Thanks cbaber,
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I haven't looked, but I am sure a 12V DC-DC converter capable of car level amperage is going to be rather expensive. You really have to know what demands your car has, and make sure your DC-DC converter can supply more.
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Absolutely. So I'll be measuring my amps usage in the next couple of days, and then I can decide on size of battery/size of dc/dc converter, etc. So far I have found one UK company who makes suitable dc/dc converters designed for charging one battery from another. They make them modularly at 12amps; 24amps; 36amps; 48amps, and so on. From memory about £70/module. They will put out a predetermined voltage and shut off with an audible alarm at a predetermined donor battery voltage. Because they make them locally they can set those voltage levels to whatever I want, within limits.
Another possibility is these dc/dc boost units available from Hong Kong on e-Bay. 10 amp units, sold as a bare circuit board on a heatsink with no case. They have adjustment screws for regulated output voltage and I believe they have a contact for remote switch on/off. Up to 95% efficient, so they claim, and any number of these 10A units can be run in parallel. They look good to me and cost £12 each delivered to the UK. Three of those units would cost £36 and put out 30A. I'm sure that would be more than adequate. (I am, after all, only having to provide the average power needs through the converter, as any peaks will be taken from the starter battery.)
I'll probably go with the UK firm though because a guy in the tech support department there is an mpg nut himself and would be keen to help me get this working well. He was giving me advice about LED lighting. I also like the idea of the automatic shut-off alarm at low donor battery voltage. I'm told it can extend battery life significantly if you don't go down below 50% charge. Also I can use that shut-off signal to poke the alternator back into life automatically.
But this all depends on the data. I was playing with the clamp meter tonight. It hooks nicely over the battery's earth cable and seems to work well. Maybe not quite as accurate as a multimeter in series, but they say it's accurate within 3% or so, and that's good enough for my purposes.
...PS - I just read your thread. So you removed the alternator/power steering belt completely, eh? Power steering delete (or fitting an electric p/s pump) is also on my list to do, but one thing at a time... I like the battery relocation you've done but I'm wanting to go down the two-battery route. I was nervous about having a long cable run to the starter in a diesel. At 300A cranking current you'd need a monster cable for such a long run. As it is, I think I can safely use the existing cable to the 12v power outlet socket in the load area as that is fused at 35A and I doubt my dc/dc converters will be rated higher than that. I'll just plug the converter in there to start with.
My 2nd battery is going to be located down in the spare wheel well and the spare wheel is going to stay at home. I thought about the 'no spare wheel' issue a long time and it does make me slightly nervous as I did lose a wheel in an accident a year or so ago in the snow. Tyreweld would not have helped in that situation. But it's rare these days. That was the first time I'd had to change a wheel by the roadside in 20 years. In that time I've had quite a few breakdowns due to faulty batteries and alternators, as well as other breakdowns for other reasons. Tyres are tough. If they're in good condition and you drive carefully I reckon they're no more likely to let you down unexpectedly than any other component. With two batteries (and all the voltage and current monitoring guages I'm going to have on the dash!) I'm far less likely to find myself with a dead battery or be caught out by a failing alternator.