[paulgato]

Just to give you all some idea of how this two-battery system is working so far, I have today driven 350 miles or so, with 12.5 hours of drive-time, and all without using the alternator.

The first 7.5 hours of that was from the East coast to Oxford in heavy traffic, with some hold-ups, in daylight with no lights on. Approx current draw: 8A.

I then had a 75 minute break at home, where I plugged the car into the mains, recharging the batteries at a rate of about 35A.

After that 75 minute 'tea, food and charging' break I drove another 5 hours to Bristol and back, this time in the dark with the headlights on, and also with CD's playing loud on the car stereo to keep myself awake. Estimated current draw at the batteries: 17A.

On my return home, the DC/DC converter had almost shut itself off (had cut back from 12.5A to about 2A) and the 'donor' battery had got down to about 40% SOC, with a resting voltage of between 12.0v and 12.1v. The Odyssey starter battery however still had plenty of life in it and was sitting at a resting voltage of 12.5v a couple of minutes after switching off the engine.

This is much better than I had expected, ...which is slightly disturbing as it might mean my calculations about current consumption are out. Alternatively (and I think this is more likely) the 'donor' battery - the Yuasa YPC100-12 104Ah deep cycle AGM - is performing significantly better than spec.

Earlier today I filled up the tank with about 58 litres of diesel, and fuelly.com reports that on that last tankful I drove 1150 miles at an average of 89.51mpg Imperial. I do drive slowly, but for a fairly large and heavy estate car I reckon 90mpg overall average is pretty good. (The odometer under-reports distance by 1.5% so the true average was over 90mpg.)