I realised I have a small 200W 240V to 12V dc mains power supply at home. So I experimented a bit more yesterday. It saved me throwing away £34 for a 40Ah starter battery for this purpose.
With stable output of 13.74V and 9.5A (roughly 130W) this is what happened
start:
VCDS oil temp = 8C
60 minutes:
Oil temp = 18C
120 minutes
Oil temp = 23C
It was promising at the 1 hour mark, 10C per hour, but something went non-linear in the second hour, only raised by another 5C.
I drove after that, and it took only 10 minutes to get oil up to 80C, but took another 5 minutes to get to 85C. I gave up measuring after that. All slow 30mph roads and traffic lights. Coolant was heated to 35C before leaving so this helped too.
I am quickly realising, this was a fun experiment, but in practice its going to be a real pain heating oil for 2+ hours before every first trip of the day. I think I will only be bothered to use this heater for only the coldest days 0C or below.
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