# Solar Golf carts, check my math



## heynow999 (Mar 2, 2008)

I am in the solar power business. I know a custom solar panel manufacturer who can make panels that fit on golf carts. What I want to calculate is how big of a panel would be needed to keep a golf cart charged all summer without having to plug it in. So I am going to throw out some numbers.

Lets say a golf cart takes 150 watts per mile and once around the course is 2 miles so we need to replace 300 watts.

We can put a panel up that can produce 200 watts/hr and in the summer in my area even laying flat it should produce about 860 watts.

It would seem to me that even if charging is %50 efficient, it should be able to keep the batteries charged for one round of golf each day.

Am I missing anything?

Thanks

Peter


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## major (Apr 4, 2008)

heynow999 said:


> We can put a panel up that can produce 200 watts/hr and in the summer in my area even laying flat it should produce about 860 watts.


Peter,

What's wrong with your statement?

I don't know the energy usage for a golf cart (watt hours/mile) and don't know how long a round of golf requires it to run. But golf carts, like the one we have at work, use six 6 volt batteries likely capable of about 200 ampere hours. That would be 7200 watt hours for the set.

A 200 watt solar panel would take 36 hours to produce an equivalent amount of energy.

I don't know how far discharged the batteries would be after one round of golf, but I think it would be more than a 200 watt array can supply in a day. But maybe we have some golfers here that can chime in 

Regards,

major


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## Sunking (Aug 10, 2009)

Yes there some problems. For one a trip around a golf course is more like 4 to 5 miles. Second problem is you are assuming the charge/discharge efficiency is 100% when in reality it is 66% at best.

Another factor is solar insolation or what we call a Sun Hours Day. Depending on what part of the country you are in, time of year, and weather conditions the sun hour day varies. Where I live (TX) in July, the peak Sun Hour Day is 5.2 hours.

So for a 200 watt solar panel the most usable power that can be dumped into a battery = 200 watts . 5.2 hours x 66% = 686 wh. That is from sunrise to sunset.

Third thing you are off on is wh/mile. Try 200 to 250 wh/mile


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## Guest (Sep 6, 2009)

1. Watts per sq foot of the panel .....................?
2. Square ft of space for mounting a panel ........?
3. Voltage output of the panel .................?
4. Amp output of the panel .................?
5. Voltage of the cart ..................?
6. Amps used driving the cart ................?
7. How to charge in the evening ............?
8. How to charge when overcast .............?
9. Will need to be able to plug in when nothing else will work.
10. Might consider battery swap system so one pack is charged while the other is powering the cart.
11. Any thing I am missing?


Pete


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## heynow999 (Mar 2, 2008)

About the panel, I have already calculated that a 200 watt panel laying flat in the Toronto area will produce on average 860 watts, may through september.

I think 200/250 wh/mile is a bit high. Doesn't a full size car use 250 wh/mile? Lets say 200 wh/mile but I will research that point.

So if a golf course is 5 miles long and we need 200 wh/mile, we will use 1000 watts. 1000 watts times .66 efficiency equals 1515 watts so I would need a 352 watt panel. I don't know how to show the math, but 100 watts would give you 430 watts.

That's a big panel. It would be a bit bigger than 2 sq meters. It is do-able but may be cost prohibative.


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## major (Apr 4, 2008)

heynow999 said:


> I am in the solar power business..........Peter


Hi Peter,

No offense intended, but if you're in the "power business", you really should get your units for power and energy straight.

Regards,

major


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## Sunking (Aug 10, 2009)

heynow999 said:


> I think 200/250 wh/mile is a bit high. Doesn't a full size car use 250 wh/mile? Lets say 200 wh/mile but I will research that point.


That is possible if using a very efficient 3-phase AC motor, but golf carts use DC series and shunt would low voltage motors.

I do a lot of stand alone solar system designs for telephone companies for their remote cell sites and a few commercial installations, and I do not see how this would be even feasible. In addition to the expense of the panels, you are more than likely going to have a custom made MPPT charge controller, not out of question, but made for the application to operate at 36 or 48 volt. You could go with a much less expensive shunt regulator, but that will cost you another 10 to 30% power loss across the regulator.

One other note; I don't know of any single panel that would provide you with a Vmp that would high enough to charge a 36 or 48 volt golf cart. You are more than likely to have to use two panels in series to get the Vmp up high enough.

One last thought a round of golf last about 4 hours, and if you are like most golfers like to hang out in the shade under trees. Your 4.3 Sun Hour day calculation to get 860 watt-hours assumes clear view of the southern horizon from sunup to sunset. IMO that is not a realistic assumption. 1.5 to 2 sun hours would be more appropriate.

It could work if you parked and left the cart outside with clear view of the sun all the time, and you do not use it everyday, but I wouldn't dare leave my cart outside exposed to the elements, I put too much money in it.

FWIW I have an EZ-GO fleet Series cart that has been modified. I upped the voltage from 36 to 48 volts by adding two more Trojan T-105's, replaced all the wiring from # 6AWG to 1/0, the reverse switch and solinoid, stock controller up to 500 amps, and the motor with a GE Raptor D-398. It will pop wheelies and go 30 MPH. I live in a gated resort community and use it on the golf course and as a NEV. 

The battery capacity in theory is 8 x 225 AH x 6 volts = 10.8 Kwh. Using crude measurements I can go about 15 miles to the 50% DOD point or 360 wh/mile. Before the modification around 320 wh/mile.


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## neanderthal (Jul 24, 2008)

I think it is a cool idea. But i definitely think it would be tough to get all your power from solar. If you built some sort of large solar sunshade/garage for them to help charge, that could be cool, or use panels to increase the reange of the cart and use less energy from the grid, i think it will be easier. 

And to help with your calculations, it is good to understand the units you are dealing with. Watts are instantaneous power. For example you can say my cart's motor uses 300 watts to drive at 8mph. Or my panel produces 200 watts in full sun. It is the electrical power that is happening at that instant. The amount of energy is called watt hours or wh. Say a 200 watt solar panel is in the full sun for 30min. It has produced 100 watt hours of energy. There is a god section on this forum that explains the units and the math and stuff. Learning that will help you tons with making your ideas work. good luck


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## ClintK (Apr 27, 2008)

Shoot you guys are being pretty harsh. He wants to charge a golf cart to go 2-5 miles a day, not one that goes 15 miles at 30 mph. Also, the actual capacity of the battery is nearly irrelevant.

I just did a quick search and found:
http://www.frugaldougalsgolf.com/solarroofpanel
120W panel with MPPT charger for 48V system, $1195
They claim 1 hour gives about 1 mile (maybe in Phoenix, but gives you an idea).
You may be able to get the parts cheaper from elsewhere though. sunelec.com has panels at $1.98/Watt.

I'm betting the rough calcs are worth a test. If you can afford it...
Measure how much power your golf cart uses for the round trip. If the number is reasonable, pickup a panel (or find one you can borrow) and measure how much power it produces in a day.


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## Sunking (Aug 10, 2009)

ClintK said:


> Shoot you guys are being pretty harsh.


No just real 



ClintK said:


> I just did a quick search and found:
> http://www.frugaldougalsgolf.com/solarroofpanel
> 120W panel with MPPT charger for 48V system, $1195


I looked at the specs, claims, FAQ of the site. What they have done is what I said earlier. They custom designed a MPPT controller to take the standard Vmp voltage (around 27 volts) of a 120 watt solar panel and up convert to switch selectable 36 or 48 volt systems.

If you read a little further, they only claim it is a range extender of about 25% and results will vary depending on location, time of year, terrain, and road condition. That I believe is an honest analogy about the best one can hope for.

I do not know about everyone else who owns a golf cart, but myself and folks around me who own them tend to use them almost everyday or every other day. Under those conditions, a solar panel cannot keep it up to 100%, and a charger will have to be used. And if folks are as proud as I am of my cart, it is not staying outside in the weather when not in use. 

So a range extender OK, replacement for the charger, I have serious doubts it would meet that need. I mean use the same analogy as an EV, you know you cannot charge them solely with a solar panel mounted to the car.


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## major (Apr 4, 2008)

ClintK said:


> the actual capacity of the battery is nearly irrelevant.


Hi Clint,

I used that as a benchmark. I said I didn't know how much of that charge was used for a round of golf.

But then again, why do they use that capacity of battery in those carts if they don't need it? And I realize that you don't fully discharge in order to get decent cycle life.

Hey, I got no problem with these golfers putting around on solar power. I wish they would. But I think the guy is off by a factor of ten. It's difficult to tell when he talks about watts/hr 

Regards,

major


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## Sunking (Aug 10, 2009)

FWIW with the newer carts with regen braking you can get about 35 miles off a full charge with new batteries. However you would never ever want to do that. With that you can get a rough idea of efficiency.

For example a EZ-GO PDS LE Freedom uses 6 Trojan T-105 batteries. They are 6 volt, 225 AH at the 20 Hour rate. So that gives you around 230 wh/mile.


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## kek_63 (Apr 20, 2008)

major said:


> ..... But then again, why do they use that capacity of battery in those carts if they don't need it? And I realize that you don't fully discharge in order to get decent cycle life..........Regards,
> major


Most golf courses expect their carts to be able to make at least 2 1/2 rounds (45 holes) each day. and that's with high school kids (with little or no EV experience) managing the cart rentals. The courses I know routinely use most of their cart's battery capacity.

Keith


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## tomofreno (Mar 3, 2009)

I can give some real data for solar panels. I have 26 nominal 215W panels mounted at about 18 degree angle with respect to a horizontal surface (slope 4 roof). On a clear sunny day in July they typically produce 40kWh +/- 2kWh energy, or 1538 Wh/panel. Maximum power output I have seen at around solar noon on a sunny day in July is 5300W or about 204W/panel, but more typical is about 4800 W. These numbers are on the AC side, after the inverter which has 96% efficiency. Location is at 4900 ft elevation in dry high desert atmosphere.

Based on the above, I would guess that during the summer months of June through August a 200W panel might supply about 1kWh/day on clear days if mounted horizontal and kept in full sun all day. If the cart consumes 200Wh/mile that would be enough to go about 5 miles.

Tom W.


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## Guest (Sep 7, 2009)

As long as the voltage is high enough for charging for that one panel. You can't charge a 48 volt pack at 24 volts. That is assuming the panel is rated at 24 volts output dc. Many are 12 or 24 volts out dc.


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## Sunking (Aug 10, 2009)

gottdi said:


> As long as the voltage is high enough for charging for that one panel. You can't charge a 48 volt pack at 24 volts. That is assuming the panel is rated at 24 volts output dc. Many are 12 or 24 volts out dc.


Gotti. not true if you use a custom made MPPT charge controller. A MPPT controller is a Switch-Mode DC-to-DC converter. It takes the incoming DC, converts it to high frequency AC, uses a transformer to step up or down, rectify, filter, and then regulates the voltage.


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## Guest (Sep 8, 2009)

True but converting to AC then back to DC to charge at a higher voltage will lower the charging amperage rate and you don't want to charge at a trickle charge. You do want to get the batteries to charge for the next round, not next week. 

Pete


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## heynow999 (Mar 2, 2008)

Yes, I understand that a panel is rated in watts, but I was trying to dumb it down a bit when I explained that the panel could produce 200 wh "per hour". I understand that you only have to say "watts" and that the "hour" is not neccessary. I just wanted to help make the calculation more clear as this is an electric car site and not a solar site. For example I have 2.54 kw of panels on my house, so in 1 hour it will produce 2.54 kw of electricity. (under perfect conditions)

About using a charge controller, as I mentioned from the start, I know a custom panel manufacturer who can configure a panel so that it puts out 1 volt and 130 amps or 130 volts at 1amp. We can produce a panel that would put out about 56 volts which would be good for charging a 36 volt system. We would only need a charge controller that would protect against overcharging. Not sure what we are going to do for that.

What I was mostly interested in, seeing this is an electric car site, is approximately how many watts/mile a golf cart would use. I could see it using a similar amout to a car as it is operating on grass and it does have tires that would have a high rolling resistance. As mentioned, it also has an inefficient 36v DC system and brushed motor. 

I thank everyone for thier input. I guess the jist of it is that it will not be possible for the 130watt solar panel I had envisioned to charge a golf cart enough to do even one round of golf per day. 

What it could do is act as a range extender, and it would help with battery life because the batteries be getting charged whenever there is sun so they wouldn't be left in a discharged state as often. 

I didn't mention it, but our market would be golf courses, not individual owners.


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## major (Apr 4, 2008)

heynow999 said:


> Yes, I understand that a panel is rated in watts, but I was trying to dumb it down a bit when I explained that the panel could produce 200 wh "per hour". I understand that you only have to say "watts" and that the "hour" is not neccessary. I just wanted to help make the calculation more clear as this is an electric car site and not a solar site.


Hi there heynow,

I wondered if you'd get back to us.

Please don't dumb us down. We, at least those of us likely to reply, understand the accepted units of energy and power. It does not matter if it is electric cars, solar energy or nuclear power. Scientific units and physics remain constant.



> For example I have 2.54 kw of panels on my house, so in 1 hour it will produce 2.54 kw of electricity.


I guess you still don't get it 

Good luck with your solar power, or energy, or whatever it is,

major


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## major (Apr 4, 2008)

heynow999 said:


> What I was mostly interested in, seeing this is an electric car site, is approximately how many watts/mile a golf cart would use.


Hi there again, heynow,

Caught you again. Or is this more dumbing down?

watts/mile ????? How many angels can fit on the head of a pin? What quantity is represented by the unit of watts/mile?

I really want to know.

major


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## Sunking (Aug 10, 2009)

gottdi said:


> True but converting to AC then back to DC to charge at a higher voltage will lower the charging amperage rate and you don't want to charge at a trickle charge. You do want to get the batteries to charge for the next round, not next week.


Well certainaly, but 100 watts is 100 watts minus the conversion loss of about 3 to 5%. The element of time remains the same which is the whole point of what we have been discussing. With a claimed solar insolation of 5 hours means a 100 watt panel will deliver about 340 watt-hours per day to the batteries once all the losses are taken into account, and that is only possible if the cart is exposed to the sun from sun up to sun rise without any shade issues.


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## Sunking (Aug 10, 2009)

heynow999 said:


> I understand that you only have to say "watts" and that the "hour" is not neccessary.


No that is not correct and the source of confusion. Watts or Power is the rate at which energy is generated or consumed at a given moment in time. Energy or watt-hours is the amount energy over a given amount of time measured in hours.


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