What would be the Voltage of a fully charged Battery with no Electrical load on it at minus 10 degrees F.

My though is if you aready start off with less Volts due to the Cold you are also going to get less Volts when you turn the Glow Plugs on.

Cold Glow Plugs also draw more amperaged till the get hot.
If you combine a minus 10 degree Glow Plug with Carbon that is going to conduct the heat away from the Precombustion Chamber keeping the Glow Plug colder longer you are going to drain you Battery much faster than you would during a nice warm day.

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84 300D, 82 Volvo 244Gl Diesel