Thanks Joe,
You can't imagine how many times I've read the data sheet.... (it's been open on my desktop for days now)
I'm clearly not quite understanding yet.

Yes, a pattern of 1010 turns on the charger, and the other 4 bits of the byte set the charge rate.

So what ( I think) I want to send is 10100101 which should tun on the charger and set it to "1 diode, 2Kohm" operation. In HEX, that's $A5, right?

So I've sent $90, which should be the register address for the charger, and $A5, which should be the code to turn it on and set it to my chosen charge mode. I even tried sending the data in binary, like so:
Code:
        rst = 1

        'Set Battery charger on and set to "1 diode, 2Kohm"
        Shiftout IO, SCLK, LSBFIRST, [$90, %10100101]  ' Set charger   [$90, $A5] 
 
        RST = 0
but with the same (no) result.

What am I doing wrong? (please!)

Thanks,
Steve