Solar cells and AA batteries

[rodeman]

Hello everyone, I just found your site and needed to ask a question. I've not taken a EET class since my first year in college which was 14 years ago.

We live in Arizona and the temp is already in the upper 80s with tons of son. We found a clip-on battery operated fan for my son's stroller that runs on two AA batteries. But I wanted to attach the fan to a flexiable solar cell so that the sun is powering it instead of the AA batteries all the time. I figured I might as well get some benefit from this great weather.

My guess is the motor is about 50mAh (I have not received the fans yet, they are being shipped currently). The solar cells are inexpensive, about $10, and have a couple of different voltage/mAh configurations.

I'm not sure which one to purchase though. They have a 3V 50mAh one which should be enough to power the fan I believe. And if I hook a diode up in the connection, that would probably be enough power to charge two NiMh batteries right?

My other choice is a 3.6V 100mAh. I don't think the 3.6V is an issue because a AA battery is about 1.65V, so it is pretty close. I'm just not sure on the mAh. I can't recall from my EET courses if the motor will only pull the current it needs or will 100mA be too much for that little motor?

Does anyone have any suggestions on this? I would really appreicate the input.

Thanks,
John

 

[itsmoked]

The motor will only draw what it wants based on the load and the voltage. Get a "big enough" panel to run it and it will take care of the rest. However, if your panel puts out more than 3V you'll probably damage that motor. Look at Jameco for a different voltage motor.

And don't be chopping off any of the kids fingers by allowing him to touch moving blades..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rodeman]

Thank you for the feedback. I thought the motor would only pull the current it needed. I was looking at Jameco for the solar cells. But I didn't see any additional motors on there.

The fan I am purchasing is at

http://kelgar.com/Stroller/Stroller.html.

It's the only stroller fan that I can find. I would have thought more vendors would make that type of product.

I was looking for solar cells from Jameco and Sundance Solar

First is 3.6V 50mAh solar cell #MPT3.6-75 from Jameco (

http://www.jameco.com/webapp/wcs/st...oreId=10001&catalogId=10001&productId=228013)

Second was the 3.6V 100mAh from Sundance (

http://store.sundancesolar.com/po310flsopam.html)

Third was the 3.0V 50mAh from Sundance (

http://store.sundancesolar.com/3vol50maflex.html)

I was leaning towardds the 3.6V 100mAh from Sundance because it had more mAh and I thought would provide enough power to run the fan and to charge the NiMh batteries I'll put in the unit. Does that sound like it would work? Last thing I want is a fire on the stroller.

As for getting their fingers in the fan, It's completely enclosed with plastic covers so I don't think this will be an issue.

Thanks again for the information.
John

 

[coconutalley]

rodeman,

First of all: "mAh" means it is the current output for a period of time in hours. 100mAh will output 100mA for 1 hour. Your fan uses two AA batteries which means it adds up
to 3.0V. I would guess that you'd need more mAh to get that motor to spin. AA batteries can put out a lot of current something like 600mA easy, though your motor might only need 100mA. You'd need to experiment. The 3.6V solar cell is probably for high noon direct sunlight. The problem with charging a drained battery at the same time with the fan is that the fan probably would not work since the battery would take all the juice.
To get an idea how long it would take to charge a battery
of say, 600mAh. Just divide it by the capacity of the solarcell.
Example, 600mAh / 50mA = 12 hours, so you'd need to use several 50mA solarcells in parallel to cut down on charging time. In this example, you'd need 12 to bring the charging time down to 1 hour. Though, the battery may also have a limit on how much current it can handle and you may not be able to reduce the charging time any sooner.

 

[jghrist]

Why would a solar cell be rated in mAh instead of mA?

 

[coconutalley]

Solar cells are rated in mA and Volts or Watts. Batteries are rated for energy storage capacity in mAh. It is an odd unit, because it is current output x time. I wonder why
Joules is not used instead. Engineers and scientist learn about Joules in physics. The utility folks use kW hour. It is also a unit of energy. I guess there must be a history and reason for these conventions.

 

[itsmoked]

You don't want a battery in the picture just run the fan directly with the solar cells as when the sun is out is when you need the fan. Trying to put a charger in the picture needlessly raises the complexity.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

How about two diodes?
A conventional diode to isolate the solar source from the battery and a zener to limit the voltage.
Comments Keith?
respectfully

 

[itsmoked]

Gaugh!

Diodes are not good here as solar cells generate power based on a diode junction 0.6-0.7V. One diode essentially "wastes" one whole solar cell.

If you must keep the batteries then just use a single pole double throw switch. One way fan-batteries - other way - fan-solar selected.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

If you must have batteries for the possible operation after sundown (trust me, it's still plenty warm in AZ after sundown!), I would jettison the charger idea and the NiMh batteries in favor of plain old alkalines for convenience. Just put a diode on the battery output and run the solar cell in parallel. It will run off of the solar most of the time and only draw off the batteries when the solar output is lower than the battery voltage. Then just get a cheap personal fan like one from this place which has the soft foam blades. Kiddos can't get hurt with those.

http://safetycentral.com/batopfanspra.html