I had that idea too. The theory seemed fine to me. But unfortunately the MPPT will suck as much current as the PSU can possibly supply, or more! In the case of more, the MPPT blows its transistors as it happily tries to pump out more than its rated for. In the other case the PSU starts glowing red hot as it tries to supply its maximum current 100% of the time.Feed that 40 amps into the MPPT controller
In the end I used a DCDC converter. This device will limit to its rated current, whereas MPPT won't. I can power the DCDC from the alternator or a mains->12V PSU (an old HP server PSU) via a switch.