Anywhere you see a twisted pair connected to an optocoupler is a prime candidate for replacement by a fibreoptic link. You separate the LED from the PT and stretch an optic fibre between them. You'll notice that it uses optocouplers (the CAN interface, not the Sistine Chapel), although bizarrely, any isolation they might have provided, is completely voided by connecting the GND references together from both sides of the optos!Īnyway, a fibreoptic link is effectively a stretched optocoupler. But before you do, scroll up to see another version of the Sistine Chapel image.
You can see the schematic Coulomb worked out for this CAN interface in this earlier post in this thread. And it's fixed at 250 kb/s on the CAN side. It passes everything to the charger at 2400 b/s and so cannot cope if CAN data arrives faster than that, even CAN data it's not interested in. We did not use CAN bus to the Elcon/TC-Charger-brand charger because its so-called "CAN interface adapter" is a joke - a Clayton's CAN interface.
That was a fairly complicated hardware mod since the comms went via the DC-37 connector you can see in the photos above, and we had to repurpose a bunch of CMOS inverters inside the DCU, changing their power supply voltage and reversing the direction of some and changing the value of pullups.īut this IFO mod only requires the addition of one resistor per IFO connector - a 47R in series with the LED (blue connector) and a 680R pullup to 3V3 on the phototransistor (PT) (black connector).
At that stage the DCU also became our BMS master, after some major additions to its software by Coulomb. Previously we had modified the Tritium DCU to talk to the charger and BMUs using an RS422-style interface over twisted pairs.
The one on the right is Coulomb, pointing and saying "Haa haa, that'll teach you - doing home maintenance on EV day."īut that didn't stop Coulomb from working away diligently and doing a beautiful job of modifying a Tritium Driver Controls Unit (DCU) to talk to both our charger and our string of Battery Monitoring Units (BMUs) using Industrial Fibre Optic (IFO) connectors, in a much neater way than before, and trying to avoid the paint flakes in his lunch. The limp wrist on the left is mine, after all the overhead paint scraping. Last EV day I was unable to contribute much, due to the need to recover, as quickly as possible, a ceiling-painting disaster I had created on the previous day, in my family's kitchen. For that reason, we decided against any scheme that "made use of" the link between cells. Even if it becomes high resistance (as in hundreds of milliohms), it gets pretty scary. There is the problem of an exception at the end of every battery box all packs have an end (we currently have two) that has to talk to the BMS master, and many have at least one break between boxes (we have six or seven, depending on the configuration).īut the show stopper is when you consider the voltage that appears across one of those heavy current links should it become open circuit, even for a millisecond, when the vehicle is running. I could eliminate one of the comms wires by connecting the 330R resistor which is presently from V+ to the opto anode, from there to V- instead! That is because the V+ of this cell is connected to V- of the next and so the heavy bus connection between cells becomes one of the comms wires.