Diesel Heaters, 2 Power Sources

I have a 12V 'Chinese' diesel heater, which are currently very popular for installations in camperized vans, RV's and even rooms or buildings. I have one in my small overland 'squaredrop' trailer and am wanting to install a second one in my garage just to take the chill out of the air in my work area when I'm working in it during the winter.

Once operating, it's imperative they not lose power as they need to go through a shutdown/cooldown cycle of about 5 minutes or so after being turned off. The one in my trailer is not a concern, since it has 12v power to it at all times whether plugged in to 'shore power' or not, using a Xantrex Inverter/Charger and two deep cycle RV/Marine batteries. But in the garage, I'm using a 12V 30 Amp power supply to power the heater and my concern is that if there is a power failure when it's running, it will not be able to complete a proper shutdown, which could permanently damage the heater. I have a 12V car battery to supply alternate power. And I have purchased a small switching device (link below) from Amazon, however the one I received says it's rated at 5 amps, not 12. The heaters require usually a minimum of 14-15 amps during startup and then 4 or so while running. I'm not sure about the cooldown but I know it uses the glowplug again to ensure all the diesel gets burned up, so it's possibly a similar load to startup(?).So I'm wondering about using 40 amp 12V relays either instead of or in conjunction with this device. It's meant to instantly switch sources from the power supply to the battery if the power supply goes down.

Thanks for any thoughts or suggestions.

https://www.amazon.ca/dp/B0DBJCNWBK

-------------
Brian, Vancouver Island, Canada
Photographer, Car Enthusiast, 4X4, Overland trailer

I think the best solution would be to use a setup similar to your trailer, possibly on a smaller scale. A dedicated inverter/charger (rather than a power supply) designed to maintain a battery/battery bank of your choosing. Then if power goes out, the battery/battery bank should keep the heater going without any need for switching equipment.

Your idea of a standalone battery may work in the meantime using the output of your switching device to trigger a relay. The issue I see here is the battery will need to be checked periodically if it's not on a maintainer.

Well, twice now I have ordered these emergency switches from different suppliers, both were advertised as 12 Amps but only indicate 5 Amps when they arrive. So I'm wondering if I just use a SPDT 30/40 Amp relay, wired 'backwards" as below, can anyone see any issues? I'm not sure if there's a short time interval between when the Power Supply loses power and when it's output drops enough to release the coil and switch to Battery power. Thoughts? Thank you.

Proposed Connections:
+12V from Power Supply to 86 and 87
+12V from Battery to 87A
+12V to Diesel Heater from 30
Ground on 85


-------------
Brian, Vancouver Island, Canada
Photographer, Car Enthusiast, 4X4, Overland trailer