# Solar Charging



## sailfish11 (Sep 15, 2007)

I'm re-doing my EV, putting in all new components. My DC-DCis going to be the Delphi 2.2KW. One of its unique abilities is that it can run backwards to provide low current charging to the HV pack.

I'm imagining hooking up one or two 100W flexible solar panels and setting them on my roof while the car sits in the parking lot at work all day. That might only get me an extra mile or two of range, but hey, it would be cool!


I'm wondering if anyone has experience using this feature.


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## Paul9 (Oct 2, 2015)

I haven't had any experience with DC converters which can charge back to the HV pack. 

I have, however, put a 90watt flex solar panel on the roof of my Suzuki Swift/Geo Metro. I wired the panel and the DC converter through a regulator into the auxillary battery. I can turn the DC converter off and on by means of a switch on the dashboard. Since doing so I have only used the DC converter 2 or 3 times a year as the panel puts enough into the aux battery to keep it topped up.

Being a retired accountant I am a bit keen on numbers so I kept detailed records of the power drawn by the accessories. Power draw from the HV pack is reduced by between 3% and 7% due to not using the DC converter giving me around 3kms to 7kms extra range.

People doing a fresh conversion could consider the possibility of dispensing with the dc-dc converter completely, particularly if they are in a position to have the car (and thus the panel) in the sun all day at work. On the rare occasion the aux battery needs a supplementary "boost", a 12v battery charger would be fine for that purpose.

The flex solar panel cost me $199 Aus, the 12v battery charger (which I already had) cost me about $40Aus and even though I spent $300 on the regulator (I wanted one with plenty of digital readouts), the cheap regulators at the same shop cost about $50Aus. The cheap regulators just had three led lights to indicate low voltage, being charged and battery full.

Just something for you to consider.

Cheers
Paul


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## Sunking (Aug 10, 2009)

Cool factor is about all you will get. Actual power you can get is pretty much meaningless and a lot of money for nothing. 

The real challenge which there is no product on the market for this? How are you going to convert a Low Voltage Current Source from the panels into a High Voltage Current Source for the traction pack? All Controllers on the market are Buck Converters made for 12.24, and 48 volt batteries. You will need a custom made Boost Converter and mo money to waste.

Cool huh?


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## Moltenmetal (Mar 20, 2014)

Wow- $995 usd for a DC/DC converter...straight up waste of money in my opinion, but to each their own. I did read the blurb- for some bizarre reason it will do exactly what you're talking about, ie both buck and boost conversion, ie bidirectional into and out of the 12 V battery. Sure, you can put a simple charge controller on the solar panels, connect to the 12V battery and hence to the DC/DC and it will dump surplus charge to the main pack. I'd do what the other poster did and ditch the DC/DC entirely, using the panels to recharge the 12 v. Use a large enough 12v battery and you will get perfect isolation and save a grand in the offing- minus the cost of a small 12 V charger for rainy days.


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## sailfish11 (Sep 15, 2007)

I'm getting the Delphi for $350. I hadn't thought about foregoing the DC-DC altogether, but I think I'll keep it just the same.

As others have said, to each their own. I'm an early-adopter kind of guy. My philosophy with this build (and with life in general) is to push the envelope a little. Not too crazy far out on the bleeding edge, but a little. 

My plan would be to use one, maybe two, 100w flexible panels. If even one were hooked up only to the 12v, it would top that off pretty quickly. If the Delphi has the ability to put any excess energy into the main pack, why not do it?

If I hooked up two panels, over the ~9 hours the car is in the lot, I might gain ~1kw (I live in sunny San Diego). Admittedly that's not a lot, but it's not meaningless either. My commute to work is only about 8 miles round trip, so if that's all the driving I do in a day (which is common), the panels could amount to a 35-40% reduction in my recharging needs.

~20kw per month equates to about $5 (at SDG&E rates), or about $60 per year. That's a pretty slow rate of return on my investment. I know. But then there's always the cool factor


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## Sunking (Aug 10, 2009)

sailfish11 said:


> If I hooked up two panels, over the ~9 hours the car is in the lot, I might gain ~1kw (I live in sunny San Diego). Admittedly that's not a lot, but it's not meaningless either.


There is where you are DEAD WRONG. You are guilty as most are fooled into thinking Sun Shine = Sun Hours. Wrong Wrong Wrong, dead Wrong. 

A panel does not produce its rated power if exposed to sun light. In fact it never generates its rated power. If the panels are fixed, oriented due Solar South, with Optimum tilt angle, absolutely no shade, from Dawn to Dust in San Diego July equals 5.9 Sun Hours. In January down to 2.9 Sun Hours. If you had a 2-axis Tracker that can point the panels directly at the sun from Dawn to Dusk you are up to 7.2 and 3.4 Sun Hours respectively. Guess what? You cannot do any of that with Panels mounted pointed straight up with no south orientation and 0 degree tilt. 

Now let's talk system efficiency. I assume you will use a PWM Charge Controller. If so those panels rated at 100 watts had better have a Vmp of 17 to 18 volts. With a PWM Charger *Input Current = Output Current*. So a 100 watt panel with a Vmp = 18 volts means Imp = 5.5 amps. Two of the 100 watts panels in parallel = 11 amps. How much power is 11 amps into a 12 volt battery? Can you say 130 watts from 200 watt panels that will never generate 200 watts or 11 amps? that is 66% efficiency. From panels to battery the best efficiency you can get with PWM is 50% assuming the Vmp is 17 to 18 volts. If Vmp is higher the efficiency goes lower. 

If those flexible panels have a Vmp of say 25 volts you are down to 4 amps per panel or 8 amps total into a 12 volt battery. 

At best in July you will get 4 Sun Hours x 200 watts x .66 = 530 watt hours. Oops for got to talk about the battery charge efficiency and wiring losses. That wil knock you down to 500 watt hours at best. 

Do we need to talk about conversion losses from 12 volt to your Traction Battery? All that money and extra weight for 1 mile-day. 

You are not saving any energy or lower emissions one little bit. Nor are you saving any money vs just buying that power from SDGE. 

Sorry to bust your chops, but I design Solar Systems for a living and have built hundreds of Off-Grid Battery system. I have even talked more people out of taking anything off-grid because of two simple facts. There is no such thing as either EROI or ROI. The power generated will cost you 4 to 10 times more than buying the power, and you become a heavy polluter. The only reason to take anything Off-Grid is if there is no other option available. Battery power is going to cost the user about $1/Kwh today and even more later with battery replacement cost. 

So be careful what you ask for. You are drawing false conclusions based on false assumptions.


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## sailfish11 (Sep 15, 2007)

Thanks for keeping me honest on the numbers Dereck - this is something about which you've obviously got a lot of knowledge and passion.

I'm using the Delphi anyway, so my original point was to double check my understanding on its capabilities. It sounds like it has the ability to do what I want, so the only decision is whether I want to take advantage.

Paul has confirmed from his experience that a 90w panel extends his range.

As I said above, I know the ROI numbers don't support the concept. Psychologically, I gain benefit from doing it.

To each his own.


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## Duncan (Dec 8, 2008)

Hi Sunking
Solar panels ROI?
My system has only been in for two months - and those are autumn/winter here - but so far it looks like about 10% return - that is a LOT better than putting the money in the bank (less than 3%) - even better than borrowing the money - 4.7%

I won't know completely until it has been operating for a year

- there is no (zero) subsidy here and the electricity company only pays 1/4 the normal rate for power I "return"


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## Sunking (Aug 10, 2009)

Duncan said:


> Hi Sunking
> Solar panels ROI?


 Not Off-Grid in the USA. Battery cost alone is going to cost some 50 to 75-cents per Kwh in constant battery replacement cost. Here in TX I pay 7.6-cents per Kwh delivered for the first 2500 Kwh in a month. Over 2500 Kwh per month cost drops to 6.9-cents. 

Grid Tied you can have a ROI depending on where you live, subsidies, Net Metering Laws, and local electric rates.


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## bigmotherwhale (Apr 15, 2011)

Sunking said:


> Cool factor is about all you will get. Actual power you can get is pretty much meaningless and a lot of money for nothing.
> 
> The real challenge which there is no product on the market for this? How are you going to convert a Low Voltage Current Source from the panels into a High Voltage Current Source for the traction pack? All Controllers on the market are Buck Converters made for 12.24, and 48 volt batteries. You will need a custom made Boost Converter and mo money to waste.
> 
> Cool huh?


did you even bother reading the post that you are commenting on? he already has a bi directional DC - DC.

I would personally buy a quantity of bare solar wafers and attach them to the roof, those flexi solar cells are expensive, not very efficient and have a very poor lifespan. I dont think its a bad plan to use solar cells on EVs it wont give much but at least it will give something and your batteries wont ever be dead regardless of parasitic loads.


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## Sunking (Aug 10, 2009)

bigmotherwhale said:


> did you even bother reading the post that you are commenting on? he already has a bi directional DC - DC.


Sure did. What about you. Just how are you going to prevent the 12 volt system from dumping all its power to the HV pack over discharging the 12 volt system, or vice versa? Fine if you want to keep a small 12 volt system charged up, but it is a fart in a hurricane on the Traction Pack. Even if you could make it work is not worth the money or time invested. 

But when it comes right down to it. I don't give a crap. Like anyone vested in the biz, I am more than happy to take your money so you feel better and look cool.


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## bigmotherwhale (Apr 15, 2011)

Sunking said:


> Sure did. What about you. Just how are you going to prevent the 12 volt system from dumping all its power to the HV pack over discharging the 12 volt system, or vice versa? Fine if you want to keep a small 12 volt system charged up, but it is a fart in a hurricane on the Traction Pack. Even if you could make it work is not worth the money or time invested.
> 
> But when it comes right down to it. I don't give a crap. Like anyone vested in the biz, I am more than happy to take your money so you feel better and look cool.


http://media3.ev-tv.me/DelphiUserManual.pdf

"• Bi-directional capability supports charging of high-voltage battery from low voltage sources such as
solar panels."

Its controlled via CAN. 
Take the time to read the post or browse the manual then you will be able to offer more accurate advice regarding this.


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## sailfish11 (Sep 15, 2007)

Sunking, as I've said in almost every post, I'm not trying to justify this on ROI alone, but for ROI purposes, why compare this to an off-grid system? The biggest cost there is the batteries, which we already have. 

I get that there's some trade off with flexible panels, but I'm not planning on mounting the panel permanently, so my thought is it would be easier to store (eg in the trunk) when not in use. 

I see buying a flexible panel as the short-term (1-2 years) proof-of-concept phase. If it works, I may try permanently mounting some small cells in the convertible top. EVTV sells them, but you can find them cheaper elsewhere. 

For what it's worth, the guy helping me convert the car agrees with Sunking that it's not worth the hassle. It's just hard for someone like me to know the car has this capability, and not try to take advantage of it.


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## baerfoot (Jan 2, 2011)

My idea for using solar panels to charge my traction pack is to match charge voltage of the pack to the output MMP voltage of a group of panels. That way I avoid having to use any DC/DC converters. I have actually already done this while working at a solar company. I used 2 panels which had MMP of 45 volts in series to give 90 volts MMP. My pack has 25 cells so during most of the charge cycle draws 86 volts. 










I was thinking of using 5 of the flexible solar panels wired in series with the right voltage. Any reason why this could not work?


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## Sunking (Aug 10, 2009)

baerfoot said:


> My idea for using solar panels to charge my traction pack is to match charge voltage of the pack to the output MMP voltage of a group of panels. That way I avoid having to use any DC/DC converters. I have actually already done this while working at a solar company. I used 2 panels which had MMP of 45 volts in series to give 90 volts MMP. My pack has 25 cells so during most of the charge cycle draws 86 volts.
> 
> I was thinking of using 5 of the flexible solar panels wired in series with the right voltage. Any reason why this could not work?


Could be a whole lot wrong with your thinking.

How much voltage does it take to charge say a 4S LFP battery? If you said 15 volts you would be wrong. It take at least 17 to 18 volts to charge any 12 volt battery. Does not matter if it is Lithium or lead acid. You have to overcome conversion, resistance, and voltage losses. 

Another trick question. If you have a 100 watt 36-cell solar panel with a Vmp of 18 volts and Imp 5.5 amps. What is the maximum Charge Current the panel can generate? Answer is either 5.5 or 8.3 amps. Do you know why?

Answer depends on what charge controller you used. If you used a PWM controller you only get 5.5 amps. With PWM *Input Current = Output Current* Now if you had used MPPT then *Output Current = Wattage Input / Battery Voltage* or 100 watts / 12 volts = 8.3 amps. With PWM you have turned your 100 watt panel into 65 to 70 watts. With MPPT you can get up to 95% of the power from the panels to the battery with proper wire and voltage management. 

Another thing you might be over looking is Battery panels are extremely expensive. A battery panel is a 36 cell panel and are antiques along with their PWM cousins. A battery panel can cost up to $5 to $6/watt with $2/watt typical. GT panels are around $1/watt. If you use a inexpensive PWM controller, you must use a very expensive battery panel. Example say a 200 watt GT panel versus 2-100 watt battery panels. A 200 watt GT panel has 54 cells with a Vmp of 27 volts and Imp of 7.4 amps. If you used a PWM controller on a 12 volt battery you have 7.4 amp of current @ 90 watts out of a 200 watt panel. Use a MPPT and you have 16.7 amps.. For PWM you would have to use the 2-100 watt panels in parallel to get just 11 amps. Not only does the 200 watt PWM system cost more, but less power. It takes a 300 watt PWM system to equal a 200 watt MPPT system. A 300 watt PWM system will cost you roughly $700. A 200 Watt GT MPPT system will cost you $400. 

The fun does not stop there. PWM also means low voltage ratio. If you operate say a 12 volt battery, you panel voltage must be 17 to 18 volts, So say a 1500 watt panel at 17 volts is 80 amps. That is some big ass expensive heavy wire. Or you could run say 1000 watts at 600 volts or lest than 2-amps on the input of the controller using very inexpensive GT panels. You get the same 80 amp output but you shave 40% of the weight, and 60% of the cost. 

So does your idea work? Maybe, I don't know or care if it does. Not my problem to design or figure out. That would cost you big bucks to figure out for you. Most likely if it does work, is the most expensive and least effective way you could go about it based on what you have stated. Could work out the extra weight and drag of the panels can actually lesson your watt hours/mile efficiency shortening your range. That is what happens to a many a golf carts when they try it. All depends on the little details. 

Good luck.


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