Adding a solar panel to MintyBoost

[kpintar]

I would like to know if anyone has added a solar panel to the stock MintyBoost and how effective it is. My thought is to use 2 or possible 3 NiMh rechargeable AA batteries in parallel (i think that's what it's called) with the solar panel. I have a 6V 1.5W solar panel and was planing on using a simple 2.5mm plug with a diode. Got the idea from another charger I saw on a website. I attached a picture so you can see what I'm doing.
My initial thought is a USB device will mostly charge from the batteries and that the solar panel is not enough to directly charge a USB device. Also that the solar panel would just slowly charge the batteries and it best not to have a device connected while charging the batteries.
I'm not an electronics circuit person and would like a little explanation of the charging process and if this will actually work as I think it might.
Thank you

[adafruit_support_bill]

That will probably work for the most part. The diode will keep the current from reversing through the panel when the sun goes down. The problem is that you are not controlling the charge going to the batteries when the sun is up. Unless you are using a fairly small panel, with no load on the MintyBoost, the panel will likely be pushing more than a safe charge current into the NiMH cells.

[kpintar]

Is the idea to use a small solar panel and not have a device charging while the solar panel is attached? What size panel would be recommended for 2 AA batteries or 3 AA batteries. I thought I read 3 AA batteries would be better than 2.
Thank you

[john444]

Hi Kpintar,

Like Adafruit-Support said:

The problem is that you are not controlling the charge going to the batteries when the sun is up.

You need some way to cut-off the charge when the batt's are full, even using a 1-watt solar panel.
I know that overcharging NiMH or NiCad or CZn or Alkaline AA-size cells will cause them to leak and fail.
Not a pretty sight.

Good Luck, John

[adafruit_support_bill]

3 NiMH cells in series work well with the Minty. The 3.6v nominal output voltage is right in the sweet-spot for boost efficiency.

It is possible to charge NiMH cells via solar, but if you want to do it at more than the 'trickle' rate, you should use a charge-controller circuit. Charge controllers for NiMH are different than for LiPo. All the chargers we sell are for LiPo.

This post over at EMSL has some good information on trickle-charge solar circuits. http://www.evilmadscientist.com/2008/si ... -circuits/

[kpintar]

A trickle charge at this time is fine for my use. So a couple things I would like to verify are:
- I would only want to connect a USB device with fully charged batteries correct?
- With fully charged batteries and a USB device connected to charge would having the solar panel have any impact on charging?
- Once batteries drain to a point of not charging a device I would want to disconnect any USB device and let the solar panel charge the batteries?
- I performed some charges on a couple different types of smartphones (HTC Incredible, iPhone3GS, iPhone 4S) that had fully drained batteries and they charged to 45% - 50% battery reading. Is this typical?
Thank you

[adafruit_support_bill]

Yes. A trickle charge rate for the batteries is not going to be sufficient to charge your phone. So you will need to charge the batteries up before connecting your phone, and disconnect the phone when the batteries are depleted. You should be able to leave the solar connected while the phone is charging to supplement the battery output.

I performed some charges on a couple different types of smartphones (HTC Incredible, iPhone3GS, iPhone 4S) that had fully drained batteries and they charged to 45% - 50% battery reading. Is this typical?

That sounds quite good. Is that with 3xNiMH? People are reporting 20-25% with 2x alkaline cells on the current generation of smart-phones.

[kpintar]

I used 2 - AA Energizer NiMH Rechargeable batteries 2300 mAh for my test.

Test 1 was using a totally dead HTC Droid Incredible 2, connected it for 1 1 /2 hours and left the device powered off the entire time. When I powered it on the battery status was 47%.

Test 2 was using a totally dead iPhone 3GS, connected it for 2 hours and left the device powered off the entire time. When I powered it on the battery status was 43%.

I just changed out the 2 battery holder for a 3 AA battery holder and will try and do similar testing and post results in the next day or so.

Thanks

[adafruit_support_bill]

Thanks for the feedback! There are a lot of phones out there. A lot of combinations to test!

[kpintar]

With 3 AA batteries we were able to get better results such as charge an iPhone 3GS from 12% to 100%, Kindle from just about dead to @ 75%, Nexus 7 from just about dead to @ 50%.
We also had varied results depending on the rechargeable battery brand and type.