Coulomb

Yes, this appears to be the case. My reading of the firmware indicates that you can use any two voltages in any order (i.e. higher voltage first, or lower voltage first, but a valid BTA1 must come before BTA2). [ Edit: the BTA commands are also sent to the Solar Charge Controller; if it's powered up, it seems to expect the higher voltage first. ] However, they have to be at least 5.0 V apart (both the measurements and the arguments to the BTA1 and BTA2 commands), and the value in the BTA1 and BTA2 commands must not be over 60.0 V, and can't be zero. In addition, the difference between what it measures and what you tell it must not exceed 3.0 V, so you can't "spoof" the battery voltage by more than 3 V (but possibly a little more or less, depending on the natural error). Any violation of these conditions results in the command getting NAK'd instead of ACK'd. It looks to me that the inverter will still be expecting valid BTA1 and BTA2 commands. I don't think you can use really low voltages, say < 40.0 V, since it won't run the inverter. You could run it from mains or PV though. There doesn't seem to be a test for a low value, other than for zero, so that might work. However, best accuracy will be had by using realistic voltages. Yes, it's CCITT, as noted here: http://forums.aeva.asn.au/forum_posts.asp?TID=4332&PID=57733&title=pip4048ms-inverter#57733 I note that this is when I welcomed Chris Hobson to the Australian Electric Vehicle Association forum, and now this is my first post here. Thanks for posting about the calibration procedure. I had noticed the BTA commands some time back (along with perhaps fifty test commands) and wondered what they were about. [ Edit: added comment re 5.0 V difference applying to both measurements and the BTA1/2 arguments. ]