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State Of Charge Manipulation


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State Of Charge (SOC) is the battery's level of charge, usually expressed as a percentage of full. It is similar to a fuel gauge, such that 25% SOC is 1/4 full. Depth Of Discharge (DOD) is another term used which describes how empty the battery is. DOD is the opposite of SOC. So, 25% SOC is equal to 75% DOD. Since in a Hybrid car the battery is recharged from gas, the SOC usually has a target of about 3/4 full. This is unlike a Battery electric vehicle (BEV) whose SOC would fall from full to empty untill refueled like a traditional gas car.

In a PHEV we usually want the SOC to act more like that of a BEV up to a point. We want a lower target SOC so that there is more room to recharge from offboard sources such as the AC power grid. Though it is also important not to reach too deep a DOD so that the car will still have electric assist power and to prevent problems with certain battery chemistries which can be damaged by deep discharges. SOC Manipulation or SOC W:Spoofing is possible by exploiting State Of Charge Drift.

[edit] Prius

Prius PHEV Implementations of SOC Manipulation.

[edit] Hybrid-Pack Method

The new Hybrid-Pack Method works much like the #Original CalCars Method although there is no longer a Battery Tap Emulator. Rather, the original Prius battery is charged from the new battery in such a way that its voltage is raised enough to cause State Of Charge Drift.

[edit] Contactor Based

A contactor (or contactor and resistor) is opened and closed with a duty cycle which depends on the SOC. The added battery parallels the original battery, and charges it simply as a consequence of the fact that its voltage is higher. See Prius PHEV Battery Options#Contactor Based for more battery pack details.

The contactors and resistor are driven in this manner to accomplish the desired SOC level of ~70-73%...

[edit] PFC Based

A PFC Charger float charges the original battery from the new battery with a target voltage below that which causes State Of Charge Drift, this voltage is raised to a point which will cause Drift based on SOC. See Prius PHEV Battery Options#PFC Based for more battery pack details.

The PFC-30 is controlled in this manner to accomplish the desired SOC level of ~70-73%...

[edit] Original CalCars Method

See also:

[edit] Charge Limit Conundrum

The CCL issue is being resolved via the Prius PHEV#Hybrid-Pack Method.

Per PriusPlus_Maillist:2006./4./0 Y!:561 & EAA-PHEV_Maillist:2006./4./45 Y!:174

A problem has arisen where EV-only mode is not always available. The problem traces back to the value of another CAN bus parameter emitted by the Battery ECU, "Charge Current Limit (CCL)". This parameter tells the hybrid system the maximum charge (regenerative braking) current that the hybrid battery is currently able to accept, and is the main way that the Battery ECU has of limiting excessive current into a mostly-charged battery. Atilla Vass' reverse-engineered Prius CAN message spreadsheet [1] lists this parameter as at ID 3CD, byte #0 (though it and "Discharge Current Limit, at byte #1, may be transposed).

  • Without building a replacement battery management system (BMS), we don't have an easy way of determining real SOC, so we would not necessarily know how to adjust perceived SOC relative to real SOC.
  • No matter what real SOC is, a major problem is that adjusting perceived SOC anywhere near 80% automatically causes CCL to decrease well below 50%, prohibiting EV operations and causing poor drivability.

[edit] SOC Spoofing

The Prius' battery management computer (BMS, called the Battery ECU) communicates to the main hybrid computers via the CAN bus. It indicates battery voltage, current, temperature, and its estimates of state-of-charge (SOC), and maximum allowable charge and discharge current.

The BMS' estimate of SOC is critical, as the hybrid controller keeps SOC within 40-80% (the lower and upper limits of the (nonlinear) display graph), and tries to keep it around 60%. When the SOC is above 60%, the hybrid controller works to discharge the battery by using battery power (and less gasoline) even during normal cruise. This increases to around 30A (~6kW) at 70% and above.

When the SOC is below 60%, the hybrid controller works to charge the battery by making the ICE work extra hard even during normal cruise. Below 40% SOC, stranger things happen and it is difficult to get the engine to put out much power at all.

Dan Kroushl did some experiments with higher voltage batteries that proved that the BMS's indicated SOC can be spoofed (Thanks, Dan!). That led Ron to do enough further experimentation to discover that it is definitely possible to do what is indicated in the above paragraph, and generally how to do it. However, the circuitry and programming to do so is still in development. It generally involves, as needed, providing a higher voltage to the BMS than the actual battery voltage.

Toyota's BMS also checks the voltage of 13 battery taps on the OEM battery. These voltages must be equal to each other or the BMS will indicate a fault. Since few PHEV battery packs, unlike the OEM pack, are divisible into 14 equal subpacks, these tap voltages must be spoofed, too. Fortunately, it has been found that a fairly simple voltage divider can accomplish this.

For a PHEV, the object of SOC spoofing is to keep the BMS's indicated SOC at 80% or above until the battery is discharged enough to accept significant regenerative braking current; then between 70-80% -- to force less gasoline use even during non-EV-only mode -- until the battery's real state-of-charge has come near its lower limit. At that point, the BMS's indicated SOC should hover around 60% to keep the battery's real state-of-charge from trending further downward (bad for the battery) or upward (thereby wasting gasoline).

We are currently using the spoofed ECU to determine the end-of-charge point (CAN-View, by the way, is currently incapable of the accurate integration necessary to do the job of a BMS). We are spoofing the ECU by increasing the battery voltage by a set amount. We adjust this fixed additional voltage so that (during trial runs referencing an amp-hour meter), with continuous spoofing, perceived SOC drops to around 60% -- the point where the hybrid system's average battery throughput drops to zero -- just when 70-80% of the PHEV battery's charge has been used up.

Because of all of the above, CalCars' overall PRIUS+ circuit diagram is still in flux, and definitive answers about it are as yet unavailable.

[edit] SOC Management

See also Prius PHEV User Interfaces & Prius PHEV Pseudo Code

Here is the minimum needed in terms of a computer for spoofing the Prius' built-in BMS:

  • CAN message reading and parsing (CAN bus writing is NOT necessary or even desirable)
  • The ability to separately close and open two reed switch contacts based on CAN information
    • one to set EV-only mode, based on it not already being set, speed <34 mph, power request <120 (out of 200), SOC >49%, and a few other parameters.
    • one to set a voltage boost (to be explained later) to keep perceived SOC within a given range until the battery is sufficiently depleted

Additional displays:

  • Amp-hour integration and display from the appropriate CAN bus messages
  • HV battery voltage and current display (both analog and digital desirable) from the appropriate CAN bus messages
  • Display of trip info (since reset): # of CAN errors (important for debugging), odometer, milligallons of gasoline used, Amp-hr and/or kWh used, trip milligallon/mi, Wh/mi, and mpg, highest peak charge and discharge currents, highest and lowest HV battery voltages, and the battery's internal resistance (beginning, current, and end)

Additional displays, desirable but not necessary:

  • Not strictly necessary, but SUPER desirable: storage of CAN trip running data on a removable medium (like a CompactFlash card) for later analysis.
  • Small graphical engine tachometer (but see rectangular suggestion below)
  • Gasoline use rate (milligallon/min and/or milligallon/mi (inverse of mpg), or just a binary for gasoline being used
  • Tiny graphical brake cylinder pressure (sum of that for each wheel), to indicate amount of non-regenerative braking being used

A very cool display would be two rectangular graphs indicating engine and electric power:

  • Engine power (e.g. blue for combustion): vertical: RPM, horizontal: torque
  • Electric (e.g. red for discharge, green for charge): vertical: HV battery voltage; horizontal: HV battery current (absolute value)

The areas can be calibrated so that they show the relative power being produced by the electric motor vs. the engine. The same pair of rectangles could display and compare the power going into regenerative braking vs. that being wasted in the friction brakes.

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