Well... finally! I've managed to come up with an Arduino algorithm that reliably maintains setpoint temperature within +/- 0.5C without using PWM. It was simpler than supposed. For reasons that escape me and leave me utterly frustrated, the prototype does just that (+/- 0.5C) using PWM.
Can I get the battery grip setup to do the same, using the same program? No!
The silly thing is, not using PWM was raised on this forum by one of the guys a while back - guess I wasn't listening. Rather than running at ~500 - 3800hz PWM, I am pulsing the output at 10hz, which has the desired effect
I guess the reduced noise provides for more reliable reading of the analog temperature reference.
I told you so! Indeed, you did! And thank you!