# charging 2 AA batteries with 1 solar panel (NO arduino)

Hello,

I've searched the web for the last day and a half trying to understand if I'm right about this thinking or not and haven't found anything.

So I'm turning to you to help me as you already have in the past.

This is the problem I'm facing:

I have 2 NiMH AA batteries (1.2v 2050mAh), 1 solar panel at 2V 0.3W (generating 150mAh).

My circuit to power a red led require 1.8V so I have to connect the batteries in series in order to light the led. that thing I already understand, the problem is that I can't charge the 2 batteries with the solar panel while the batteries are in series as the voltage of them both will be above 2V that the panel can give.

My question is if there is any way for me to connect the 2AA batteries in parallel to the solar panel but in series to power the led?

my plan not to over charge the batteries are that if they are connected in parallel the trickle charge of the solar panel won't exceed the 1/20C rule so it would be good enough. But if you have any good advice on how to prevent them from dying (as I know NiMH don't do well when undergoing deep cycles) that would be great as well.

A schematic will be great for me to follow but an explanation can work as well.

Sagi.

I would use a second solar panel in series and limit the voltage using a zener diode. I wouldn't worry to much about limiting the charging current, 150 mA (NOT mAh) is probably the short-circuit current.

lg, couka

I don't have the space for another panel on my project this is why I wanted to tackle this in this way.
And yeah sorry about the mistake...

In that case, use a little step-up converter between the solar panel and the batteries and set the output voltage to the charging voltaeg of the 2 NiMH-batteries in series. (Afaik 2x1,45V = 2,90V)

lg, couka

hmmmm this is a new term for me (still new at this in general).... do you have any good link on where I can learn about this?
or better yet a link to a product like this?

Thank you for all the information!!!

Sagirokach:
Thank you for all the information!!!

Happy to help, but I think you can google “step-up converter” yourself

lg, couka

ok just to see if I'm on the right track....

is this what you were talking about?
DC-DC converter

I'll continue to search if this is the thing I actually need to search for.

Yes, but I would use one with a variable output voltage, though 3v probably won’t kill your batteries too soon.
The choice of parts depends a lot on the desired efficiency, size and lifetime.

lg, couka

yeah I get that.... ok so if that is the way to go I'll keep looking.
I noticed the efficiency is low so I'll probably go for a better one, and I'll look for a variable output option.

But what do you think I should put on the output voltage for this? 2.9 exactly? what will 3V do?
I was sure you need more voltage to put on the batteries to charge them.... :o

Thanks again for everything!

Well, charging batteries correctly is not that trivial, really. If you want to do it right, it becomes complicated really quickly: NiMH battery technology, how to charge Nickel Metal Hydride Batteries tutorial for design engineers, as well as NiMH chargers.

I don't know much about your project, but I assume that putting a current-limited voltage close to the charging voltage on the batteries is proper in your scenario.

I just looked the NiMH charging voltage up again, Wikipedia says 1.4-1.6V per cell, so you are probably fine with the 3V converter

65%-88% is far from great, but probably better than other parts of the project (Solar panel efficiency, it's power point, the resistor for the led etc...)

lg, couka

yeah ok that's what I thought... so I'll look for an efficient one.

Thanks a lot!!!! you really helped me!

will that work if I put it on 2.9?
The thing is that I don't know how much current I'll get...

Both should work. I don't know if these are more efficient than the first one you found, so try it.

Sagirokach:
I don't know how much current I'll get...

Less than 2A.

lg, couka

But is it directly proportional to the voltage?
If I'll put 3V will the current be 2A/8? to be 250mA? or is it going to be 1.9A or something...

I want to understand if I got that right....

Step-up voltage converters do not work well with solar panels. If they work at all, the output current will be MUCH LOWER than the short circuit current provided by the solar panel.

The very best option to charge those batteries is to buy another solar panel and use two panels in series.

Thank you for the response but if I don't have the area to place another solar panel?
I want the imprint of the project to be as little as possible. with 1 solar panel.... is there anyway to do anything with just 1 solar panel?

Yes, buy a higher voltage solar panel and reduce your current usage...

No current specs from you...so we are now just guessing.

@Johnny010 All the specs I do have are on the older posts, nothing is connected yet and I'm searching for the right solution.

I have 2 NiMH AA batteries (1.2v 2050mAh), 1 solar panel at 2V 0.3W (generating 150mAh).

My circuit to power a red led require 1.8V so I have to connect the batteries in series in order to light the led. that thing I already understand, the problem is that I can't charge the 2 batteries with the solar panel while the batteries are in series as the voltage of them both will be above 2V that the panel can give.

That's what I have and that's what I can do in terms of space and maximum output from the solar panel... so I was asking if there is a way to recharge the batteries.

I'll manage if the current will be lower (to a point) but I want to understand how much I'll get and if there is a better way to connect everything to preserve energy it would be great.

Thanks again for the responses.

Sagirokach:
Thank you for the response but if I don’t have the area to place another solar panel?

Use 2 smaller panels.

lg, couka

The 2 panels won't give me the power that I need for this or won't fit in terms of a space.
So what can I do?