# [Q] Battery trailer possible?



## Ziggythewiz (May 16, 2010)

Entirely possible. Likely not economical. For lithium to pay for itself, it needs to be used quite regularly. If you don't need it often enough to include it as part of the vehicle, you'll end up with lots of $$$ just sitting around most of the time.

Also, the solar on it would be awesome, though also not economical. I've calculated that with 40% efficient panels I could run my bug on 100% solar and never plug it in. Unfortunately, the best panels currently available are around 20%. With a trailer (which would likely add 25% weight/drag to the car) I could have more than double the power, so it would be doable with today's cells.

However, dragging around your investment whether in lithium or PV is probably not the safest thing to do as it will be more likely to get in accidents\dragged over curbs, etc. The best place for solar is staying put on a roof, and lithium would be best kept in your car.


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## Nic (Oct 27, 2011)

Thanks. I didn't think of the safety aspect. An accident totaling the trailer would be messy, and very expensive. I doubt my insurance company would like to cover a trailer with $1000's worth of batteries anyway. Also, my main thought for the solar cells would be for charging the batteries while the car is parked. I can't go with cells on the roof since I have a convertible, but I could load up the hood and trunk lid.


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## Ziggythewiz (May 16, 2010)

Yeah, practical or not I plan to cover my bug as soon as I have time/figure out how. 

Aside from the additional 2-3 miles I could get from the cells I can afford, I like the buzz factor. The only time anyone really asked about the car was the one time I had to charge away from home, so was sitting at a gas station for an hour. After adding a 5 watt solar panel to handle all the ACC charging it got loads of attention at work.


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## Nic (Oct 27, 2011)

Yeah, solar panels would definitely add some cool points. I have also thought about a propane powered turbine generator made out of a turbo. I read about those a long time ago and you can get some pretty good power out of them. Would be cool to fire it up and blast some fire out of the back of the car, hehe.


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## Ziggythewiz (May 16, 2010)

lol...Fire! The Vasimr ion engine that is planned to help power the international space station uses about 10x the power of my bug, so I figure with lithium that would be a doable amount to stuff in, and would provide around 450 hp and a rearward ion blast to escape pursuit.

About propane, have you seen this one? http://www.instructables.com/id/DIY-Plug-In-Hybrid-Car/?ALLSTEPS


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## dougingraham (Jul 26, 2011)

Nic said:


> I am just getting started with researching my DIY, so I am trying to plan out my needs. I am curious if it would be possible to have a removable battery pack light-duty trailer for making extended trips. I am specifically wondering if the power can be carried over a pair of large wires, or if there would need to be several wires. I would not be using lead-acid batteries, so that would keep the weight down. I am also curious how much charging ability I would have if I covered the trailer with solar cells. From what I have found, so far, it would almost be pointless to have solar cells on it because the wattage is so low. I haven't done a lot of research on solar cells, so I imagine there may be some out there that are significantly more powerful than what I have found.
> 
> My plans would be for a trailer around 4'x7'.


I get asked about adding solar cells all the time so I did some computations.

If your car takes 200wh to go a mile at 60mph that means 200 watts of solar cells would take your car one mile in an hour. 200 watts of solar cells is about half a sheet of plywood. So if you want to go 60mph you need about 30 sheets of plywood worth of solar cells. That will require a pretty good size trailer and if you have a trailer that size you no longer will get 200wh per mile. Its a case of diminishing returns. As for covering the car with cells, under the best of conditions you can get 20 watts per sq ft. Those conditions are cool temps, facing the sun, high noon. Since you cant angle your car you will never get them facing the sun. And at best only two sides will sort of face the sun so I am guessing you will see only 30% of ideal or 6 watts per sq foot. This means for a typical car it is pretty worthless. It would look cool though!

As for a trailer with batteries on it I figured I wanted a 350 mile range I would need a total pack size of 70kwh (assuming 200wh/mile). Since I have a 17.3kwh pack in the car the trailer would need a 52.7kwh pack. A pack that size is going to weigh a little over 1160 pounds. Now adding 1160 lbs plus a trailer to your car you are not going to get 200wh per mile anymore. It would probably go down to 300wh per mile which means your actual range would only be 233 to a 100% DOD and I am guessing this would be highly optimistic at best. There most likely would not be a lot of additional wind resistance because the trailer would be drafting but the additional rolling resistance would be there. Such a pack would cost $46000 at todays prices. A 30A level 2 charger would take over 11 hours to charge the car + the external pack.

Since this pack is just used as a range extender I didn't include the extra 20% needed to keep the cycle life high.

And yes you could use 4/0 cables to connect this to the car. I wouldn't use anything smaller than that as the runs will be pretty long.

So the idea of a trailer with a range booster battery pack is an interesting one but not very practical. I certainly would consider renting one for long trips if some company made such a thing available but owning one would be out.

A trailer with a 40hp motor generator set however would be more practical and it would most likely be able to add a small amount of charge to your batteries and maintain your highway speed at the same time. This would at this time be a much better deal than a huge bank of batteries. I wouldn't want to own one of these but I wouldn't mind renting one for long trips.

I have been thinking a small solar panel on the roof of the car to run a fan in the summer to keep the internal temp down would be a good idea. Thats about all I would expect to do with a solar panel. Draw air in through the battery boxes maybe.

Best Wishes!


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## Ziggythewiz (May 16, 2010)

dougingraham said:


> If your car takes 200wh to go a mile at 60mph that means 200 watts of solar cells would take your car one mile in an hour. 200 watts of solar cells is about half a sheet of plywood. So if you want to go 60mph you need about 30 sheets of plywood worth of solar cells. That will require a pretty good size trailer and if you have a trailer that size you no longer will get 200wh per mile. Its a case of diminishing returns. As for covering the car with cells, under the best of conditions you can get 20 watts per sq ft. Those conditions are cool temps, facing the sun, high noon. Since you cant angle your car you will never get them facing the sun. And at best only two sides will sort of face the sun so I am guessing you will see only 30% of ideal or 6 watts per sq foot. This means for a typical car it is pretty worthless. It would look cool though!


A couple things:

Unless you're a solar racer, the goal isn't to run directly off solar, just to charge enough while at work to get you back home (or possibly back to work again). So to accomplish that you don't need 30 sheets of plywood, just 1 per ~9 miles of commute (assuming 250 whs/mile).

Also, solar tracking systems only make about 10% more power than stationary PV, so by not having the ideal angle you won't lose 70% of your possible power, more like 10-20.

Of course, everything depends on your local sun conditions. Somehow I manage to get about 45 whs/day out of my 5 watt solar panel which is never ideally oriented.


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## dougingraham (Jul 26, 2011)

Ziggythewiz said:


> A couple things:
> 
> Unless you're a solar racer, the goal isn't to run directly off solar, just to charge enough while at work to get you back home (or possibly back to work again). So to accomplish that you don't need 30 sheets of plywood, just 1 per ~9 miles of commute (assuming 250 whs/mile).
> 
> ...


On a regular car I about the best arrangement would be to have the hood hinged at the front and cover it with your solar cells. Park facing south and tip the hood up at an appropriate angle for your latitude. This would probably give you between 12 and 16 sq ft of solar cells or 240 to 320 watts. In the winter in the continental US the number of sun hours varies between 1 and 5 hours (google Solar Insolation map). In Houston it would be around 3.5 hours. Where I live it is about 3. So if you have a largish hood with good placement you could hope on the average in the winter to get 1.12kwh from the panels about 80% of which you could probably get into the batteries. If you get 200wh/mile this would give you 4.5 miles of recharge. Covering the rest of the car would gain you very little and increases the charging complexity greatly. These are rough estimates and I suspect the actual winter average would be worse than this. As they say, "Your Mileage May Vary!"

I would not consider this where I live because we have at least one damaging hailstorm a year and you can't always get the car out of the weather. If you do everything yourself you might be able to cover your hood with solar cells and make a charger for $1000. That $1000 could buy enough electricity to drive 45000 miles.

I do think it would have a huge cool factor.


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