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Automatic series/parallel configuration
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PlasmaArcEng:
I have found that Mosfet gate voltages are usually a max of +-15V for the common varieties.

I did a schematic for you of what Paul Moir suggested.

The relay can nearly be anything. If you chose this relay https://www.digikey.com.au/product-detail/en/omron-electronics-inc-emc-div/G2RL-2-DC12/Z2926-ND/368870 it turns on at 70% of 12V= 8.4V.
The zener has to dissipate 33mA*14.4V=0.5W. This one would work fine https://www.digikey.com.au/product-detail/en/on-semiconductor/1N5351BRLG/1N5351BRLGOSCT-ND/3462220
The diode can just be a 1N4001 https://www.digikey.com.au/product-detail/en/comchip-technology/1N4001-G/641-1310-1-ND/1979675
With this circuit the heaters are in parallel when the relay is de-energised (12V) and puts them in series when the voltage is above 22.8V, but probably will be a bit lower as the relay will likely actuate before this voltage. If this is too high, reduce the voltage of the zener by a couple of volts to have the relay turn on sooner.
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