'07 Prius won't start - suspect Hybrid battery contactor (relay) may be at fault

My 2007 Prius had been running fine until just recently, despite the fact that is has nearly 250,000 miles on it. Suddenly, one day, the red triangle warning showed up and my OBD2 code reader said the Hybrid Battery was bad. After watching a few YouTube videos about Hybrid battery diagnosis, I decided to order a Panlong OBD2 bluetooth scanner to use with my Android cell phone with the Torque Pro diagnostic app. I wanted to see whether the battery really needed to be replaced, or if it had just one or two bad blocks that could be replaced.

After setting up and running Torque Pro, I was very surprised at the results. I had set the app up to show voltage readings on all 14 blocks, as well as HV State of Charge. I had read that blocks showing voltages between 14 and 16 could be considered good. What I found was that only block 5, at 13.9 volts, was lower than the others. Numbers 1 and six showed 15.0 volts, and all the rest showed 15.1 volts. State of Charge showed 0.0, which is about what I expected since the engine would not start and the dashboard view of the hybrid battery shows zero bars. Since the voltage readings were better than what I expected, and with the one block being just a hair below normal, it seems strange that the battery would not take a charge and leads me to believe something else is at fault. If the hybrid battery had been going weak, it seems that my miles-per-gallon would have noticeably diminished, but it didn't. My mpg has remained up around 47 for at least the past 2 or 3 years, and continued that way right up until this problem occurred.

Just before the hybrid battery lost all of its charge, I noticed that I could hear a couple of loud clicks, in rapid succession, when I pushed the Start button. To me, that sounded like a large relay attempting to engage, but then failing and dropping the contact. I know that there are three relays for the hybrid battery, and that these are referred to as "contactors." Here's a photo showing these relays. One is a "Pre-charge contactor," and is used in conjunction with the auxiliary battery to supply an initial amount of power to the system so as to prevent a huge surge and otherwise resultant arcing when full power comes on. That's why that relay has a ballast resistor, and also is said to utilize a capacitor. The other two relays connect the positive and negative hybrid battery cables to the inverter. Thus, if any one of the three relays is not operating correctly then that would explain why the hybrid battery failed to stay charged.


Of course the proper operation of at least the "Pre-charge contactor" depends upon the charge of the auxiliary battery being sufficient to activate the relay, so I checked the aux battery (Exide Edge AGM type, 440 CCA) with a load tester. It checked out good, and at 12.6 volts. I figured it would because it was replaced just 2 years ago. If I press the Start button with the key fob in and my foot on the brake pedal, I can hear the relay clicking as if attempting to engage, but nothing else happens.

While I have easy access to the hybrid battery now, I will pull the disconnect plug and remove the battery cover so that I can inspect the bus bars and clean them. Then I'll see what's up with the one module block that is slightly on the low side from normal voltage to determine if that should be replaced. Then I'll need a suitable charger to use in bringing the hybrid batter up to a sufficient state of charge so that I can do further testing and adjustments. So that's the plan, and I'd welcome the thoughts and suggestions of others who may have had a similar problem and cured it without having to replace the entire hybrid battery.

 

Thanks for the diagram, and sharing your knowledge. The code reader showed P0A80. Yes, it is quite likely that the relay clicks seem loud simply because I had already removed the rear seat backs and access panels so that I could get at the hybrid battery. Later today I removed the battery cover to take a look at the bus bars, and found that they could use a good cleaning, so I'll work on that tomorrow before doing anything else.

 

@ TMR-JWAP: You said, "Your car has likely detected (and you even verified) that you have a single cell failure in block 5.

Fix the obvious first before you go digging yourself into a rabbit hole. There are likely other weak modules, but more in-depth testing is needed to determine those."

Yes, I had first studied the composition and specs for the hybrid battery, so had assumed (after seeing the Torque Pro results) that the low module could possibly have a single cell failure. That makes perfect sense. After removing all the copper bus bar plates for cleaning, I took voltage readings for all 28 modules with my multimeter, and naturally I tested block 5 modules # 9 and 10 first to see what the problem was. Module 9 proved to be the bad boy, with a reading of 6.36 volts. Numbers 2 and 11 were the next lowest readings, at 7.52 and 7.55 respectively. All others were at, slightly under, or slightly over 7.60 volts. Overall, I felt that the battery modules were remarkably well balanced for a 16-year-old Prius with nearly a quarter million miles on it. If the voltage readings of the modules are summed together, that comes out to 211.63 volts. I'll attach an image showing the individual module results.


You had also stated, in a second reply, "Also, it is highly unlikely the battery is actually at a zero SOC. It's much more likely to be a false indication, either due torque settigns, the safety disconnect not having step 3 performed for the installation (pushing handle down to satisfy the interlock), or perhaps damage at the voltage sensing harness ecu socket."

I too feel it is unlikely that the SOC could have been at absolute zero, though with the last run of the engine - before the no start problem - I had started the engine and saw that the battery monitor showed that the battery was charging normally so I decided to take a short test drive on the road. After driving only about 1,000 feet, the battery monitor showed that the charge level was continuously dropping, so I turned around and drove back home. The car died about half way into my driveway, showing no bars on the battery monitor, and would not start again. The safety disconnect plug was definitely installed correctly before I used the Torque Pro app for diagnosis, and before running the app I had checked the connectors to the control unit. No damage or corrosion. Same for the fan connection. and the fan tested okay. I removed and cleaned the fan as well. I also checked all fuses related to the hybrid system (14, 22, 31, 33, 37, 49, and 50) and all were okay. Any further thoughts? With only one bad cell, and 167 good ones reading over 211 volts total, it seems that charging and discharging would have continued normally unless there is a fault in the charging circuitry. I will order three replacement modules having at least 7.6 volts each, as that will bring the battery modules back into excellent balance again, but will have to give some thought as to what is causing the no charge / no start problem. I had wondered if any of the 3 relays are malfunctioning, but if that's not where the trouble lies - as Mr Chapman suggests is probably the case - then I suppose looking into the Inverter / Converter unit should be the next step once the replacement modules are in and the cleaned bus bar elements are reinstalled and torqued properly. If something else would be a more logical approach then please feel free to offer that advice. Thank you.

 

@TMR-JWAP: You said, "Unfortunately, I'm starting to lose track of your initial symptoms. Which of these are initial symptoms and which occurred after you started working on the battery?

Suddenly, one day, the red triangle warning showed up and my OBD2 code reader said the Hybrid Battery was bad.

The code reader showed P0A80

though with the last run of the engine - before the no start problem - I had started the engine and saw that the battery monitor showed that the battery was charging normally so I decided to take a short test drive on the road. After driving only about 1,000 feet, the battery monitor showed that the charge level was continuously dropping, so I turned around and drove back home. The car died about half way into my driveway, showing no bars on the battery monitor, and would not start again. The safety disconnect plug was definitely installed correctly before I used the Torque Pro app for diagnosis, and before running the app I had checked the connectors to the control unit. No damage or corrosion."

The RTOD was the first thing noticed, and I was headed home at the time (about a 30 mile trip from where I was when the RTOD showed). I switched to the energy monitor screen and could see that the hybrid battery was not charging normally. It appeared to be getting worse as I drove further, in that the blue bars became green bars, and there were less bars. When I came to a slight hill right after a stoplight, The Prius was barely moving and I wondered if I would make it home, but once the road leveled off things became a bit better and I was able to limp home. As soon as I pulled into the driveway, I hooked up my OBD2 code reader, and it showed the POA80 code.

At that point, I decided to rent a vehicle for one week while we searched for another car. I didn't have time to mess with the Prius because my wife needed a car for driving 40 miles to work and back, and we were due to move out of the home we were renting in just 2 weeks, moving back to our rebuilt home which had been burned to the ground 10 months earlier. I needed to take the Prius to the garage of our rebuilt home (the garage had not burned, thank goodness), and since my daughter was coming for a visit I had her follow me while I drove the Prius. That was the day after the RTOD incident. Strangely enough, the Prius ran just fine that day, and showed that it was charging normally, until I was about 3 miles from my 30 mile destination, and then the bars started going green again and diminishing. I made it to my garage and parked the car inside, where it remained by its lonesome until about 3 weeks later, before I had any time to spend on it.

In thinking about why the Prius might have gone haywire just 3 miles before my destination, I wondered if it was because I was passing over a rough road at that time. It seemed logical that the rough ride over that stretch of road could have caused an electrical connector fault if the connector was loose or corroded. In searching the Internet, I found that the hybrid battery fan connector, as well as the battery ECU connector, were prone to corrosion problems due to dampness from a leaking hatchback seal. So, when I could find the time, I checked out the fan and fan connector, and all was well there. The ECU connector was also good, and so were all hybrid related fuses, so that shot down my hopes of a simple and easy fix. That's when I decided to get the bluetooth OBD2 reader and the Torque Pro app. And just after taking the voltage readings on the 14 blocks, I attempted to start the Prius and found it appearing to be charging properly again. That's when I gave it the short road test, and headed back home very quickly when I saw the charge level dropping off rapidly. That was just a few days ago, and the Prius hadn't started again after that, so that's when I decided to remove the battery cover and inspect the bus bars. Naturally, the copper contact bars looked mostly black after all those miles, with a bit of green or blue here and there, so it made sense to remove and clean all of them, which I have done. My last effort was in obtaining the individual module voltages to determine which one, or ones, I would need to replace. While I only plan to replace #9, if 2 and 11 turn out to be okay, I have ordered two extras just in case they are needed now or in the not too distant future.

 

@TMR-JWAP: That's correct - no other codes. I used a wired scan tool for reading the trouble code, before I obtained the bluetooth unit and the Torque app, and I did not use Torque to read trouble codes - only for the battery block voltages. I had also downloaded the Dr Prius app to test charge/discharge and battery life expectancy, but of course didn't get to do that because of the no-start issue. I did have Techstream installed on a laptop computer using a USB interface, but lost the interface and the laptop computer in my house fire. Surprisingly, I was able to salvage the laptop's 1TB SSD which has Techstream on it because when the folding table the laptop was on collapsed, the laptop fell straight down to the floor, thus protecting the drive. I prefer using a laptop for diagnosis, so will see if I can get Techstream working on my new laptop.

 

Yes, I'm familiar with this method of load testing after watching just about every YouTube video there is concerning Prius hybrid battery testing and servicing. I'll definitely work up a spreadsheet showing my results for each module after doing 3 to 5 charge/discharge cycles. Being retired, and somewhat situated in the new home now, I can find the time to check all of the modules. I agree, of course, that doing anything further until getting the battery squared away would be premature and pointless. I also agree that module #9's dead (or nearly dead) cell was probably enough to cause the sometimes charging/sometimes not malady, and as it became worse could have taken out the entire battery. With the cells and modules all connected in series, it would be much like what used to happen with series connected Christmas tree light strings. If one light quit then all the lights went out. Finding the bad one then required starting at the first bulb and spending some time to look at each successive one for a broken filament.

 

Just waiting for ordered items to arrive, which could be either tomorrow or Saturday. I'll remove the hybrid battery from the car and disassemble it this afternoon so as to be ready for what comes next. Yesterday was too darned cold to do any work. It actually snowed at one point during the day, and the overnight temperature was 36 degrees. Today is much better, at 54 degrees currently.

 

I was surprised to find a module in my mailbox today. This one measures 8 volts and appears to be in good shape. Clean housing and no swelling. What say you? As all my good modules are at over 7.5 volts already, should I try to bring all of them up to 8 volts as a first step, or would it be better to connect all modules (except for #9, of course) in parallel for a couple of days to let them all even out first?