# 9V Power Supply using Solar Panel

#### ziin

Dec 2, 2013
9
Hello everyone,

I am building a power unit of 9V from "6" x AA_batteries (1.5V each), and a solar panel to charge them. I have 2 solar panels, and each of them has 2.5W, 8V output, and 310mA.

I am thinking about using a simple trickle method to charge the batteries using a diode connects in series between the solar panels and batteries. I am aware that there are many other ways with low cost to connect the panels and batteries to make it work better and protect my panel safer as well. So do you have any recommendation for me to it?

Also, can I use only 1 solar panel (8V, 310mA) to charge 9V= 4 batteries? or the voltage supply from solar panel need to be much higher than the total voltage of batteries in order to charge the batteries?

Thank you very much.

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#### davenn

Moderator
Sep 5, 2009
14,011
hi ziin
welcome to the forums

your maths is a bit out

4 x 1.5 doesn't = 9 it only equals 6

so would you like to rethink your problem and come back with a better plan for us to help you

Dave

#### ziin

Dec 2, 2013
9
Hi Dave,

My bad, I have 6 slots for 6 AA batteries, not 4; and it adds up to 9V total. I am currently using a testing power supply unit of 9V to run my device, and really hope to build my solar power unit soon.

Let me know what you think, Dave.

Thanks.

#### BobK

Jan 5, 2010
7,682
Your math is now right, but there are other problems. What kind of batteries are you talking about? I do not know of any batteries that are rechargeable and 1.5V per cell. NiCad and NiMH are 1.2V per cell. So 6 batteries will get you 7.2V, not 9.

Then there is the problem that you cannot charge a 9V battery from an 8V source without stepping up the voltage. Simply connecting it through a diode will not charge the batteries to anywhere near full.

Bob

#### ziin

Dec 2, 2013
9
Hi Bob,

Yes, that's the right problem. I am not sure what type of batteries I should use; if you can recommend me one, it would be great.

My original idea is using 6 regular AA batteries (1.5V each) to create 9V voltage supply for my device since it needs 9V-12V to operate.

Later, I decide to upgrade my device to use solar panel so that my power unit looks nicer. I guess my goal here is to create a 9V power supply from the solar panel, and I already have a 2.5W solar panel in my hand.

Any suggestion for me?

Thank you.

#### BobK

Jan 5, 2010
7,682
The best rechargeables are Li Ion. These have a voltage of 3.7V / cell. so a 3 cell battery would put you in the 9-12V range (11.1V). Second best would be NiMH which are 1.2V, so 7 of them to get 9.4V. If you have higher current requirements, a 12V sealed gel lead acid battery might be best.

How much current does the device require? How much run time do you expect to get from a day of charging? These effect the size of the batteries and the solar cells.

And for best performance, you will need a solar charge controller appropriate for charging the battery type you use. Each has different charging requirements.

Bob

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#### ziin

Dec 2, 2013
9
Thanks Bob for the information,

I need to power 4 small motors (which I expect no more than 1 Amp, please correct me if I am wrong), and my circuit board with 5V input and 50mA max (I already built the step down voltage to 5V from 9V-input supply). And I need to run the device for a hour, or as long as it could be, but no pressure on it. So I think the 3 cell Li Ion batteries (add up to 11V) should be good enough? I will use 2 solar panel (2.5W each) to charge for these batteries. What do you think?

The solar charge controller is the one I concern the most. I was thinking just use an simple diode to control the current direction, but it seems to me not enough. Any suggestion? I would love to build one, not buy one; but not sure where to start.

Thanks.

-
Ziin

#### BobK

Jan 5, 2010
7,682
Lithium is a good choice. You would want about a 2000mAH one to gaurantee 1 hour at 1 Amp on a charge. That would be using 11 Watt Hours.

To charge them from a 2.5W source would take about 6 hours of peak sunlight, or realistically, more like 12 hours since it will not produce peak power over all 6 hours. You would need about 14 or 15V to charge them, but a charge controller might be able to step this up from a lower voltage.

You can actually buy the charge controller fairly cheaply, probably cheaper than you could build it, look on Ebay. They are not simple devices.

If you want to just charge it simply, without a charge controller, a lead-acid battery would be better. You would again need about 14V and a relatively simple circuit to do this, though more than just a diode.

Bob

#### ziin

Dec 2, 2013
9
Thank Bob,

It seems like 3x 2000mAH lithium batteries with a smart charger is a good idea for me, except the this type of battery is quite expensive.

There are lot of good-price charge controllers on Ebay, but most of them work only with the lead-acid batteries (correct me if I am wrong). I am not sure why it works with lead acid batteries, but not lithium batteries.

I am thinking about using the LiPo Charger Basic - Micro-USB as the charger controller, with 3 lithium batteries 2000mAH. It costs me more than $70 totally. Let me know what you think? Thanks #### BobK Jan 5, 2010 7,682 You can get 18650 cells and an AC powered charger for <$10 on ebay.

Have you given up on the solar panel then?

Bob

#### ziin

Dec 2, 2013
9
Hi Bob,

I just found out a controller for the Lithium batteries with about \$30 on ebay, and I am wondering if I could actually build one instead buying it.

Ziin

#### BobK

Jan 5, 2010
7,682
I guess that depends on your skills. Try googling for one and see if it looks doable.

Bob

#### ziin

Dec 2, 2013
9
Thanks Bob,

I came up with my new design in which I only use 1 solar panel (since the panel is big and take much space), 8 AA NiMh batteries (1.5V, 2000mAh each) group to 4 big-batteries (by combing 2 batteries to have 3V output/each big-battery), and some dip switches.

Basically my device runs using only 3 big-batteries, and the 4th is for the backup purpose. By using the dip switches system, I can choose which big-battery is used for the backup mode and which batteries are used to run my device. When the battery is in the back-up mode, it will be connected to my solar panel for charging process.

So my solar panel (2.5W, 8V output at the maximum) only charges one big-battery per time. I use a diode connected between the solar panel and the battery to prevent the current from battery that can damage my solar panel.

What do you think?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
What do you think?

I think you'll overcharge your batteries.

Fortunately you're using NiMH, not a LiPo. The former get hot and die quietly. The latter can explode in flames.