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paulgato · 04-10-2014, 07:29 PM

I think solar CELLS could be incorporated into the bodywork of a car with zero drag penalty and almost zero weight penalty. I did some rough calculations a while back (sorry, I must have lost that evelope) and worked out that it was feasible to more or less replace the functionality of an alternator by incorporating about 400w of solar cells. (Or was it 300w?) Solar cells have a life of roughly the same as the life of a vehicle (20 years+?) so building them into the paint layer at manufacture kind of makes sense. Obviously the flatter panels are easier to deal with (roof, bonnet, hood...) but individual cells can be incorporated into curved bodywork far more easily than even 'semi-flexible' ready-made solar panels. (And how thick are solar cells? They weigh almost nothing.)

They could be used to charge a deep cycle battery which in turn charges the starter batttery (or starter super capacitor?) via a DC/DC converter/charger. That way the starter battery stays fully charged all the time and the 'donor' battery can be at any state of charge between flat and full, depending on sunshine levels, levels of use, length of journey, the time since last mains plug-in, etc.

I say, "more or less replace the alternator". I don't think you can do without it completely if you still want to be able to drive unlimited distances at night, but it would remain switched off for 90% of the time or more for most people. The alternator could then be thought of as a back-up generator, and could perhaps be coupled via a clutch pulley such as is used to engage an air conditioning compressor.

I'm currently working on a 'donor battery' system for a 2002 diesel Golf (the battery or batteries to reside in the spare wheel well) and although I haven't got any immediate plans to use solar cells for charging, and will be relying on a pair of 10A mains chargers for the foreseable future, I do really like the idea of a predominantly solar-powered vehicle electrical system. At some point I may mount a pair of 20w semi-flexible solar panels on the inside of the rear side glass in the load area. (It's an estate/wagon and the driver can never see through those panes anyway.) I'd have one panel charging the starter battery and the other charging the donor battery.

I am currently using a donor battery with a DC/DC converter/charger but haven't got it properly installed yet. (The test battery, converter and the various volt/amp displays are all bundled together in the front passenger footwell like some doomsday device ticking its way towards zero hours. I dread to think what the Police will make of it next time I'm pulled over for driving suspiciously slowly. Hopefully it will all be tucked away safely in the spare wheel well soon.)

I haven't worked out yet quite how to automate the switching, but I reckon the judicious use of voltage sensitive switches could control the whole set-up, including shutting off the converter (at 12.2v?) to protect the donor battery from excessive discharge, and also determining the starter battery voltage at which the alternator finally kicks back in.

At present I have a 90Ah AGM starter battery, charged at night off the mains, and I expect to fit a 110Ah AGM in the rear as a donor. Taking both batteries down to no further than 50% State of Charge, I reckon I'll get between 3.5 and 12 hours driving time depending on time of day/night, raining/not raining, etc. With just the one battery I'm currently getting between 1.5 and 5 hours alternator-free drive time. Not using the alternator seems to have put my overall average mpg up from about 70mpg (Imperial) to about 80mpg.

With a 'donor battery' system, even if the alternator kicks in on a long, rainy, dark night drive, it will never charge the donor battery itself, so a good amount of fuel will still have been saved on that journey, just as on shorter journies.

I think ultimately a small light starter battery or super capacitor for cranking the engine, combined with a large deep-discharge battery (AGM? LiFePo4?) as donor is the way to go, but that would be expensive: I would then need a much more powerful DC/DC converter (30A? 40A?) necessitating the running of dedicated high current wiring, ...and I can't afford a LiFePo4 donor battery.

So I'm compromising and going with the two large AGM batteries - one as starter and one as donor. I'm finding that a little 12A DC/DC converter/charger works really well, as I want to spread the load across both batteries. 12A is so low that I can safely use the existing wiring that goes to the rear 12v power outlet, which is permanently connected to the starter battery and is rated (fused) at 30A. Some days (or nights) the starter battery takes more strain than the donor, and some days the donor battery maintains the starter battery fully charged at between 13v and 14v the whole day, but overall it seems both batteries are working well within their 'comfort zones' for 95% of the time, so both should hopefully last a reasonable number of years.

04-10-2014, 07:29 PM	#17 (permalink)
paulgato EcoModding Apprentice Join Date: Sep 2013 Location: Oxford, UK Posts: 126 Black Beast - '02 VW Goff Estate S Thanks: 27 Thanked 51 Times in 31 Posts	I think solar CELLS could be incorporated into the bodywork of a car with zero drag penalty and almost zero weight penalty. I did some rough calculations a while back (sorry, I must have lost that evelope) and worked out that it was feasible to more or less replace the functionality of an alternator by incorporating about 400w of solar cells. (Or was it 300w?) Solar cells have a life of roughly the same as the life of a vehicle (20 years+?) so building them into the paint layer at manufacture kind of makes sense. Obviously the flatter panels are easier to deal with (roof, bonnet, hood...) but individual cells can be incorporated into curved bodywork far more easily than even 'semi-flexible' ready-made solar panels. (And how thick are solar cells? They weigh almost nothing.) They could be used to charge a deep cycle battery which in turn charges the starter batttery (or starter super capacitor?) via a DC/DC converter/charger. That way the starter battery stays fully charged all the time and the 'donor' battery can be at any state of charge between flat and full, depending on sunshine levels, levels of use, length of journey, the time since last mains plug-in, etc. I say, "more or less replace the alternator". I don't think you can do without it completely if you still want to be able to drive unlimited distances at night, but it would remain switched off for 90% of the time or more for most people. The alternator could then be thought of as a back-up generator, and could perhaps be coupled via a clutch pulley such as is used to engage an air conditioning compressor. I'm currently working on a 'donor battery' system for a 2002 diesel Golf (the battery or batteries to reside in the spare wheel well) and although I haven't got any immediate plans to use solar cells for charging, and will be relying on a pair of 10A mains chargers for the foreseable future, I do really like the idea of a predominantly solar-powered vehicle electrical system. At some point I may mount a pair of 20w semi-flexible solar panels on the inside of the rear side glass in the load area. (It's an estate/wagon and the driver can never see through those panes anyway.) I'd have one panel charging the starter battery and the other charging the donor battery. I am currently using a donor battery with a DC/DC converter/charger but haven't got it properly installed yet. (The test battery, converter and the various volt/amp displays are all bundled together in the front passenger footwell like some doomsday device ticking its way towards zero hours. I dread to think what the Police will make of it next time I'm pulled over for driving suspiciously slowly. Hopefully it will all be tucked away safely in the spare wheel well soon.) I haven't worked out yet quite how to automate the switching, but I reckon the judicious use of voltage sensitive switches could control the whole set-up, including shutting off the converter (at 12.2v?) to protect the donor battery from excessive discharge, and also determining the starter battery voltage at which the alternator finally kicks back in. At present I have a 90Ah AGM starter battery, charged at night off the mains, and I expect to fit a 110Ah AGM in the rear as a donor. Taking both batteries down to no further than 50% State of Charge, I reckon I'll get between 3.5 and 12 hours driving time depending on time of day/night, raining/not raining, etc. With just the one battery I'm currently getting between 1.5 and 5 hours alternator-free drive time. Not using the alternator seems to have put my overall average mpg up from about 70mpg (Imperial) to about 80mpg. With a 'donor battery' system, even if the alternator kicks in on a long, rainy, dark night drive, it will never charge the donor battery itself, so a good amount of fuel will still have been saved on that journey, just as on shorter journies. I think ultimately a small light starter battery or super capacitor for cranking the engine, combined with a large deep-discharge battery (AGM? LiFePo4?) as donor is the way to go, but that would be expensive: I would then need a much more powerful DC/DC converter (30A? 40A?) necessitating the running of dedicated high current wiring, ...and I can't afford a LiFePo4 donor battery. So I'm compromising and going with the two large AGM batteries - one as starter and one as donor. I'm finding that a little 12A DC/DC converter/charger works really well, as I want to spread the load across both batteries. 12A is so low that I can safely use the existing wiring that goes to the rear 12v power outlet, which is permanently connected to the starter battery and is rated (fused) at 30A. Some days (or nights) the starter battery takes more strain than the donor, and some days the donor battery maintains the starter battery fully charged at between 13v and 14v the whole day, but overall it seems both batteries are working well within their 'comfort zones' for 95% of the time, so both should hopefully last a reasonable number of years.