Solar for NiMH Batteries with Minty Boost

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ecodad
 
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Solar for NiMH Batteries with Minty Boost

Post by ecodad »

I'm working on a project to create a solar recharger for the batteries with my MintyBoost v3.

Currently, I have a pair of 6V 0.5W solar panels (from the solar pocket factory) wired in parallel. I've replaced the 2 AA battery pack with a 3 AA battery pack and am using brand new eneloop 1.2V NiMH batteries.

I have the panels going through the a pair (in parallel) of BAT-46-TR Schottky Diodes (100V, 150ma, Vf 450mV) to prevent the batteries from discharging over night.

I've only tested it for a few days, but I'm having mixed results. One day, I was able to charge my iPhone 5 fully from about 40% charged. The next two days, I've only been able to charge about 10%.

Any ideas if: Other suggestions?

I want to use the NiMH batteries to keep costs down and avoid needing the LiPoly charger.

Thanks,
Jon

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john444
 
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Re: Solar for NiMH Batteries with Minty Boost

Post by john444 »

Hi Jon,

I'll try to answer a couple of your questions.
ecodad wrote: The Schottky diodes limiting my current to the batteries too much?
The diodes will not 'limit' current until they blow - as a fuse.
I am not entirely clear as to how you wired the diodes.
One diode in each panel's + lead?
That would be good. They will prevent the panels from feeding
back into each other when one panel is partially shaded.
ecodad wrote:I should be looking to do something like the pseudo mppt here: http://learn.adafruit.com/usb-dc-and-so ... sign-notes ?
MPPT is not very practical for such low currents.
You will waste more energy in the control-circuitry than you
will put into the cells.
ecodad wrote:Need to increase the number of panels?
One watt is not much power. I saw eneloop 1.2-V cells with 800
to 1,900-mAhr capacity. Since you are only putting out 167-mA/hr,
you will need 4.8 to 11.3-hrs (at full sunlight) to charge the cells.
So... yah, increasing the number of panels will provide more current.
ecodad wrote:Get lower voltage panels?
This question is a little more involved.
For the same power, lower voltage panels mean more current.
By matching the panel to the number of cells,
you can achieve fairly good efficiency.
For example, with your 6-V panels, 4 series cells in series = 4.8-V.
You cannot get more current from the panel than it is rated for.
So, if you only use 1 cell it will charge at the same rate (167-mA)
as 4 cells. The same for 2 cells or 3 cells, the same 167-mA rate.

The problem is not that the panel cannot charge the cells.
It can, even if it takes a while. The problem is disconnecting
the panel when the cells are fully charged.
It is not hard to ruin the cells by over-charging them.
That is what Lady Ada's Solar LiPo charger does for LiPo cells.
You will need to watch your cells very closely (with a meter) to
tell when they are charged and disconnect them from the panels.

Hope my comments were helpful, John

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ecodad
 
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Re: Solar for NiMH Batteries with Minty Boost

Post by ecodad »

Thanks John
john444 wrote: The problem is not that the panel cannot charge the cells.
It can, even if it takes a while. The problem is disconnecting
the panel when the cells are fully charged.
How does one tell that rechargeable cells are fully charged?

The datasheet for the eneloop cells I'm using lists various Cell Voltage characteristic lines based on temperature.
http://www.eneloop.info/fileadmin/EDITO ... R-3UTG.pdf

Am I reading the chart right to think that I should stop the charging when it reaches between 1.6-1.7V per cell (4.8-5.1V for 3 cells in series)?

Thanks,
Jon

p.s. on the diodes, I plan to wire them the way you described, but haven't gotten around to switching it. I originally had just one connecting both panels to the batteries to prevent discharge through the panels, but since they were rated for 150mA I put a 2nd in series to avoid blowing them. Eventually each panel will have it's own diode.

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john444
 
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Re: Solar for NiMH Batteries with Minty Boost

Post by john444 »

Jon,

The best way is to use a temp-sensing peak detector.
If you notice, once reaching full-charge, the voltage drops slightly.
The peak voltage shifts somewhat with temp so
a fixed voltage does not work well.
Another factor is that the cells will noticeably warm once charged,
as the excess current is converted to heat.

Building this type of charger for the low currents involved is not very
difficult if you understand op-amp peak-detectors and thermistors.
Commercially available charges of this type have to be manually
reset after each charge cycle. That could be a problem unless you
intend to remove the cells when charged.

Good Luck, John

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