8 data bits, 1 parity bit. I said 9 because that's how I looked for the word size: look for 1/0 (stop/start) transitions at regular intervals. It always had a 1/0 then 9 bits, then another 1/0. The parity part is easy: just do a parity check, and if they all check out there must be a parity bit. The chances of an accidental match fall by half every byte.
The longer BATTSCI packets look related. They seem to trade off every other packet (0xAA or 0x87 for the start byte). All the ones I've looked at have summed to 180, 200 or 280 hex. Also, it seems like bit 7 (the last data bit, just before the parity) of the first byte of a packet (METSCI or BATTSCI) is always 1, and all other bytes of the packet have 0 in bit 7.
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