Step 4: Choose Your Solar

Picture of Choose Your Solar

If we use two rechargeable AAs that put out a total of 2.4Vs we're going to need a solar panel that is at least 3 - 4Vs just to meet basic levels of charging. The higher the voltage of our solar cell (or cells) the less light we need to charge up our batteries.

Now we're also trying to fit this into an Altoids Tin, so we're limited in space. I have found some great 4V solar cells that perfectly fit into Altoids Tins. They're the same ones I use with my Solar AA Altoids Charger.

Sure, a bigger and better solar cell would give us added power, but it wouldn't fit into our tin. (Something that has annoyed me with nearly every Altoids Solar Guide out there.)

You could also use a combination of several smaller cells to get your four volts. For example, 2V cells are very cheap and small on ebay. You could easily connect two of those in a series to get your 4Vs.

Just remember that when charging NiMh batteries we don't want to throw more than 10% of their capacity at them at any one time. For instance if your battery has a capacity of 2000 ma we can only use a solar cell that puts out 200 ma or less of current. This isn't usually a problem unless you're using a massive solar cell or a big combination of cells. None the less, keep this in mind.

Alexjoy3 years ago
Do you know if excessive heat damages the solar cell? And if so, how hot does it have to get for that to happen?

Also, I know heat will damage the batteries for sure, however, again I'm not sure how hot exactly they can get before it damages them.

My situation is that I live in Phoenix Arizona and I really want to make one of these, however, I don't know how long I will be able to leave the charger in the sun without it getting damaged due to excessive heat. It's really the solar cell that I am concerned about as I am sure I could find a way to keep the batteries fairly cool in the shade while the solar cell is in the sun.

Do you have any suggestions?
swimfan24893 years ago
Since your saying keep the input current at about 10% of the rated battery capacity, is that 10% for just one battery? Or is that for the total capacity if batteries are wired up in parallel?

For example, if i wire 2 packs of batteries, each cell with 1.2V and 2000mA together in parallel to make a combined total of 2.4V and 4000mA, should i buy a solar panel with a current output of 200mA or 400mA for best results?


JoshuaZimmerman (author)  swimfan24893 years ago
2 AAs in a holder are in a series and will come out to be 2.4V at 2,000mAh

Parallel = 1.2V at 4,000mah
Series = 2.4 at 2,000mah

10% of total capacity.

I've gone over 10% before, it really depends on your batteries and how trusting you are of them.

So in your case you'd want to use two battery packs, with a total of 4 batteries. Giving you 2.4V at 4,000mAh.

Go with the bigger cell. You'll never ever reach your max power for solar anyways.
I figure i might as well ask too.. I'm going for a bigger setup and want to use a 12v that puts out 100ma to charge 8 AA rechargeables. (that's 9.6V, i haven't bought said batteries yet so I'm not certain what mah i'll have)

will the 12v be sufficient? ... too much? will the 100ma allow me to charge the batteries in a reasonable amount of time given that it's solar trickle charge we're working with?
JoshuaZimmerman (author)  Matieland883 years ago
From what I've read you should always go half again as much voltage as needed.

Meaning a 12V system should have at least 18V of solar panel in it.

So if you're doing 9.6V you should have at least 14-15V of power.
a number of sources have mentioned needing 1.4-1.6V/cell which calculates to 11.2-12.8V for the 8 cells.

It sounds like I can't very well blow anything up with 12v so I'm going to proceed with wiring the mock up and testing on the next sunny day.

also I might add, the panel only puts out 100ma, and that for 2500mah batteries would take forever so i've got two panels. Should suffice. The idea is that i can strap the device atop my backpack while hiking, trickle charging as i do, and then flip open to reveal both panels when the opportunity presents itself.
gwbonline4 years ago
Do you mean the total capacity of all batteries together when you say not to throw more than 10% of their capacity at one time? And when I want to use for example six batteries, do I need a solar panel with more volts and an other charging circuit?
Alright, so what did you get your solar cell from? I'm not following this at all. Haha.
JoshuaZimmerman (author)  Bakenshake094 years ago

You can also take one out of a solar garden light, provided it is at least a 4V cell.

I get mine in bulk from China.
Gotcha. Thanks, man.
If I'm not mistaken, those cheap solar walkway or landscape lights you can get at walmart or harbor freight tools (around $5 each) have good potential for this project. They each contain a PV cell, 2 AA rechargeable batteries (the smaller ones might have AAAs) and a charging circuit with diode. As an added bonus, you get a photo-sensitive switch and LED plus reflector.
JoshuaZimmerman (author)  fryingsquirrel4 years ago
I found the cheapest in my area was at Menards. They had $0.99 ones. The solar cells were 2.5V and easy to remove. Plus you got a 300 mah AAA battery and dark detecting circuit.

A lot of it depends on how easy they are to remove... I've broken a few solar cells due to them being too tough to get out of the solar light.

Also, you need to be aware of the current on these cells. They may have a high voltage, but probably have a low current. Not a big deal for the lights as they have small batteries, but we want to change up larger capacity batteries. This is why getting a solar cell with a higher current (miliamp) really helps things out.