DIY plug-in conversion Gen1 Prius

welcome to priuschat, looks like a tough nut to crack, all the best!

 

Welcome to the forum! You may also want to investigate the YahooGroup, Prius Technical Stuff, as many of the technical issues have been addressed there over the years. Sad to say, the interface has become more difficult a couple of years ago.

What I will do is BOLD your questions and insert my insights in normal font:

I saw many people criticizing Gen1 (NHW11, European spec) for its PHEV conversion abilities in favour of Gen 2 and higher. Bus as an owner of Gen1 i'm committed to plug-in my 2000 Prius.

There are many conversion kits available on the market. But everything comes with its price. Thus DIY budget conversion seems to me a way to go.

The NHW20 and later models have an "EV enable" pin on the hybrid vehicle ECU, the primary control computer. This pin does not exist in our NHW11 so alternate ways have to be pursued. The existing Prius plug-in kits exploited this EV pin as well as defeating the 25 mph (40 km/h) EV speed limit and the 42 mph (67 km/h) hybrid-mode, speed limit. Above 42 mph, the engine will run even if no power is needed. For our NHW11, it will have to be DIY but that is not a bad thing.
. . .

Since i want to use DC-DC converter free system, to avoid losses on conversion, i will connect my add-on battery pack directly to HV traction battery cables. However, not to confuse hybrid control unit, the voltage from add-on battery should match the 273,6 Volt, right?

Early plug-in experiments failed when people used relays to 'dump in' the extra batteries in parallel to the existing traction battery. This would trigger numerous fail-safe systems as well as scrambling the battery controller. In contrast, a DC-DC converter simply adds power, often with a 'soft-start' so a voltage spike is not induced. There is also an AC signal on the battery power lines used for ground-fault detection. If either the B+ or B- ever gets enough conductivity to the car body, the ground-fault is detected and the battery controller puts the car in "safe" mode (i.e., shutdown.)

BTW, the traction battery voltage varies quite a bit not only by State of Charge but also the load. I've seen as low as ~250V and as high as 330V. Then the traction battery controller is designed to work with NiMH chemistry. If another battery chemistry is put in parallel, it is likely to suffer a fatal under or over charge. So the DC-DC converter usually incorporates safety ranges to protect the external batteries from damage. They also work in discharge-only mode from the extra batteries.
. . .
Normally, (correct me if i'm wrong) SoC working range is between 15% and 60%, so to which nominal voltage should i set my final add-on battery pack?

I would like to suggest you get a Prius-aware scanner that works with our ISO-9141/KWP, NHW11. In particular, look for one that has a data recording capability to monitor the traction battery voltages, current, and temperatures. This will give you accurate information and be critical for integration and test.

Factually, the SoC varies between 40-80% with 60% being the nominal middle. If the battery SoC is above 60%, the car will use the extra charge to offload the engine. If the battery SoC is too low, the car will start the engine to put a charge on the traction battery. Also, if the engine coolant temperature reaches 64C, it will start the engine to keep it warm enough for hybrid-mode, normally 70C, when the engine can turn off.

. . .
When the voltage is set, then comes the question of regenerative braking and charging by petrol engine. If I have my add-on pack directly connected to HV system, would it not overload the generator? Would it simply slower charge extra capacity battery pack?

There are half a dozen control computers operating our NHW11 but three actually handle driving power:

1. Hybrid Vehicle ECU (HV ECU) - this one operates MG1, MG2, and communicates with the engine and battery ECUs. The accelerator pedal pots connect to the HV ECU that then coordinates everything else that has to run.
2. Engine ECU - this is a normal engine controller that operates the fuel injectors, throttle plate, valve advance, spark plugs, and reads the engine sensors. It gets a 'requested power' signal from the HV ECU and operates the engine to deliver the requested power.
3. Battery ECU - more of a safety and monitoring system, it lets the HV ECU know if the traction battery SoC, temperatures, and ground fault.

The HV ECU operates the transmission and with the brake controller, never lets regeneration overcharge the traction battery. It smoothly coordinates using the hydraulic brake if more braking power is needed.

. . .

Making our NHW11 into a plug-in faces a number of technical challenges:

1. When parked or moving forward, the NHW11 will run the engine until the coolant reaches 70C and the HV ECU goes into 'hybrid-mode' that allows the engine to stop.
2. When moving in reverse, the engine will normally stop even if the engine is cooler than 70C.
3. If the car is shifted into "N", the HV ECU will not operate MG1 and MG2. If the engine is off, it will remain off. If the engine is running, it will remain running.
4. When the car speed exceeds 42 mph (67 km/h), the engine will come on even if descending a hill in "D".

These control laws are built-in to the HV ECU. But there have been different approaches to defeat these limits:

1. Turn off the fuel pump - this prevents the engine from running so the HV ECU uses what remains of power from the traction battery. However, the check engine light comes on and there is a risk of over-spinning MG1. When the engine is off and the car is moving, MG1 will turn faster and faster. There is a rumor that there is a 6,000 rpm limit (?) to MG1 rotor speed and 10,000 rpm on the NHW20. Some of us have 'tickled the tiger' and gone faster than 42 mph without MG1 flying apart (like in what a hand grenade does.) We do not recommend this experiment but wanted to let you know there is some pad.
2. Disable fuel injectors or spark plugs - same effect as above.

Let me suggest taking a step back from one solution, "plug-in," and look at an easier goal . . . reducing NHW11 fuel consumption. This can be accomplished by:

• Mastering the HV ECU control laws - knowing when the Prius has to run the engine and making sure it runs only when necessary for as little fuel as possible.
• Reducing aerodynamic drag - in cool weather, blocking the front bumper air inlet. But there are other enhancements IF high speed is important.
• Reducing rolling drag - tires, lubrication, and alignment.
• Warm-up - engine and transmission heater.
• Pre-charge - bringing the traction battery up to 75% before the car is started in the morning.

You may want to find the Prius shop manuals and electrical diagram. These are your roadmap to the car systems. But if you are committed to making it a plug-in, there are more extreme options:

1. Remove the engine and radiator and rebuild the transmission so MG1 and MG2 work in parallel. Replace the HV ECU and master the brake ECU interface for regeneration and safe braking.
2. Soft traction battery assist that adds power to the traction battery.

You might read the sticky about who should and should not own our 11+ year old cars. They can be rewarding or a nightmare depending upon what you want to do with the car.

GOOD LUCK!
Bob Wilson

 

Hi Bob, just a small thing -- it's probably just a test to see who's paying attention lol -- your MPH (km/h) conversions are off, e. g. 42 mph is 68 km/h. I'll give you your due though, it is consistent all through your post. Can you take a look and adjust. Thanks.

 

Sorry, long day and a lot of other draws on my time. I did a divide instead of a multiply by 1.6 and I tend to truncate instead of round. So my 42 mph * 1.6 = 67.2 giving 67 km/h instead of 68 km/h. But heck, what is 1 km/h between friends?

I also miswrote ISO-9141/KWP which is also fixed. Sad to say, my English wasn't a precise as I would normally prefer but let's see if the OP comes back.

Bob Wilson

 

der_Denis,
I like the idea of a plug-in Gen 1, I enjoy mine, and would like to convert it also.
I thought about a second battery in parallel matching the same voltage as the primary battery pack.
And also connecting the sense lines from the primary pack, to the secondary pack, essentially making it a battery pack with twice the energy, so it would be 4kw capacity instead of 2kw.
I purchased the wiring diagrams for The Gen 1, and was thinking of adding a switch to disable the start line. But am not sure where to add the switch, there a two lines that feed the start line, one is from the key, and other is from an ECU. This may be a good place to start experimenting, with EV mode. There is a black and white wire that goes to the stop light switch, and black and yellow from there to the ST- on the Hybrid Vehicle Control ECU.
Stop switch S3 is located near the brake pedal. This line shuts off the motor, but may also disable power to the drive motors. Another way to disable the gas engine may be from a red and yellow wire from Hybrid Vehicle Control ECU, to the Engine Control Module, it's is labeled ESTP. This might be the way to go. You can try this before adding the second battery. It is pin 8 on the HVC ECU to pin 16 on the ECM . It may need to be pulled up to activate the ESTP function. Please let me know what you find out experimenting.

 

Denis I'm glad to see your follow. Now to the details:

The first time is the hardest and the rest soon become boring. <GRINS>

Excellent! You are mapping out the control laws. This is the first step.

The additional battery pack needs to be managed too based upon the chemistry. This is a lot easier with a microcontroller operated DC-DC converter. But useful power ranges are a challenge and expensive. Efficiency is negotiable but some of the synchronous, DC-DC converters have efficiencies approaching 95%. So let me share some useful data: 2003 Prius

Thanks to Ken@Japan, we got Toyota's formula for NHW11 drag as a function of speed:

Use 745W/hp and your target top speed and that defines the DC-DC converter power limit. So if 45 mph (72 km/h) is the target, it would need no more than 10 * 0.745 kW = 7.45 kW as the highest DC-DC converter limit. In reality, the load will be shared with traction battery but now you've got a target.

Sorry, I have to run off to work but we'll discuss more later.

The scangauge is good but has only four displayed metrics and no data recording. A video capture somewhat mitigates the capture. I would recommend taking a look at the miniVCI because it has a data recording capability and larger number of metrics that can be recorded. The only problem is it really handles only one ECU at a time.

Let me suggest one plug-in approach is to keep the engine running at minimum fuel burn idle and use the plug-in, extra power for early motive power. Then when the engine is capable of 'hybrid mode', bring it back online. Speculation but it may be disconnecting the communications link between the HV and engine ECU after the engine starts may be all it takes. IF reconnecting allows the car to resume normal operation, problem solved. This is something I might test.

No problem. I wanted to share some of my earliest musings. There was a report of a first generation, Honda Insight owner who was adapting a Prius drivetrain to make it into an EV.

No problem but I do need to scamper into work.

Bob Wilson

 

Hi Denis,
I have a NHW11, 2001 Prius Gen 1, I was thinking of using another set of Prius batteries, for the second set.
And just connecting it in parallel. I think getting the motor to stay off first, would be a step forward, before adding the second set of batteries.
Is it that much extra weight? It weighs around a hundred pounds, for 38 cells. Here is a diagram that I was looking at, that may be useful. I purchased the wiring diagrams off of eBay.

 

I'll chat about the battery approach later but want to address the chart. It has been awhile since I did the math converting SAE units to metric. An easy source:
Horsepower to kilowatts (kW) conversion calculator
1 hp = 0.745699872 kW

Now about the chart:

The important parts are:

• "190 + 4.2*(V**2)" is the drag force in Newtons and velocity in m/s
• "red" line is the required power, right-side HP, as a function of speed
• x-axis is the speed in SAE units, miles per hour

Reading the 45 mph (72 km/h) velocity, the required power on a flat, standard day would be 10 HP or 7.45 kW. This would be the power needed from the plug-in battery (PiB) system to the traction battery (TB) system to completely replace the vehicle drag energy. I choose 45 mph because it made the math easier. However, 40 mph (64 km/h) is close to 7.5 hp or ~5.6 kW, getting closer to 5 kW, one of the more readily power ranges for a DC-DC converter.

The reason this chart is so busy is I want to compare the theory versus trusted benchmarks. There are two other lines:

• "blue" - this is the vehicle miles per gallon, left y-axis, at an assumed 31% efficient engine and transmission. The 31% came from my metrics of NHW11 specific fuel consumption times my calculated transmission efficiency, a much longer subject than this line description. However, it does not include the vehicle electrical power overhead.
• "gray" - this is the "blue" line adjusted for a fixed ~450W electrical power overhead. This includes all of the ECUs and the transaxle coolant pump. So this is the line used to compare to the benchmarks.

About the PiB system, you've suggested going high-voltage series that the DC-DC converter reduces the voltage. But never forget that series batteries are sensitive to discharge limits:

• reverse charge - most commonly discussed, this destroys LiON and NiMH batteries 'instantly.'
• too low voltage - seldom discussed, if the voltage drops below this level, even if not reversed, the internal battery chemistry suffers a loss of capacity. This can be resolved, sometimes, by cycle charge-discharge often called 'conditioning.' Staying away from too low for any given cell becomes more complicated at the number of cells in series increases. The weakest cell defines the power limit of the total pack.

The other extreme, less risk to the cells, is to run them all in parallel. But at typical 3-3.7V of a LiON cell, the current for just 3kW would be 1,000 A and worse, the power loss is (I**2)R. So let me suggest something Tesla pioneered, a federation of PiB modules. I understand the Tesla patents are available and might give some insights to this design approach.

I've got to head into work. I'll try to follow-up with more suggestions later.

Bob Wilson

 

Denis,
I was planning on using the EV mode for a short period of time only, and at low speeds, due to the limitations of the Gen !.
What other diagrams do you need? I can scan and post them as needed. I have the 2001 Toyota Prius Wiring diagram manual.

 

Hi - this is Roy from Canada - Do you have the 2001 prius wiring manual in PDF form?

 

It sounds like you've got a good approach but a couple of humble suggestions:

• Inverting DC-DC converters have the advantage of a wide, inverted, buck-boost voltage range without their complexity. My understanding is they have similar efficiencies to a buck converter.
• Solid-state fuses avoid the arc risk of melting link fuses.
• One weakness of the Prius traction battery are the series, 38 modules where one failure spoils the series and it was never really designed for module replacement. Two or more parallel strings that could isolate a failed string and safe, field replacement of a weak cell or module should provide a 'fail soft' capability.

Also, I wanted to briefly cover engine warm-up modes:

• ~45-55 seconds O{2} sensor warm-up - during this time, car draws heavily on the traction battery. It tries to minimize drawing engine power. I've accelerated to 35-40 mph before the O{2} sensors start working and it starts drawing significant engine power. Then I shift into "N" and minimize part of the 5-6 minutes warm-up fuel burn.
• 70C engine coolant temperature is required before hybrid-mode can start. However, the engine coolant temperature can be spoofed after 40C to show 70C and not stall. This can save 2 minutes of the warm-up fuel burn.
• Engine power requests MAY use the direct communication lines between the HV and engine ECU. It may be that disabling this communications could let the engine run a minimum fuel burn, warm-up idle while the extra battery energy provides motive power.

All in all, I think you've got a good understanding. It is not the approach I am after but still good.

Bob Wilson

 

Roy, I have the Wiring Diagrams in a soft cover paper back manual, and can scan them as needed.