VOLVO+ Development Story (2), low power design

After the 1st prototype broken, I made several prototypes again. I'm not good at soldering so I re-do the prototypes several times to get a working board. To make VOLVO+ become a real product, I have to do something difficult: low power mechanism

VOLVO XC60 used AGM battery which costs ~300USD and many people complained the lifetime is only roughly 2 years. It means 150USD expense per year. The basic idea to make lead-acid battery healthy is simple: make the battery fully charged

I don't want VOLVO+ kill my battery, so I shall design low power mechanism carefully



As engine on, the VOLVO car's alternator can output >100A while VOLVO+ only drains 13mA, which is so small to neglect. As the engine-off, VOLVO+ keeps draining the battery as it's plugging to OBD2. So the real challenge is: how to minimize the standby power

I defined a useful metric to measure low-power performance: how long does VOLVO+ standby power drains 1% battery. The longer time, the better low-power performance. Picture below is my car's battery, EFB 70Ah. So 1% battery in this article is 70Ah * 1% = 0.7Ah




Static Power Consumption (2019/05)

With some engineering effort, I've reduced the standby power to 0.385mA. This means 1% battery can last 75days, or 2.5months. The self-discharge of lead-acid battery is 3% per month. The number is far below the self-charge, and that's sufficient low-power performance

0.7Ah = （0.385＊0.001）A ＊24h＊75.75days





Low Power Mechanism (obsoleted)

We designed the wake-up and sleep mechanism according to car's CAN bus behavior. In the picture below, the left column unit is seconds. As car is awaken, the packets appeared every 0.001 (1ms)



As MCU wakes up, it monitors the CAN bus for 10ms. The time should be sufficient to cover 5 packets. If receives any valid packets, then keeps waking up for 20 seconds

If not receiving any packets, it means the car is sleeping. To detect the CAN bus is alive, I set MCU wake-up period at 3 seconds. It's expected to be sufficient to capture any human operation

According to the parameters above (wake-up period=3000ms, wake-up-time=10ms), the weighted power consumption is 0.385mA
  • 3000ms standby, 0.345mA
  • 10ms normal operation, 12.430mA
    • Weighted average (3000*0.345 + 10*12.430) / (3010) = 0.385mA = 385uA

Further analyze the standby power, the TI CAN transceiver is of ~95%
  • TI CAN TRANSCEIVER, 370uA
  • CPU, 3.2uA
  • LDO, 15uA
TI had several CAN transceiver models. I planned to use SN65HVD231, whose standby power reduces to 0.04uA. After switching to this IC, 1% battery can last 4 years. The number is actually sort of meaningless if considering the lead-acid self-discharge at 3%/months


After designing proper low-power strategy, I'm willing to place VOLVO+ prototype on my car (I'm quite picky to low-power design)
  • The PCB now doesn't have flying wires
  • The backside of PCB, which had 12v-to-3.3v component
  • Let's try the prototype on the car


Static Power Consumption (2019/10)

Final static power consumption is <30uA, which drains 1% battery in >2years. So the power consumption is no longer a problem now


Comments

Post a Comment

Popular posts from this blog

OBD-II and ELM327 (1)

Image
I bought a Volvo XC60 2017, and the car had an OBD-II port. I always feel the port is inviting me to do some hacking. I had power supply, oscilloscope, soldering equipment, multi-meter. I can read English, and had some embedded system development experience, but no hacking car ever. Maybe with some study, I can learn to do that My adventure starts with ELM327, the equipment is quite cheap. I want to talk pro/con of the cheap China ELM327 unit, and when you will feel disappointed to it ELM327 is basically a Microchip PIC18F2480 with customized FW, which supports various OBD protocol. Picture below is from its datasheet Left-upper corner, MCP2551 is the CAN transceiver It communicates to car by CANL/CANH signals It outputs CAN signal to PIC18F2480 (ELM327) (PIN1/4) PIC18F2480 (ELM327) It accepts the output signal from CAN transceiver The internal CAN controller send/receive from CAN bus It had UART interface in PIN17/18 Option1: FTDI UART to USB interface Conne...
Read more

OBD-II and ELM327 (3), try to decode CAN package recorded by ELM327

Image
Volvo XC60 2017 doesn't offer the capability of closing tailgate by remote control. I have to press the button in the tailgate (see below) to hand. That's probably because of security reason, but I feel so stupid. M assumption is, the button sends some message to CAN bus. If I can sniffer the signal, I can close the tailgate remotely Challenge Accept !! I connect the ELM327 with my XC60, and successfully read VIN code . Then I tried ATMA (Monitor All) command to sniffer CAN bus... Then... nothing happened !! What the fuck ?! So now I can tell you when the cheap ELM327 makes you disappointed In complete AT command Because Torque App doesn't use ATMA command, so the cloned ELM327 don't offer such capability Poor bluetooth connectivity As sending ATRV command, sometimes I got incomplete response. I guess that's because bluetooth module doesn't work well Even the cheap ELM327 dongle offers ATMA capability, the data rate is far exceeding the blue...
Read more

OBD-II and ELM327 (2), the underlying interface of ELM327

Image
Go to Google play and search keyword of "ELM327 Terminal", all of them were similar, you can just pick arbitrary of one. If you really had some difficult of selection, this one  looks beautiful and just updated recently After connecting the tool with ELM327 and car, type "0902" in the tool to read the VIN code . These PIDs (0902) were industrial standard , and all cars shall implement it. Here's some useful PIDs. That's why HUD (head up display) can connect to all brands of cars with OBD2 connector 01 0D: car speed 01 0C: engine RPM Here's the 0902 query response of my car in the APP. If interested, decode it :-) 7E8 08 01 14 49 02 01 59 56 41 7E8 08 21 44 5A 41 38 42 44 48 7E8 08 22 32 30 37 36 36 38 32 A lot of people read CAN bus by Torque, but only few people read the ELM327's capability document. Check out ELM327 website and download their AT command list , that's all the capability it had. It had different capability...
Read more