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Solar recharger for small batteries

pgoeleven

New Member
Hi,

I'm new to electronics (so bear with me :rolleyes:) and I'm trying to build a simple solar recharger for a battery or a series of batteries that will power couple of LEDs.

I've looked around the internet and I found some resources to get me started, such as AA Battery Solar Charger and this forum.

I currently have a solar panel hooked up to a AA 1.5V rechargable 1800 mAh NiMH battery (output rated 1.2V). I placed a diode in between. I tried to recharge a battery yesterday, and it appears to have worked.

There's some stuff I can't figure out though. Here are my questions:

1. How exactly does the charging work? I searched around for this, but I can't find a simple conclusive explanation for this. Does the voltage coming from the solar panel have to be larger than the voltage of the battery for it to actually charge?

For instance, do I need to match the solar panel's output to the battery being charged? I know it shouldn't be (much) higher, but can it be lower? I currently had a panel hooked up that put out a maximum of 2V in bright full sunlight to a 1.5V AA battery. Could I hook up a 0.5V panel to the same battery and expect it to work? Or would this just charge the battery to 0.5V and then stop charging?

2. I read about solar charging batteries and I understand that the type of charging I'm using isn't exactly ideal, but that it is safe (if not good for the battery though). I'm looking to charge AA batteries or 9v batteries. What are your thoughts on this? Should I use NiCd batteries or NiMH ones?

3. I don't know the exact type of the diode. I asked for a Schottky diode in the electronics store. The receipt shows "BAT85 Diode" which is also printed on the diode itself. I was told I could find the exact specifications on Velleman nv. On that site the closest match is this one I believe. However, the model number doesn't match I believe and a Google search yields this: BAT85 Datasheet pdf - Schottky Diodes - General Semiconductor.

I do believe the diode is ok, but I'd like to have the exact specifications. The datasheets from the second URL says "continuous reverse voltage 30 V, continuous forward current 200mA".

Does this type sound ok? Which source should I consider as correct?

Would this limit the output of a 0.5V 800mA (because of the "continuous forward current 200mA")?

4. This question is sort of a catchall and ties into question 1. I have access the following components: the diode I was talking about in question 3, the following solar panels: link, and various types of batteries.

Let's say I wanted to charge a pack of 2 1.5V 1800mAh AA batteries. What kind of setup would you build?

The setup needs to fit into a confined space, so I'm afraid it's not as easy as attaching as many panels as I like.

5. Could I change the AA batteries for C or D type batteries? Would this impact the circuit any way other than increased power capacity from the batteries?

6. I read on some battery I had lying around (type C if I remember correctly) that it is "protected against overcharging". Is there any truth to such claims?

I appreciate any and all feedback! :)
 

kpatz

New Member
1. How exactly does the charging work? I searched around for this, but I can't find a simple conclusive explanation for this. Does the voltage coming from the solar panel have to be larger than the voltage of the battery for it to actually charge?
Yes. A 1.2V battery will need probably need around 1.4V from the panel to charge. In simplified terms, electricity flows from higher voltage to lower voltage, so in order to push electricity into the battery (to charge it), you need a higher voltage than the battery puts out.
For instance, do I need to match the solar panel's output to the battery being charged? I know it shouldn't be (much) higher, but can it be lower? I currently had a panel hooked up that put out a maximum of 2V in bright full sunlight to a 1.5V AA battery. Could I hook up a 0.5V panel to the same battery and expect it to work? Or would this just charge the battery to 0.5V and then stop charging?
A 0.5V panel won't be able to overcome the 1.2V coming from the battery, so it won't charge at all. Instead, the blocking diode would act as an open circuit so nothing will happen. If you use the 0.5V panels, you'll need to connect at least 3 in series to charge one NiMH cell.
2. I read about solar charging batteries and I understand that the type of charging I'm using isn't exactly ideal, but that it is safe (if not good for the battery though). I'm looking to charge AA batteries or 9v batteries. What are your thoughts on this? Should I use NiCd batteries or NiMH ones?
NiMH batteries are generally superior to NiCd but it depends on the application. Charging either is a similar process.
3. I don't know the exact type of the diode. I asked for a Schottky diode in the electronics store. The receipt shows "BAT85 Diode" which is also printed on the diode itself...I do believe the diode is ok, but I'd like to have the exact specifications. The datasheets from the second URL says "continuous reverse voltage 30 V, continuous forward current 200mA".

Does this type sound ok? Which source should I consider as correct?

Would this limit the output of a 0.5V 800mA (because of the "continuous forward current 200mA")?
Putting 800mA through a diode rated at 200mA is going to burn out the diode. But read on...

4. This question is sort of a catchall and ties into question 1. I have access the following components: the diode I was talking about in question 3, the following solar panels: link, and various types of batteries.

Let's say I wanted to charge a pack of 2 1.5V 1800mAh AA batteries. What kind of setup would you build?
Use 2 of the SOL4 panels (2V @ 200mA). Connect them in series to get 4V. Add the diode which drops up to 0.8V at 100mA and you'll have 3.2V of charging voltage. The safest way to charge a pair of NiMH cells (according to How to charge Nickel Metal Hydride Batteries.) is at 1.4V/cell at 1/10 the mAH rating. So, that would be 2.8V @180 mA. So, you should add a resistor that allows 180 mA and drops 0.4V, so R=E/I = a little over 2 ohms. This isn't really mandatory though with 200mA panels, you're not going to overcharge the batteries at that rate, so 2 panels in series, diode, and 2 batteries in series should do the trick.

You have enough extra voltage that you could use a regular (cheap) silicon rectifier diode instead of the Schottky diode. A 1 amp one will be more than sufficient.
5. Could I change the AA batteries for C or D type batteries? Would this impact the circuit any way other than increased power capacity from the batteries?
Bigger batteries have a higher mAh rating so they'll take longer to fully charge, unless you build a charger that can provide more current.
 
Last edited:

pgoeleven

New Member
Thank you for your swift reply! Everything is a good deal clearer to me now.

I'm going to tinker on and take a more in depth look at this beginning next week. I'm sure I'll have more questions then. :)
 

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