Update and technical details on my PHEV project

The shunt balancers I'm using are all safe and will never dissipate enough current for a fire. They simply allow top balancing. You are more likely to destroy an expensive pack if you run without BMS. Additionally, every cell string is monitored for voltage and temperature and will tell the BMS ECU if anything goes wrong. If the BMS ECU doesn't hear from all the modules constantly, then it will set a code, and also disable charge/discharge.

You can't use someone's fire as a reason not to use a BMS, that's just silly!

 

At some point I'll shoot some video, but I'm low on spare time right now.

 

My car was never equipped with Nav, and the Map button isn't available anywhere on the CAN bus, so I went "old school". Amazingly, there are a few spare unused conductors on the "clock spring" assembly that connects all the stuff in the wheel, so I just ran some wires and modified the button assembly.

The Engine ECM (ICE ECU) is what needs to basically be fed erroneous information to prevent ICE run, so it was the right choice for isolation. The Hybrid ECU is the "main brain" in the Prius and essentially calls all the shots, and sends and receives a lot more information than any other modules, so it wouldn't make sense to isolate it. (you'd have to pass-through 99%)

No, I don't think I want to tackle that job, that's the bulk of the Prius right there! I've replaced the Battery ECU with one of my own design. This wasn't that difficult, but a replacement Hybrid (HV) ECU would be a monumental undertaking.

When I get this all dialed in and have time to write it all up, I'll share the details. (there are many) But basically, the 0x038 frame is how the Hybrid ECU requests engine power from the Engine ECM, the 0x348 frame controls whether the ICE is allowed to run, warmup, and the RPM request, and the 0x3C8 frame is how the Engine ECU tells the Hybrid ECM that it wants to warm up. So if you intercept these frames and replace the appropriate values and bit flags, you can force the engine to either stay off, or spin w/o using any fuel for high speed EV-only mode. Right now I'm working on the right scaling to use as much of the battery as possible, but still have the ICE available if you need it. The Prius can do pure EV mode without using any fuel at highway speeds, but has very little left to accelerate or climb hills. (~20kw limit because of the boost converter) So the best way to do it is allow the engine to come on when you need the power, but otherwise have it using no fuel.

You also can inhibit the warmup, by blocking the warmup requests from the Engine ECM (0x3C8), but you don't want to allow a cold engine to try to deliver any power, so I let it warm up when the speed hits 35mph unless I'm making a short or low-speed trip.

 

The A123 cells have a nominal voltage of 3.3v, and I complete my top balance at about 260v.

I did some experimentation and found the Prius works quite well at around 240 volts, and doesn't seem to suffer any issues and can still do over 100A of regen, so that's what I chose. The stock pack's range is quite high, even on my low mileage 2008, I saw the voltage swing well over 260v on hard regen.

The boost converter is the limit on the Prius, but it's also got a current limit, so by having a higher voltage that insures that I get the most wattage through the converter, and the efficiency goes up with the smaller delta.

 

After settling several hours after the charge, my trips start out at about 255v and then slowly run down to 235v. I clock about ~25ah out (10% SOC) before I revert to standard Prius mode, and then I allow close to a kwh of pack use where I report 40-70% SOC. When I'm right off the charger, (260v & 100% SOC) my Batt ECU does not allow much regen, and forces EV mode until it's down to about 99.5% then it allows full regen.

If it's a short trip (and don't need heat), I just leave it in full EV mode until 10% SOC then it does a warmup and becomes a normal Prius. If I'm going farther, right now I simply report 75% SOC and leave it in Prius mode. This basically pulls a constant 20-40A off the battery to assist the ICE, and typically will get over 99.9mpg if you drive carefully (assuming highway). I still need to do some work to get a better "hybrid hybrid" mode, and allow use of the ICE in EV mode if I need more power. Right now it's all manual, which works, but not as cool as it would be if it was automatic.

 

I haven't really looked at a Gen3, but I have spent some time with a Hybrid Camry which I've been told is similar. In that car they have eliminated the separate Engine ECM and Hybrid ECU and instead have combined the functions into one ECU (Hybrid Vehicle ECU). Also, the inverter now has only digital signals in/out, there are no analog signals that run into the HV ECU as on the older systems. If that's the case, then my trick for getting high speed EV mode is not going to work, as you don't have the 2 separate ECUs to alter messages between.

They have also eliminated the Battery ECU and instead have a (similar) device, but it's called a Battery Monitor and does not sit on the CAN bus, but instead has a dedicated comms line right into the HV ECU. The Inverter also has dedicated digital comms lines, so they've eliminated the critical CAN bus messages I would imagine.

This also means that Gen3 may not be spewing all the cool high-rate passive data,which sucks for my instrumentation system.

On top of that, I don't like the Interior at all and the exterior styling looks too much like they hired Cadillac's stylists. hwell:

 

FYI Toyota calls the ICE ECU an "ECM" (Engine Control Module) and the "main brain" in the Gen2 is the Hybrid Vehicle ECU or HV ECU, but I guess hECU is fine.

The HV ECU makes all the decisions in the Prius, it's where the Accel pedal feeds into and it receives info from the Battery ECU on the CAN bus, and it talks to the ECM and tells it how much engine power it requires. It also listens to the ECM for warmup requests, etc.

 

That's the open-circuit voltage. It will indeed sag lower. I do not use voltage as any kind of SOC indicator, I count coulombs.

I didn't cut anything. There are 2 CAN bus bridge connectors in the Prius, so if you disconnect these, you can isolate every device on the bus. There is one located in the right side kick-panel that isolates the ECM. I simply pulled the 2 CAN H/L wires from this connector and ran them back to my new Battery ECU where it has it's own CAN port. The battery ECU receives frames from the HV ECU and either modifies them (Full EV) or sends them as-is (Normal mode). Additionally, it listens to the ECM and does the same thing in reverse for messages destined for the HV ECU. (i.e. Warmup requests)

What greatly helped my analysis of the CAN bus was to build a dual-port logging device. It hooks into any ECU between it and the CAN bus and passes all frames as-is through the device, but it allows me to see "who said what" and having that info makes it much easier to reverse-engineer the messages.

If I didn't already have the battery ECU in back, I simply would have put a device right up front so I didn't have to run the extra shielded twisted-pair.

The 2 CAN bus junction connectors are of great value for learning! 2 points where you have access to every individual device on the bus.

 

Response from another thread:

Thanks, Yes I know the PHEV stuff (especially done as I did) has a limited market, but a Monitor (Priuscan) has a much wider market and can benefit more people. Plus, you pretty much need it (or something similar) if you have a PHEV.

I too am using an old Brusa NLG4 that's been modified. It puts out 3.5kw @ 240vac for a ~2 hour charge and can still plug into 120 for the slow charge. Brusa makes great stuff, too bad it's so expensive.

I agree on the large-format lithiums. The reason I used A123's is because I got them from another project. Thundersky's have come a long way! I think I'd use the 40ah as well.