diff options
author | Daniel Silverstone <dsilvers@digital-scurf.org> | 2018-03-21 21:01:28 +0000 |
---|---|---|
committer | Daniel Silverstone <dsilvers@digital-scurf.org> | 2018-03-21 21:01:28 +0000 |
commit | 10307e67c72f3d2715d34bede7b9d8fb840d7b8e (patch) | |
tree | a83eb306f31f22fb6fd617a0bed222dc1876b7d3 | |
parent | 9657e99b2bd1d2540cf78f4b33509903195d65dc (diff) | |
download | blog-10307e67c72f3d2715d34bede7b9d8fb840d7b8e.tar.bz2 |
initial draft of evse stuff
-rw-r--r-- | posts/evse-tester-prototype.mdwn | 77 |
1 files changed, 77 insertions, 0 deletions
diff --git a/posts/evse-tester-prototype.mdwn b/posts/evse-tester-prototype.mdwn new file mode 100644 index 0000000..678310f --- /dev/null +++ b/posts/evse-tester-prototype.mdwn @@ -0,0 +1,77 @@ +[[!meta title="Building an EVSE test box"]] +[[!meta author="Daniel Silverstone"]] +[[!tag draft]] + +A while ago now, I was having "fun" learning about [SAE J1772][] signalling and +started to examine the complexities in [ISO 15118][]. Realising that the +documention available on the Internet on ISO 15118 was less than edifying, I +contacted my brother whose company is involved with electric vehicles in the +vain hope that he might have a copy of it which I could read. Some hours later +we'd had quite a discussion which ended up with me agreeing to create a +prototype test box for J1772 signalled chargers as an urgent activity. + +I sketched out some schematics, picked components, ordered some stuff I was +missing, and on a bright Saturday morning, I began to assemble things. By +Sunday evening (I'm very slow) I'd built and tested something I was quite proud +of. I thought I'd share the journey with you, so you can share my pain :-) + +SAE J1772 Control Pilot +----------------------- + +Before we begin with the tester box, let's first think about the control pilot +signalling. This signal is a bidirectional negotiation between the EVSE and +the EVCC and allows the EVSE to communicate the amount of current available, +and the EVCC to communicate when to provide power and if the EV requires that +the EVSE ventilate the area (e.g. if it's underground and the battery tech +gives off hydrogen when charging). + +The control pilot is a +/- 12v signal with a moderate amount of current behind +it (maybe a few hundred milliamps). It starts life as a DC +12v signal and +J1772 refers to this as state 'A'. This is the "not connected" state. When +the charge cable is plugged into the EV, the EVCC adds a resistor between the +control pilot and ground, which brings the 12v signal down to 9v. The EVSE +detects this and transitions to state 'B' (vehicle attached). + +When it enters state 'B', the EVSE switches from a DC signal to a square wave +between +12v and -12v. Naturally, since the EVCC has added the resistor to +bring us into state 'B', the signal is +9v not +12v. Naïvely we might expect +the signal to be +/- 9v, but as the first major safety check, it turns out that +the EVCC has a diode in series with the control pilot meaning that the signal +is +9v / -12v. If the EVSE notices that the negative half of the wave doesn't +go properly to -12v then this is considered a 'diode failure' and it should +enter an error state and refuse to charge. + +This particular safety feature is designed to cope with situations where the +charge plug might be dropped into a bucket of water. You don't want the EVSE +to be confused into turning the power rails on in this situation, so the diode +means that the EVSE is able to detect the difference between a serendipitously +resistive bucket of water, and an EV. + +Assuming the diode test is passed, then the EVCC looks at the duty cycle of the +control pilot wave and uses that to determine how much current it is permitted +to draw. Assuming the EVSE is offering enough power, and the EVCC wishes to +charge, then it switches another resistor into play which brings the wave to ++6v/-12v. At that point the EVSE switches in the power and the EVCC can charge +the battery. This is called state 'C'. + +There's more to it, but that's the essentials. + +Testing an EVSE +--------------- + +The vast majority of the behaviour and safety features of an EVSE can be tested +by manipulating the control pilot. In order to test the EVSE you need to be +able to pretend to be a car, (to transition from state A to state B), request +charge (transition from state B to state C), request ventilation (B/C to D), +and then you need to be able to trigger the various safety checks. To do this +I put together a test box which has a number of switches, resistors, etc, +and looks like this: + +TODO: picture of final product. + +The circuit looks something like this: + + + +[SAE J1772]: https://en.wikipedia.org/wiki/SAE_J1772#Signaling +[ISO 15118]: https://en.wikipedia.org/wiki/ISO_15118 |