State of Charge

[Piusmilitis]

I was wondering, how does the prius monitor the state of charge of the hybrid battery pack? Does it monitor the voltage of the whole system, individual cells, or does it track the current drawn from the battery?

Thax

[carz89]

Excellent question! I don't know exactly how the Prius does it, so I did a quick Google search and discovered there are many methods, some more accurate than others. One method, developed about 10 years ago, uses an artificial neural network monitoring many input parameters over time.

I don't know if this is the method Toyota uses, but it's good reading anyway: IEEE Xplore - Login

Another good technical article: http://www.iop.org/EJ/article/0957-0...6-16b2f01f7656

More good reading specifically about the Insight & Prius batteries (don't know which model): http://www.99mpg.com/resources/artic...rypacksexpose/

Hopefully someone will post the exact answer on this thread.

[Danny Hamilton]

Originally Posted by Piusmilitis

I was wondering, how does the prius monitor the state of charge of the hybrid battery pack? Does it monitor the voltage of the whole system, individual cells, or does it track the current drawn from the battery?

Thax

Don't know how reliable the source is, but I read this previously here at PriusChat:

Originally Posted by miscrms

The Prius' battery controller functions as a Coloumb counter, meaning it derives SOC by measuring how much charge comes in and out of the battery not by voltage. There is a good reason for this, as voltage is a very poor indicator of NimH SOC. The upshot is, you can park a big battery in parallel all day, but the HV controller won't use any of the extra charge available from it. It already "knows" how big the battery is supposed to be, and only lets out as much current as that battery can handle and then tells the HV controller to put it back.

Fortunately it was discovered that there is a built in "recalibration" routine that kicks in at 242V. The OEM battery is nominally 201.6V (1.2V/cell*168) but at full charge its more like 1.4V/cell or 235V. At 242V it assumes things have drifted over time (since coulomb counting is sort of a "dead reckoning" approach) and it revises its SOC value upwards to compensate.

Nimh HV battery and discharge 40 amps


I also believe that I've read that the Prius monitors the voltage in each 7.2 volt pack to make sure that they don't drift too far apart from each other. Since I can't find the link for that at the moment, I'm less confident about it.

[David Beale]

The Prius tracks the power into and out of the battery. Voltage is not an accurate way to determine state of charge. It varies a lot just from temperature changes.

Now, I'm sure just tracking the power into and out of the battery would eventually get you lost on where you are re SOC, so I suspect there is another "calibration" method used. For example, at almost exactly 80% SOC NiMH batteries really start to heat up. The Prius system may use that to -confirm- its' at 80% SOC, once in a while. It does have temp. sensors in the battery pack.

I would guess the method Danny just posted is probably the one used. It would make more sense over time.

[miscrms]

Originally Posted by Danny Hamilton

Don't know how reliable the source is, but I read this previously here at PriusChat:

I'd consider it pretty reliable

here is one page that touches on the subject. There is a lot more info on how the whole thing works buried in the pages on that site, but its tough to point to any one spot for a concise description.
State Of Charge Drift - EAA-PHEV

The BMS does monitor the voltage of each pair of modules, as well as the overall voltage of the pack and has several temperature sensors. The individual voltage measurements are mostly used to monitor the health of the pack as it ages. When these voltages get too far apart from one another it indicates that the pack is getting out of balance and the weakest modules are starting to fail. The temperature sensors are used to monitor for potential failures, as well as factored into the safe charge and discharge currents (CCL/CDL) based on ambient conditions. This is part of what is coming into play at cold temps for example, when people can't get into EV mode. They may have enough SOC, but the temps are out of the safe range and the BMS limits currents to protect the battery. The primary pack voltage is used to look for gross SOC errors and trigger recalibration if things get too far off one way or the other. There are current sensors on the HV lines leaving the battery that are integrated to determine how much charge is going into and out of the battery. This is the primary means of determining SOC. There are also current sensors at the MGs, and maybe in the inverter. The main controller monitors all of these currents and makes sure they all add up to zero at all times. If the sum is off by as much as a few milliamps a possible short is suspected and the HV relays opened disabling the whole Hybrid system.

At least thats the understanding I have come to based on quite a bit of research toward a future PHEV conversion.

Rob

[Piusmilitis]

Now is the SOC determined by the battery computer and reported to the car computer, or by the car computer itself? Does anyone know if anyone has measured and mapped the signals between the Hybrid battery ecm and the cars ecm?

[miscrms]

Originally Posted by Piusmilitis

Now is the SOC determined by the battery computer and reported to the car computer, or by the car computer itself? Does anyone know if anyone has measured and mapped the signals between the Hybrid battery ecm and the cars ecm?

The plug in hybrid crowd have figured a lot of this out. My understanding gets fuzzier as you move away from the battery though. As I recall there are 3 main computers. There is the HV Battery ECU (BMS), the ICE Engine ECU, and the main HV controller. The BMS determines the current SOC, and sets the CDL and CCL current limits. The HV ECU is responsible for determining the most efficient way to use the ICE and Electric motors based on inputs from the driver as well as secondary controllers (Cruise Control, ABS, VSC, TC etc). I believe the Engine ECU is responsible for managing the details of the ICE to give the HV controller the power it requests. I believe the HV ECU manages the electric motors and inverters directly, enforcing RPM limits etc. All this communication is done over the car's CAN bus.

Here are some more details on the bits of the CAN traffic that have been figured out:

The new 2004 Toyota Prius : My CAN Project
Instructions for Capturing Data from the Serial Port of CAN-View
Formatted parameters
Prius PHEV TechInfo - EAA-PHEV

At this point there is a good enough understanding that people are using the data in several ways. The CAN-View product linked above monitors the CAN bus and can display graphs and values of lots of interesting parameters in real time. They can also use these values to perform simple logic operations and turn relays on and off to control the cal-cars plug-in conversion. The extended Scan Gauge can view many of the same parameters but text/values only. The cal-cars folks are now working on a board that can not only listen to the CAN bus, but broadcast on it as well. In this way they are able to listen and hear the real battery SOC, and quickly rebroadcast a "spoofed" value to trick the HV controller into doing what they want. Fortunately it seems to only care about the most recent value received so it ignores the real SOC as long as the spoofed one follows directly after. A number of the commercial converters have a complete enough understanding that they are replacing the OEM Battery ECU all together. Most of them do not share many of their details though, Davide from Hybrids Plus in CO being a notable exception.

What hasn't been done to my knowledge is any real work on replacing or modifying the main HV controller. I think the complexity and potential risks of seriously damaging the car or causing an accident have kept people out of that for now. One big lure though, is that we are pretty sure Toyota was able to get their plugin testers to cruise EV up to ~60mph strictly by modifying the software RPM limits in the HV ECU. Supposedly the 6k rpm limit is pretty conservative, and Toyota has done enough testing since to be confident that 10k is safe.

Rob

[Piusmilitis]

thats roughly what I expected. Making an entire stand in for the battery computer a little out of reach for a couple micro controllers and garage tinkerer. Oh how nice it would be if some one with a 'Linux' mentality autopsied the HV controller and BMS and posted open source solutions.

Anyway, lots of good info here. So although a complete stand in BMS might be a little far fetched, some simple logic might be able to get your own signals in there to say tell the HV controller that the batter is at full charge, or low charge regardless of how many amp hours have been drawn. Something that would come in handy if you where to want to add battery capacity.

Not that I am going to do that, I don' even own a prius, but I am fascinated by them and considering one, and as a type of thought experiment have always wondered how hard it would be to either build a stand in battery or add battery capacity to the prius.

I just sold my last project car, a SRT-4, and to be honest I think I've outgrown the fascination of getting an engine to cram and burn more and more fuel and am more and more warming up to tinkering with electric drive vehicles. Not that this would be something I'd do, but there are so many interesting things people seem to be doing with these cars that I believe I could handle.

Thanks for the info