I agree with all Fisherman has said.
A clue may be in your comment that you have
solar and the sun has been shining?
Diesel heaters may have different control options. If running to maintain a set temperature measured by a thermostat the heater may cycle on and off. So if you have set 20C the thermostat may turn it off at say 21C then on again at say 18C and at each “on event” use the glow plug to ignite the diesel fuel. This is what uses lots of amps for a short period but the amps drawn can pull the
battery voltage down if going on and off all night.
Some heaters can be set to run at a low rate continuously so that they are not repeatedly stopping and starting. This can use less electrical power as the glow plug is not used to restart the heater. The downside is that it will give a continuous heat output but if say the night turns very cold the heater will not be able to automatically keep pace with the heat required to maintain a set temperature.
Did you look to see the
battery charge when you first found that the heater would not run? Was the sun shining then? If so the
battery voltage whilst under charge from your
solar can look “full”. Don’t know where you are or how much sun you are getting but I might guess that the alternator can charge the
battery at a higher rate of amps than early morning
solar.
Do you have a simple multimeter? A very useful tool as to get a realistic measure of the state of charge of a
battery measurements with the
battery disconnected, so not being charged or discharged, are more accurate.
State of Charge Sealed or Flooded Lead Acid
battery voltage
100% 12.70+
75% 12.40
50% 12.20
25% 12.00
0% 11.80
Cannot find a better way to post a table. The danger to your
battery is discharging repeatedly to below 50% which reduces the number of discharge cycles and so shortens the
battery life