Monday, September 7, 2015

AA NiMH Ambient Solar Indoor Charger and Night Light

Indoor Ambient Solar AA NiMH Charger & Night Light

I had previously worked on an indoor (ambient light) solar charger for 4 AA NiMH batteries, but finally have something that works fairly well for me. Turns out I have borrowed circuits from at least three other designs, and modified them. The goal was to work for four AA 1.2 volt NiMH 2500 mah batteries indoors under ambient light. After getting it working, I thought it would be good for a long lasting night light when I do not need to charge batteries.

I have seen other circuits for two or three AA batteries, but could not find anything that would charge four in parallel.I wanted to charge in parallel so that the charging would be done evenly, without the need to create a separate battery monitoring circuit.

The circuit also works in direct sunlight, but be warned, the batteries can get warm under direct sunlight. It works well if set near a window that gets at least a couple of hours of sun.
Let me see, credits should go to author P. Marian at (for his 3V boost converter), mrpiiggs and Dick Cappels at (for his 1.5 volt Solar Garden Light), and to Afrotechmods video on YouTube (for his Reverse Polarity circuits).

I plan to add a 1.2V AA low voltage indicator, but have not yet decided which circuit will be appropriate for this design.
  Main driver circuit for regulated output voltage:

AA NiMH Ambient Light Solar Charger Circuit #1 - The Krell Lab

Modifications included the addition of 547 transistors in an effort to increase the amps, the use of a toroid, and the use of a slightly larger (6V) solar panel.

  The main AA 1.2 volt indoor night light circuit:

AA NiMH Ambient Light Solar Indoor Night Light Circuit #2 - The Krell Lab
The above circuit connects to the charged AA battery bank. Alternatively, it can run from a 1.2 volt AA NiMH battery. I modified this one by using a toroid, using NiMH batteries (in parallel), the addition of a Light Detecting Resistor (LDR), and the use of a capacitor.

For a breadboard view, the overall circuit looks like this:

AA NiMH Ambient Solar Indoor Charger Night Light - The Krell Lab

That's a view from the kitchen table during daylight hours, the window is about 8 feet away. The white Tenergy batteries on the left were used to power the voltage meter only. The charged AA NiMH batteries are near the upper right hand corner.

The volatge regulation circuit is on the bottom left, the night light is on the right, and the polarity check circuit is on the upper left. The batteries are charged in parallel on the upper right.

The above voltage regulation circuit looks like this on the breadboard:
AA NiMH Ambient Light Solar Charge Regulation Circuit - The Krell Lab


The polarity check part of the circuit is below:

AA NiMH Reverse Polarity check circuit, the batteries are in parallel - The Krell Lab

And a view of the batteries in parallel, being charged from the reverse polarity check circuit:

AA NiMH battery bank of four, the batteries are charged in parallel - The Krell Lab

These are 2500 mah NiMH batteries.

This would be a view of the Night Light on a breadboard:

1.2 volt AA NiMH Night Light on a Breadboard - The Krell Lab

The circuit is able to run an 'Ultra Bright' or 'Super Bright' 10mm LED.
 Warning - Do not look directly into an 'Ultra Bright' LED.

And this would be the same night light on a perferated board:

1.2 volt AA NiMH Night Light on a PerfBoard - The Krell Lab
 This version on the perf board has a toroid wound differently, but it still works. I just happened to have this toroid on hand and haven't taken the time to wind another yet.
In this picture, I have the windings backwards, as L1 should have the more windings. Just goes to show that it still works when the windings are backwards. However, the LED is not nearly as bright, so if you are not getting a bright light, then check the windings.

I have not tried the ferrite core, as I have no ferrite cores available.

So, there you go. If you want to charge 4 AA NiMH batteries indoors, or just run an indoor night light from rechargeable batteries (or do both), then this should work for you. 

The main problem is I see is regulating the amperage, as in using direct sunlight vs. ambient light. Which is to say, without appropriate amperage, the batteries will not last long under a normal load. So, what is needed next is to test for longevity. I am thinking I may test the number of coil windings and type of coil used (Joule Thief vs. Inductor) for the voltage regulation part of the system. Or, otherwise find a way to increase the current when not in direct sunlight, and keep the default current under direct sunlight.

Update 09/16/2015:

 Using one 6 volt, 1 watt panel, it does not charge properly, and does lose charge over time.
My current thinking is that the main problem is the lack of sustained current (on the order of 250 ma).

Some of the less common parts:

 Warning - Do not look directly into an 'Ultra Bright' LED.

YouTube Video:

                      AA NiMH Ambient Indoor Solar Battery Charger and Night Light


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