My silly battery charging idea

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Peter C in Canberra
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My silly battery charging idea

Post by Peter C in Canberra » Fri, 29 May 2009, 20:17

Can we all join in this game?
I'm just speculating idly here.
I know it makes more sense to put solar panels on the roof at home than on the car, and I do have them on the roof at home.
However, if I were to have some in/on the car and wanted to charge the main traction battery....
I could put some panels in series to get up to 144V and let it trickle charge whenever the sun was out. However, that means I would have high voltage connections and wires outside the car so making it safe could be a hassle. Alternatively, I could have a bunch of panels paralleled making only 12V or so. Can you get DC/DC converters that go the opposite way to what we usually have? I have a converter that takes 144V volts DC to 13.6VDC and I know you can get inverters to make 120V or 240V AC with more or less sine-like output. However, can you get a DC output you could connect directly across the traction battery? I am thinking perhaps a lower quality US voltage inverter might make something more like square wave output and some rectifying diodes might not lose too much and give about the right DC volts if that were feeding the traction battery.
Thoughts?
Thanks,
Peter.
Daihatsu charade conversion 2009-18, iMiEV 2013-2019, Holden Volt 2018-present, on the ACT's 100% renewable electricity. Kona on order.

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Johny
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My silly battery charging idea

Post by Johny » Fri, 29 May 2009, 20:47

It seems to me that just doing the series thing and using double insulated cable for the 144V would be the simplest. It's common to run 240 VAC extension cords through the rain.

However, for a scalable system, running 12V from the solar (mean you could add more as required) and a 500W 12V to 240VAC inverter, while inefficient, would let you interface directly with your on-board charger.
As long as the "Modified Sine Wave" output (errhem) didn't blow up your charger.

Unless solar panels have improved a lot since I last saw them, this is a bit of a waste of money and effort. You would be lucky to collect one kilometer worth from a (car's) roof load of panels in a day (IMO).

Edit: Added (car's)
Last edited by Johny on Fri, 29 May 2009, 10:47, edited 1 time in total.

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weber
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My silly battery charging idea

Post by weber » Fri, 29 May 2009, 21:19

Johny wrote:Unless solar panels have improved a lot since I last saw them, this is a bit of a waste of money and effort. You would be lucky to collect one kilometer worth from a (car's) roof load of panels in a day (IMO).

It's not quite that bad. I'm installing 5 Sunpower 210 W PV modules tomorrow (on a house) which together cover a car-sized area of 1.5 m x 4 m and in Brisbane will average 4 kWh per day. About 25 km worth of EV travel per day for 25 years, for a little under $10,000.
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Johny
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My silly battery charging idea

Post by Johny » Fri, 29 May 2009, 21:26

Thanks weber, better information than mine. You would have to have some kind of roof rack arrangement for that many. But at least for one panel you might get about 800Wh or 4 to 8km for about $2500.
Dunno...

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My silly battery charging idea

Post by aversion » Fri, 29 May 2009, 21:34

Thinking a bit further down the track, perhaps it might be possible to use polymer or dye-sensitized solar cells as a window tint (providing they meet window tinting regulations). It might a cheaper and lighter alternative to sticking a large panel to your roof racks Image

Peter C in Canberra
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My silly battery charging idea

Post by Peter C in Canberra » Fri, 29 May 2009, 21:57

I think if I were to be serious about this, and I don't think I will be, I would be looking at the flexible sort of solar cells or the sort that get embedded in resin over the top of the solar challenge-type recumbant bicycles. I would think those would add the least to weight and wind resistance and might go over most of the roof and bonnet. However, the killer argument is that the same money would buy more output in a conventional panel on the roof of the house at home and leave some over for an extra few cells of storage in the car. Still, it's fun to speculate and people keep asking me about solar panels for the car. They seem disappointed when I say I have panels at home, the power is sold into the grid and I charge from the grid. I can understand that a tighter coupling seems somehow more satisfying.
I agree some portable panel plus 240VAC inverter would be easiest to implement, if least efficient.
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weber
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My silly battery charging idea

Post by weber » Fri, 29 May 2009, 22:28

Peter C in Canberra wrote:I agree some portable panel plus 240VAC inverter would be easiest to implement, if least efficient.

No. Easiest to implement is enough panels in series to give enough voltage to charge the battery directly. But you must allow for when the panels are hot and their voltage is significantly lower than the 25^C max-power-point voltage given in the specs.
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acmotor
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My silly battery charging idea

Post by acmotor » Fri, 29 May 2009, 22:54

I ran a 5W PV panel on the red suzi for a while to charge the 12V aux battery.
Everyone thought it powered the vehicle.
Peter, it saves having to answer the 'why don't you fit a solar panel' question. Most people don't know how to do the sums anyway.

Image

I can charge the traction battery directly with 48V of PVs but they would be worth more than my battery pack !

I did remove the roo bar to reduce weight and wind drag !
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Paul9
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My silly battery charging idea

Post by Paul9 » Fri, 29 May 2009, 23:20

I would welcome more silly ideas - you never know when two silly ideas put together could work! Your idea sounds slightly silly whereas mine may have been in the fairly silly category.

I have a property in the Blue Mountains west of Sydney which is solely powered by solar panels and a wind turbine. After watching the first system for the house being installed I have self installed two further systems for the sheds on the place. One of these systems recharges my electric "Envirotrike" (48v DC, three wheel, motor-bike-front/ute-tray from China).

If money is no object then you can go ahead and put panels on the roof to charge the main battery bank though I can see no way of getting more than a kilometre or two per day. The restriction I see is matthematical.

You will not be able to put more than 200 watts of panel on the roof. One of my house panels is 180watts (12v) and I doubt it will fit on the roof of the Suzuki Swift my mate and I are starting converting. Two 180watt panels definitely would not fit on a Swift's roof.

If you are doing a 144v dc conversion then 200watts/144volts = 1.25amp hours. If 4 hours of direct sun per day then 1.25 x 4 = 5 amphours per day. If, say, 100 amp hours capacity in battery bank then you are increasing your range by 5/100 = 5%. And none of these calculations alow for converter losses.

My mate and I have not however discounted doing something similar. Our reasoning is that a DC-DC converter may cost $300 - $400 (somebody correct me if I am wrong as I only have one quote so far) whereas you can get a 20 watt or 30watt (12v) panel for the same price. This panel feeds, say, 2.5 amphours into the auxillary battery your donor vehicle came with. It runs low voltage auxillaries (hence my questions about CFL's) and leaves the traction batteries for moving the vehicle.

Just my two bobs worth
Regards
Paul


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My silly battery charging idea

Post by Nevilleh » Sat, 13 Jun 2009, 20:01

The energy density from the sun at the earth's surface, at midday, summer and at the equator is approx 1 kw/sq m.
I measured the roof top of a medium car and I don't think I could fit a solar panel bigger than 1m x 1.2m for 1.2 sq m of collector area. Commercially available solar panels can reach 20% efficiency, although I believe efficiencies as high as 40% have been achieved in lab tests. So my 1.2 sq m of solar cells can produce 240 watts of power under the above conditions.
As you depart from the equator, the energy drops off and at 35 deg South its about 80%, so the max output is down to 192 watts. You might get near to this from 10 am to 2 pm say 4 hours and allow a bit extra for the rest of the day, but realistically you could expect about 4 x 192 = approx 800 watt hours for the day.
I could make a converter that would charge 144v batts direct from the solar panel output with an efficiency of better than 90%, so I could reasonably expect to put about 3/4 of a unit of electricity back into my battery pack in a day, provided the weather is nice!
The 45 cell, 90 AHr TS battery pack stores 45 x 90 x 3.2 = 12960 watt hours, or 12.96 units of electricity, so I need to park in the sun for 12.96/.75 = 17 days to get a full charge!
If you designed a car to run off solar power, you could conceivably cover the entire top surface with solar cells and that could be 1.7m x 4.5m = 7.65 sq m which would give 6.3 times as much output as my "medium car" roof described above. And that is about 4.8 units of electricity in a day. Enough to propel a very light, aerodynamically efficient, low rolling res. vehicle along the road at maybe 70 or 80 kph. Which is about what the solar powered vehicles that have been built so far manage to achieve.
Let's face it, you just don't get enough energy from the sun to propel a realistic sort of car and the cost of getting the little bit that can be got is pretty high!
edit
A further thought: If I had a stack of solar cells at home and they produced power for charging my EV, how much area would I need? I'd like 20 units of electricity per day to run my Town Car. My 1.2 sq m of panel produced .75 units, so I'd need 32 sq m of solar cells. I wonder what the cost per sq m is?
Last edited by Nevilleh on Sat, 13 Jun 2009, 10:16, edited 1 time in total.

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acmotor
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My silly battery charging idea

Post by acmotor » Sat, 13 Jun 2009, 20:13

I usually try to park in the shade. Image
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