Charging Lipo Cell from AA NiMH Batteries Using 5V USB Rectifier—Charge Time & Safety

Hello.

I am building a solar phone charger and have decided to use 3 Nimh batteries, a 5/6 v solar panel, a usb 5V rectifier and also a Lipo credit card rechargeable cell.

 

My reasoning is limited for this build as the more I read, the more conflicting info I find...

(Eg, charging AA batteries is easier with solar than Lipo's, then I read its harder)...

 

So Im somewhat confused but my design has been decided on.

 

What I am really after is info on how quick I can charge the Lipo cell off the AA batteries and will that cut off, overcharge function be built into the rectifier.... I am buying a rectifier specifically for phones. (Although not sure if it will do Iphone or not)

 

And also any assistance I can get that would make this project amazing instead of just ok.

 

Cheers

Hi

First, choosing the battery you should check this "factors('')":http://www.solarpowerworldonline.com/2015/08/what-is-the-best-type-of-battery-for-solar-storage/ Second, charging time is just approximately 2.5 times the Watt-Hour of the battery divided by the wattage of the solar panel. That is if you have 2W 2V panel and a battery 1Ah, 3.7 V, it will take 4.6 hrs to fully charge it. Lastly, like the attached image, you can always use a transistor to cutoff the current to stop charging when its full charged.

Hi,
Yes it is Nimh AA batteries I am charging.

I have various solar cells ranging from 3v to 7.2v outputs so I've still to play around with them all.
I also have 2 DC-DC regulator modules that say will take input from 2.5 - 5v and give a constant 5v 1amp usb output.

So what I really want is 2 conditions, USB loaded or unloaded..

For unloaded I want the solar panel to charge up the AA batteries (until full) then stop (and perhaps recheck every hour, for example)

Until there is a load, in which case I want the USB module to take power from either the solar panel, or if not sunny enough, then take power from the AA cells.
The complete system would also include a Lipo power pack to fully charge off the USB, like a phone, so this part is simply an additional feature rather than part of the main system.