The New And Improved Unofficial R65 Forum V2
Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: MPKaier on May 11, 2008, 11:48:20 AM
-
Hi All,
My commute to work entails city traffic on the first leg, a small roll down the interstate, and then city traffic again, the same on the way home. Lots of time spent at idle at traffic lights keeps my GEN light on. I decided I would give solar a chance. I have installed two 12 volt solar cells on the back of my unused krauser tail rack. I got them off ebay for 25 bucks. In the sun they are generating 15 to 16 volts at 4 watts. Since my bike sits in the parking lot all day at work, I figure this should keep the battery charged up, an possibly keep me from having to use the battery tender. Has anybody else done anything similar? I was wondering if my theory will work? Seems like it should. Here are a couple of pictures. 8-)
-
Brilliant idea.
Follow-up news will be very interesting.
-
Good idea. Is it regulated somehow to prevent over charging the battery? I hope you're not seeing 16 volts at the battery....
When I saw the title of the post, I thought maybe somebody had converted to an electric bike. I wouldn't mind; I broke the 20 dollar mark filling up my R65 tank the other day.
-
Very cool.
-
Yes,
It is supose to have an internal diode that prevents back flow and discharge at night. I'm not an electrical expert by any means, I know enough about my digital meter to be dangerous. I do measure 13.4 volts at the battery when the solar chargers are disconnected and 13.8 when I connect the solar chargers. I was told by the supplier that they can be connected all the time without worry of overcharging.
When I measure the output from the solar panels themselves they are generating between 15 and 16 volts in full sun. Don't really understand exactly how charging works, or why the difference in volt readings, but it looks like the chargers are adding some voltage, and that given enough time, would likely charge the battery. The instructions do say that they are not capable of charging a dead battery, only maintenance. If anyone has any insight as to what exactly is going on, that would be great. I just did the solar project and have not ridden it to work yet.
Thanks,
mike
-
You're getting a lower voltage during charging, because the panels are connected to a 'load', at 13.8 volts, and 4 watts, thats around .29 amps going into the battery.
If your drive has the charge light on a lot, I don't believe you are going to overcharge your battery, if you have a serviceable type battery, keep an eye on the electrolyte level in the cells.
Interesting idea though, keep us informed on your experience with this installation.
-
It seems that with a load the solar panels' voltage dips enough to keep you out of trouble, but I agree with the others that you should keep an eye
on the electrolyte levels if it is a "wet" serviceable" battery.
A simple series diode in their design may prevent battery discharge, but it doesn't provide voltage regulation. Ideally there should be a basic voltage regulator
on the output of the panels to prevent the output voltage from going over 15V which could damage the battery. You may well get away without it when using these specific solar panels, but in a commercial design that could use multiple types of panels or panel designs would have one.
A simplified "matchbook" explanation of solar cell charging, if you are interested:
Solar cells with light shining on them produce an electric field within themselves which is somewhat self-regulating - with nothing wired up to the solar panels and impinging photons from the sunlight, the PN semiconductor junction in each solar cell has a maximum electric field potential as it tries to make donor electrons migrate across the junction from the N-type material to the P-type. Very, very,very little current results within the cell, as the electrons want to find an easier path without fighting the electric field. Once the output wires are connected to a load (you connect the wires of the solar cell to your battery) the electrons have another easier path to follow, and many of them do. However, this net migration of electrons in this easier path causes the electric field potential to decrease across the PN junction, which lowers the voltage that the solar cell generates. The more current that is directed through the output wires (more net electron migration) the lower the voltage of the solar cell goes. IF too much current is demanded by connecting too low of a resistance, or load, to the device, the voltage output drops to its minimum, which then can stop the output of current as the electric field isn't strong enough.
As long as the battery's effective resistance to this current (which is related to it charge level) isn't too low, then the output voltage of the arrangement of the solar cells in that panel is high enough to maintain the charging current to the battery. As the battery charges up and provides higher resistance to this electron flow, the solar panel's voltage starts to increase. But as soon as the voltage gets high enough to try to cause more charging current to flow to the battery again, as soon as the current begins to flow the solar cell's voltage goes down again - eventually an equilibrium point is reached. Once the battery's voltage equals the output voltage of the solar cell array minus the protection diode's voltage drop, the charging will effectively stop. The designers of those panels obviously chose the number and wiring arrangement of the solar cells so that they could be used in this type of application with minimal external circuitry. But, as time goes by and a number of cells become damaged/inoperable, the panel's ability to charge the battery will deteriorate, which a more sophisticated design could still accommodate and work effectively.
IF you tried connecting a dead (0V) battery to the solar cell, that would essentially create the situation described above with too low of a resistance - the solar cells try to output too much current, which causes the internal electric field to degrade, lowering the output voltage and stopping the output current. It cannot keep up with the demand, because of the characteristics of the device, and thus cannot charge the dead battery. The voltage you measured (13.8V with solar panels connected) is higher than the disconnected battery (13.4V) voltage indicating that current is flowing into the battery, charging it up.
Hopefully the above rambling makes some sense.
-
Dang! I actually followed that! :o
I really like this idea. Seems to me that it will work. Let us know how it goes.
-
Thanks,
What a great resource of information. I actually followed it too! I will keep a post of my results.
Thanks,
Mike
-
There lurks an electrical wizard up in NH.
I'm thinking of questions already. Might this be moved to the "technical" area?
-
As no one has taken the time to mention, That is a very nice looking R65!
The solar stuff is interesting too, of course.
::)
Ed
-
Hi All, *UPDATE*
Thanks Ed for commenting on the bike. I have rode to work with my solar panels, and brought my volt meter to work to read the volts. When I left the house this morning I had 13.44 volts at the power socket. I had the battery tender on, so I expected pretty good voltage. When I got to work, after hitting every light and being stuck in some pretty heavy traffic 34 minutes to go 12.5 miles, the volts read 12.88. I parked the bike in the sun, measured the volts when I left and had 13.2. So some charging did occur or would this have occurred naturally with the warming of the battery in the sun anyway? I had a good brisk ride home, but still hit several lights before getting home. Once at home measuring the volts showed me 12.88. Interesting, exact same as when I got to work. So my thinking is that the real test might be to not use the solar charger during the day, and take the same interval of readings as a comparison. In any event, I still like having a full charge so I put the battery tender on for the evening.
So the jury is still out.
Thanks,
Mike
-
That is a gorgeous R65! The solar is a great idea and I am very curious to hear if it works out for you.
Bill
-
I relly like this idea, makes you wonder why no ones thought of it before
-
... I broke the 20 dollar mark filling up my R65 tank the other day.
Those were the days... I broke the 45 dollar mark filling my R65 tank the other day! >:( >:(
Going solar all the way might be the solution ;)
greetings from a sunny north with excellent riding conditions
trolle
-
Hey Mike! How is the solar experiment going? Is it continuing to work well? The jury was still out on your last post.
Really interested in the outcome. I'd like to set up something like that if it works.
-
The solar power is working amazingly well. The battery has been staying fully charged. When the bike is not being used I back it up to the window in the garage and can get about 4 hours of sunlight on it to keep the battery charged.
An eco-friendly experiment worth the 24 dollars.
Thanks,
Mike
-
Fantastic! I'll have to work on setting up one of these. Thanks for the feedback!