Battery ECU secrets and teardown (warning: geek content)

I'm presently working on an outright replacement for Toyota's Battery ECU for my PHEV project. I'm sure others are also curious, so here's my initial teardown and analysis of the hardware:



The ECU sits on the end of the CAN bus, so it has a 120 ohm termination resistor. (on bottom of board) The Battery ECU has no other communication systems.

The Battery ECU monitors the Traction battery main voltage along with 13 tap points spread evenly along the pack. This is done to detect pack balance. Battery Current in/out is also sampled using a hall-effect current sensor, and this signal comes on on pin C-15. This is used internally for "coulomb counting" to keep track of battery state-of-charge (SOC). There are 4 Battery temperature probes that use temperature-dependent resistors, and finally the battery fan voltage is sampled.

The ECU also controls the Battery Fan Relay, and outputs a PWM signal to that is used by the fan motor controller to adjust the speed. There are no other control outputs used, but of interest is an additional 1A output that sits unused. (pin A-11) There are also several additional inputs that are also not used in this application.

They use little optically-coupled solid state relays to select which of the 14 battery voltage taps they wish to read, and then use another pair as a switched-capacitor sample system to preserve electrical isolation. It's interesting to note that this board was designed to support 6 additional voltage monitor taps. (components not populated however)

Note that the Battery ECU does not control precharge or the main battery relays, that is done by the Hybrid ECU.

There is a 4K non-volatile EEPROM on this board, and in addition the microcontroller memory remains powered up at all times unless the +12v battery is disconnected. It doesn't seem as though SOC is stored in the EEPROM, as it seems to go to an arbitrary value if +12v is lost. This then begs the question as to what is being stored in there?

I have not yet determined the microcontroller used, as it has house markings. It's similar to models offered by Fujitsu, so this is a possibility. I am not sure if it contains a masked ROM or flash, but there is a high probability it's Masked ROM. I will need to decap the part to inspect the silicon directly if we are to learn more.

FYI: You may repost this picture and text only if credit is properly given.

 

For reference, Here is the simplified schematic of it's wiring in the Prius:

 

While I know little of the Gen 1 system, I was under the impression that it did not use the CAN bus for these communications. If that is indeed the case, then it couldn't have been this board. It's possible though that Denso designed it for use with the Gen 1 pack, which later in the design cycle was shortened.

Yeah, my first guess was that it is used to store the DTCs as well. It's just a theory at this point, and I'm not too concerned with figuring it out, as for my purposes it's not relevant. Still, if you wanted to determine this, the serial data between the micro and the EEPROM could be easily monitored to see what read/writes are being done, and when.

Any Gen 1 geeks care to comment on the architecture of the battery ECU? (Bob? )

 

Thanks Ken,

But I didn't see any reference to those pins in that document. How do you know this, and do you have a schematic or some other diagram that shows how these connect?

 

Thanks Vertex! Yeah that switched-cap circuit is pretty neat! If I get the time, I may finish your schema.

I didn't find the full datasheet, but there is enough to understand how they work.

Here's the part # breakdown and the specs: Panasonic AQW PhotoMOS Relays

Here's a spec sheet: Panasonic AQW PhotoMOS Relay Catalog

I'm sure Panasonic will give you the Datasheet if you ask them, but there's really no need.

 

Interesting! This is useful, as it shows that the Battery ECU shuts off the external charger. They clearly show a relay in there!

Could be useful for those that are adding in extra Prius packs and wish to to a minor plug-in PHEV.

 

BTW, someone asked if I would do a similar analysis of the Hybrid ECU. If someone is willing to send me one, I'd be happy to! (I don't really want to take the only one I have out of my car)

 

I had a look at the Gen I battery ECU, similar functions but implemented way less efficiently and much higher component count. I don't see the need to do a more in-depth analysis at this point.

Next is to do a Gen III. Anyone have one they care to donate yet? I'll even settle for some good pictures.