[paulgato]

I just received a 100Ah Yuasa YPC-100-12 cyclic AGM battery. (30kg. Ouch!) That will go in the spare wheel well soon, although I'll test it out in the passenger foot well to start with, so I can monitor voltage/current, etc. easily. It cost £138 and should last at least two years of daily use and probably more. It should pay for itself in fuel savings.

I've been using the DC/DC converter/charger with a spare non-AGM car battery for a little while, and the system does work well. The converter provides a steady 12A at precisely 14.0v into the (90AH AGM) starter battery and maintains the starter battery system at between 13v and 14v, depending on load.

On dry days the 'donor' battery supplies all the power needed by the car (approx 9A) until its voltage drops below the DC/DC converter's battery protection threshold of about 12.0v, at which point it modulates the current gradually down while maintaining the donor battery's voltage at no lower than 11.8v.

On wet or dark drives, with higher electrical load, I reckon the two batteries share the load roughly 50/50, and if it's both wet and dark the starter battery provides more than the donor battery does, although when I park up I can leave the donor battery to re-charge the starter battery through the converter. My hope is that adding the second battery and DC/DC converter will bring both batteries back within their 'comfort zones' so they'll both last much longer. With just the one battery I was often using a full 50% of the battery's capacity on routine daily drives, and that puts a bit too much strain on the battery. With two batteries, the typical daily discharge of each battery should be between 15% and 25%, which puts far less strain on them, whilst more than doubling my maximum alternator-free driving range on the occasions when I need it. The extra weight is an issue, but I think it's worth it.

The converter runs cool (and therefore efficient) until it begins to modulate the current, at which time it does get quite warm for a short while until the current drops below 5A, and then it becomes cool again. I plan to add a separate adjustable voltage-sensitive switch to cut the converter out at a donor battery voltage of about 12.2v. That will limit daily DOD to about 50%, and will keep the converter in its cool and efficient non-modulating mode. I'll still be able to overide that 12.2v limit manually if I need the maximum range on a long drive or if I ever need to recharge a flat starter battery.

Today I started taking the glove box, etc. out so I can run cables through the firewall from engine bay to spare wheel well. One big advantage of using the DC/DC converter rather than simply connecting two batteries in parallel is that I only need to run normal 12-14 guage cable or so. No scary monster cable required, nor any scary 300A multiple fuses and isolators. I'm protecting things with standard inline blade fuse holders and 15A fuses. In fact I'm using the existing (30A fused) wiring to the rear 12v power outlet socket, and that seems fine and everything feels very safe. I just need to run the charger cable through to the rear really, but at the same time I will lay in a low amps multi-core cable for various control and monitoring puposes.

Just one other thought is that it would be really cool to add a couple of small solar panels to the system - perhaps one 20w panel for each battery, perhaps mounted inside the two rear side windows in the load area. More watts would be better but more watts would have to go on teh outside of the car, whcih compicates things, and even 40w should extend range and generally support the batteries to a useful extent.

I have to say this project is taking up a lot of time and I'm looking forward to getting it kind of complete and all hidden away in the spare wheel well where it belongs. I'm getting a solid 10% mpg improvement from it, so definitely worthwhile, but I'll be glad when it's all complete, all running automatically, and I can move on to the next mpg mod on my list.