charging the Modules

a. To charge all the modules, I set the charger to charge/discharge. I kept the charge settings the same, and then set the discharge voltage to 8.5V. By discharging the modules to the same voltage, it makes it easier to balance all the voltages.

9. Balancing the Modules

a. Once all the modules were charged, you need to balance the voltages. When you put the battery back in the car, you want the module voltages to be as close as possible. I think I read within 0.02 V (but I’m not sure).

b. To balance the voltages, I took the modules out and flipped every other one so that all the negative terminals were on one side and positives on the other. Once they were arranged like this, I tightened the battery cradle and wired the modules in parallel. To do this, connected every negative terminal with the bus bars from the wiring harness. They should overlap; you want every negative terminal connected to every other terminal. To connect all the positive terminals, I stripped a solid core, heavy-gauge wire, and laid it on all the terminals then snugged the wire down with the nuts. You want this to sit for a day so the voltages can normalize.


10. Load Testing the Modules

a. I read a lot of different opinions on how to order the modules in the pack once they are charged. Some say by internal resistance, some say pair the lowest capacity with the highest, and some say you want to normalize the voltage drop under load of each block. The last one made the most sense to me so that’s how I decided to do it.

b. To load test the modules, I attached a headlight bulb to the positive and negative leads of my multimeter. This allowed me to touch the leads to the terminals of each modules while also completing a circuit between the modules and the light.

c. When you do this, the voltage should settle after a second or so. Record this voltage for each module. I used a spreadsheet to calculate the voltage drop (voltage after balancing-load voltage). I then paired the ones with the lowest drop to the ones with the highest. After this it worked out that my voltage drops for the 14 blocks were all within 0.01V.

d. At this point I checked to make sure the modules were still the same voltage. They were. If they weren’t, I would have let them balance a few more hours.


11. Reassembling Battery

a. Follow the second part of the video above.

b. To clean the copper bus bars, I popped them all out of the plastic harness. Be careful not to bend or mess up the voltage sensors leads.

c. Put the copper in a small container with a lid. Pour vinegar in the container so the bars are submerged. Put a decent amount of salt in there too. The vinegar and salt cause the Copper oxide to break down. Shake the container and let it sit for up to 30 minutes. This solution can eat copper pretty quick depending on certain factors so keep an eye on it and DO NOT FORGET ABOUT IT.

d. Drain the vinegar and rinse off the copper. Put a decent amount of baking soda and enough water to dissolve the baking soda in the container. Shake it around for 30 seconds or so. This is to neutralize the acid.

e. In a sink, I used a piece of scotch brite to clean the copper. The varnish and buildup came off super easily.

f. I used a tooth brush sized wire brush to go over the terminals on the modules when they were in the pack to clean off any oxidation or rust. IF YOU DO THIS, MAKE SURE YOU DO IT WITHOUT ANY OF THE MODULES WIRED TOGETHER. YOU COULD SHOCK YOURSELF OR HURT A MODULE.

g. With the battery back together, put it in the car.


12. Start the car

a. When I went to start the car, I had techstream ready to hook up.

b. My hybrid system still showed the triangle of death, so I had more work to do.

c. Normally, I think most Priuses would be good to go at this point.

13. Replacing Voltage Sensor and Harness

a. So all the things I ignored came back to bite me.

b. The Prius was throwing a P0AA6 and a P0B47. These were current and permanent codes. Permanent codes will go away after the car has warmed up and been driven 3 times without throwing the code. If you see a permanent code and it doesn’t say current/pending, you’re probably fine.

c. At this point, I thought back and the blocks 1 and 2 going to zero, the codes, the slightly burnt voltage sensor wire made me sure there was a short somewhere.

d. I took the battery cover off and did all safety precautions. I took out the Voltage Sensor ECU looking thing. The pins in the Sensor looked burnt. I took it a part and it had obviously been VERY hot at some point. Some of the pins were non existent.

f. I ordered a used sensor and wiring harness on eBay. When they came, I switched everything out, and it started flawlessly.

g. It is weird that this seems to have randomly burnt up, but after some research, it seems that these pins are susceptible to arcing if there is a good amount of dust or humidity in the case. I feel like Toyota should have potted this board, but oh well.

h. When I took apart the sensor, I also got an overwhelming whiff of burnt electronics that was very similar to the mildewy smell I thought the car had. I believe the previous owner let it sit for a while, and when they went to start it, enough humidity was in the car that the pins were able to arc and melted.


14. Things I would do differently

a. This is a long process. I probably would have tried to find a charger with more than 2 simultaneous chargers. But it wasn’t that bad. My electricity bill did jump a bit though!

 

I definitely went through the code explanation a little too quickly. Thank you for the clarification. I sort of intended on going back and making some of the sections a little more explicit and maybe creating a list of old forum posts as sources. Just did not have the time to go back and do that. When I do, I'll try and do that.
Calling this a guide was definitely a little presumptuous of me though for sure!