[paulgato]

So, just in the last few days I've been driving around with a dual battery system comprising a 68Ah Odyssey AGM battery in the standard engine bay position, and a 104Ah Yuasa YPC100-12 AGM in the spare wheel well. Both batteries are charged up every night and the rear battery feeds power to the front battery through a 12A DC/DC converter at 14.0v while driving.

I had been testing out this system using a spare 70Ah Bosch car battery instead of the Yuasa with reasonable results, but I have to say I am seriously impressed with the Yuasa YPC100-12 104Ah battery. It seems to pump out power for hours without significantly dropping voltage. The YPC range are designed as deep cycle batteries for traction use, and the thick lead plates show in the weight (30kg).

(When I have all the wiring done and completed in a permanent kind of a way I'll post some pics of it here. It's all a bit messy right now.)

So, my dilemma is the following. I have made provision to fit TWO YPC100-12 batteries in the spare wheel well, either attached separately to two DC/DC converters (one each to give a max of 24A) or else simply in parallel, both feeding the one 12A DC/DC converter, ...or else again, simply in parallel with the starter battery with no DC/DC converters involved.

The DC/DC converter has some real advantages over a direct parallel connection by the way...

1. The current flow is automatically limited to a nice safe 12A, so no need for heavy cables or large high current fuses for safety. Also the fact that 14.0v is available means that a moderate voltage drop across cables is not a big issue, again meaning that the car's existing light guage wiring can be used without problems.
2. Current can only ever flow in one direction - from rear to front.
3. Voltage in the car's main system generally remains quite high, which means headlights, etc. run at close to full power almost all the time.
4. The two batteries are always isolated from each other.
5. This converter automatically reduces output as the donor battery voltage drops, so it limits Depth of Discharge to about 60%, which protects the donor battery from damage.

However, the converter is only 'up to 92%' efficient, so I am losing about 10% in the conversion. I have tested and found that in fact the efficiency is at least 90%, and the modest heat generated by the unit is not a problem in the enclosed space of the spare wheel well. But it would be nice to have a more efficient system. (In fact, even on the hottest day with the car stood for hours in the sun, the temperature in the wheel well - under the load area floor - remains remarkably cool and even. It really is the best possible place in a car to install a battery, ...or to keep your picnic hamper or your road kill I guess. Or even a spare wheel.)

If I had two of the Yuasa batteries I would not need to worry about the 10% efficiency loss.

[Edit: oh no wait - mistake! The conversion loss is worse than that because to produce 12.5A at 14v that corresponds to 14A at 12.5v, so the total draw from the battery in amps is something like 20% higher than the output. In other words, if it is to last the same number of hours as a directly connected battery, a battery using a DC/DC converter will need to be 120Ah instead of 100Ah.]

So, is one rear donor battery enough, or do I add a second Yuasa battery in parallel? The usual rule of thumb with lead acid batteries is, "the more you use, the less you lose". Battery longevity is closely correlated with a low average Depth of Discharge. For everyday local driving use, a single (new) Yuasa is more than enough, especially as the car will be plugged back into the mains for recharging as soon as I get home every evening. But on longer journies both batteries (donor battery and starter battery) could become fairly deeply discharged, and may remain that way overnight at my destination unless wherever I am staying can provide me with overnight mains power to the car. With two rear Yuasa batteries, an overnight stay with no charging facilities would present no problem as the Depth of Discharge will be much lower (20% rather than 40%?) and damaging sulfation is much less likely to occur. So, for example, on a weekend beach trip with no facility for plugging into the mains I should be able to get there and back on one charge without any issues.

The weight of a second Yuasa battery will in theory have a cost in terms of slightly reduced mpg, but since I live and work in a fairly flat area, and since my car is already quite heavy I don't think that extra 30kg is going to cost me very much extra fuel at all. (30kg is less than 2% of the vehicle's total weight, so the maximum theoretical, worst case scenario mpg hit is 2% - and that's if I am climbing hills constantly and never coming down again! - but in practice it's going to be well under 1%.) Also, I'm thinking of long, cold, dark and wet or snowy winter work days with headlights on, windscreen wipers and cabin heater going full blast, and I kind of like the idea of not having to ration my electrical equipment use for the sake of saving the batteries. And, I know that if I ever feel the battery charge is getting low, and especially if it is dark and rainy and horrible out there, I can get a little nervous, and that means I am liable to speed up a bit so as to get home before the voltage drops too low. Overall I think more amp hours is going to be better for mpg as well as for safety and comfort. (Another advantage of having more lead than I absolutely need is that I can safely forget about the regenerative braking idea!)

(More of an issue than the very slight mpg hit from the extra 30kg is that the rear of the car will sit a little lower, and will LOOK laden, which is a concern for me because I keep my work tools in there and the last thing I want is for someone to break in to see if there is anything worth stealing. But I can fit uprated rear springs to this car very, very easily and very cheaply, so that is hardly an issue. In fact I had been thinking of replacing them anyway, partly so the load area always looks empty even when it's full of tools, and partly because the springs creak annoyingly. I had thought the creaking was coming from the (leaking) shock absorbers so had those changed last winter, but no, it's the springs or the spring mounts.)

OK, so I am leaning towards fitting a second Yuasa battery in parallel with the first one. The two might not last TWICE as long (in years) as a single battery, but should last significantly longer. If I lived in a hilly area I might hesitate, but an extra 30kg on the flat isn't going to make much difference.

(Did I mention I was impressed by this Yuasa YPC battery? So far, I'm more impressed than by the Odyssey battery, even though the Odyssey 68Ah battery was £200 and the Yuasa 104Ah battery was only £138. Both are good I guess. Time will tell.)

If anyone has any advice or ideas on this I'd be glad to hear...