The Chinese Diesel Heater. So nice. So bloody dangerous! If the power is interrupted suddenly, it melts down.
You need to detect the power loss and then power the heater briefly from the vehicle starter battery to allow it to cool. Here are some ideas.
If the power goes away, the relay switches over to the starter battery. The relay also interrupts the fuel pump. This causes the heater to shut down with an out of fuel error. No melting!
Simple to build, but the relay sucks some power while it's switched on. You need a heater on/off switch to stop the sucking. Never switch it off while the heater is still hot!
It's not very good if the power drops too slowly (capacitors in the inverter holding it up etc.) The relay holds on until only a couple of volts, the diesel heater could reset and forget it's melting. Need to test it with your particular set up.
Need a double-pole double-throw (changeover) relay. Also a double pole power switch.
Power switch Relay
/ NC
Starter batt >-----o/ o-----------o\ COM
| NO \o--------> Heater power
| +-----o
| |
/ | |
Lithium batt >-----o/ o-----+ |
| NC |
| o\ COM
| NO \o--------> Fuel pump
Heater fuel >---------------------o
pump output | |
| + -
+-----\/\/\---+
COIL |
Ground
Use a thermostatic switch on the heater body. Similar kind you get in clothes dryers etc. Use a self-resetting normally open switch. Trip at a temperature a bit above what the heater normally gets to.
Open up the heater and cut the two wires to the fan. Wire them through the relay and back to the fan. Be careful you don't reverse the wires, the fan polarity must be correct.
Relay
NC
Fan output + >---------------------o\ COM
NO \o--------> Fan motor +
Starter batt >--------------+------o
| |
| |
| |
| NC |
Fan output - >---------------------o\ COM
| NO \o--------> Fan motor -
+-----------o
| | | Thermo switch
| | + - / NO
| +------\/\/\-------o/ o--+
| COIL |
Ground Ground
Heater may get sooty. No glowplug or slow ramp down when it stops. But it can't melt now!
Build this circuit with a 4011B quad 2-input NAND gate chip. It draws almost no power at idle. Power is only consumed during the few minutes when the heater is shutting down.
If the power is interrupted, it will switch over to starter battery power for a few minutes. The feed to the fuel pump is interrupted at the same time, forcing the heater to shut down. After a few minutes, power is switched back to the lithium battery. If the heater doesn't shut down for some reason, you won't risk draining your starter battery.
The circuit also handles brief glitches in the lithium battery supply well. If it dips below about 6-7 volts, it runs through the complete shutdown sequence. It will not switch back to lithium battery power until the shutdown has completed. So there's no danger of "chipping away" at your starter battery every time there's a small power glitch.
Ideally you should pot the circuit in epoxy, or at least spray conformal coating over the circuit board. It prevents moisture from your filthy van screwing up the circuit. Otherwise seal it in a box with some silica gel.
The circuit is for 12V only!
You can also make it using two automotive-style changeover relays instead of the DPDT (double-pole double-throw) relay:
Here the fuel pump's negative (ground) connection is switched, instead of the connection between the heater and the pump.
If you're really paranoid, put a 70-100C thermal fuse in series with the heater's fuel pump connection. Tape the thermal fuse to the relay that switches the heater's power (RL1). If the relay contacts get fucked up and decides to melt, the fuse breaks and shuts down the heater safely. Relays don't melt often but it happens sometimes.