The 36 V 350 W power brick for the improved M2 HBP arrived and seems to work fine, apart from a distinct smell of hot electronics under load. Dialed back to 30.1 V at the terminals (to match the HBP spec) and with the HBP connected through the same length of 12 AWG wire as before, the supply draws 150 W from the AC line.
It draws 160 W at 31.7 V and stabilizes at about 100 °C. The heater resistance is 7.6 Ω before it has a chance to cool off, so the heater runs at 4.17 A and 132 W. The supply efficiency is 83% = 132/160, about what you’d expect. The fan runs intermittently with that load.
In order to dissipate 150 W in the panel at the same resistance, the voltage must be 33.5 V at 4.5 A. I’d want to install it in the M2 and make some measurements before jumping to any conclusions.
The DC-DC solid state relays work about as expected, too. This is a 60 V 10 A brick, turned on with one of those old Tenergy RTU NiMH 8 cell packs:
The SSR’s forward drop runs around 1.0 to 1.1 V at 4 A, which suggests a drain-source resistance near 0.25 Ω, rather more than you’d expect for a bare MOSFET, but probably about right for an up-armored device. Or it could just be a crap MOSFET inside there…
So I think the brick will wind up at about 35 V to make up for the SSR drop. The SSR will dissipate about 5 W and won’t need much heatsinking; just bolting it to an aluminum chassis may suffice.