Trickle charging AA NiMH batteries from solar *inside* the house

[DamonHD]

Hi,

For my little stand-alone night-light that ran happily for 6 months on 4x 2000mAh NiMH AA cells, I can't leave a good thing alone, so I want to trickle charge it via a little solar PV panel to that it's completely standalone if possible.

I've ordered a 6.7V nominal (8Voc I think) 0.2W panel.

Note that I don't expect this ever to be exposed to bright light for extended periods, though it could be stood on a window ledge to charge I suppose.

The aim is a simple and safe trickle charge while inside (so illumination maybe 100th direct sunlight levels or less).

My WAG is that a diode (either Schottky or small Si) in series with (say) a 1K--10K resistor, from PV to battery, should avoid ever damaging the cells, even when full and with the panel in strong sun, but should help trickle charge even inside ~1mW during the day.

Any thoughts?

Rgds

Damon

[TomW]

Any thoughts?

Rgds

Damon

Yes, seek help! The addiction has passed "minor" and become chronic.

Tom

[Bruce S]

Damon;
1st what TomW said  .
Then what I've done, but mine are NiCds rated at 600mA and are in a cute little Dollar General "camping lantern" Norm sent me to play around with.
I use a small 6V PV rated at 0.5W setup that only has a Diode on it. The output while inside is so low I've not yet seen any problems with the batteries being warm or look to be overcharging.
With your at 2000mA I'm betting you will not have problems either.
Hope that helps.
Bruce S

 

[DamonHD]

So if I'm feeling paranoid I could keep the 1K resistor to keep charge current to ~3mA max even if the thing is popped on a sunny window sill...  (Or maybe 470R for 6mA max which is about the 0.0033C trickle charge limit suggested on Wikipedia.)

But otherwise you think I'm OK (not you TomW: you know I'm off somewhere warm in a handbasket).

Rgds

Damon

[Bruce S]

Yep.
Had to get my handy solar powered calculator out of the 470R numbers, but I'd say your batteries should be safe.

[DamonHD]

Kewl, thanks.

And I think I'll dig out a IN4148 to minimise reverse leakage current to ensure that the PV provides a net gain!

Rgds

Damon

[taylorp035]

What about the little solar panels that come in calculators?  I had one and it measured 3v in full sunlight.  I was going to hook it up to my driveway alarm so the batteries wouldn't keep dying.  Unfortunately, there is too much shade in that area.

[Bruce S]

What about the little solar panels that come in calculators?  I had one and it measured 3v in full sunlight.  I was going to hook it up to my driveway alarm so the batteries wouldn't keep dying.  Unfortunately, there is too much shade in that area.

Those have sooooo little current available the sun would need to be near supernova to get much more than a microAmp.
Bruce S

[RP]

I'm not sure what the float voltage is for NiMH but you could consider making a max voltage clamp out of a zener or a stack of standard silicon diodes.  Then you wouldn't have to limit the current in less than ideal lighting conditions.

[dnix71]

I have a civil defense cdv-700 geiger counter in the window running off a pack of AAA batteries charged from sunlight through the window. Sunlight through the glass is weakened a lot because window glass is not low iron. The solar cell is a Radio Shack regular item that puts out 6v in full sun. There is a schottky protecting the batteries. The CDV normally runs off D-cells, but I gave away the metal battery clip when I bought a spare cdv for my sister, and the D-cells fall out without a clip.





Obviously it's normally turned around the other way. This window faces west. The arrangement works fine so far. Good primary cells might be hard to find after the Apocalypse anyway.

[DamonHD]

RP: my 4xAAs seem to have a Voc of about 5.0V, but (1) as they are going 6M between charges over the summer right now (3M in winter) this is merely an experiment and (2) I don't think that that a CV clamp would be worth the complexity or actually protect anything much since I might have to temp-compensate it!

Rgds

Damon

[DamonHD]

Rather unexpectedly the stuff turned up today!

I see a soldering iron in my near future!  B^>

Rgds

Damon

[ghurd]

Damon,

Don't bother with a resistor.

Inside the house will drop the PV output like a rock.  In perfect onditions, the PV will make maybe 30ma.  In the house, maybe 10ma?  And that 10ma will not be many hours (minutes) per day.

It takes "X" ma to hold nimh at float voltage. 
A 30ma PV is NOT going to hold 2000ma of fully charged nimh at float voltage.  I would be very surprised if 30ma of solar (outside, in perfect conditions) could get them up to 1.3V/cell.  And the PV will not be making anywhere near 30ma in the house.

Might try a quick test.
Charge the nimh up completely, so they are at 1.4 ~ 1.44 V/cell for a while.
At noon on a good solar day, take the 1.4V/cell batteries out of the grid charger, put them in the 30ma solar charger outside, and watch the V/cell drop because there is not enough ma to hold the voltage up.
Remove the batteries, and check the Isc of the PV in the house.  The reduction in ma will make the overcharging worry even less of an issue.

There just will not be enough ma to overcharge/overvoltage the 2000mah nimh pack.
Been a long time, but seems like my ~100ma PVs into 1200 to 1850mah nimh would reach about 1.37V/cell then just sit there until the sun started drifting down.  Seems like even 3 x 100ma PVs would act about the same, and those PVs are 12V into battery packs from 3V to 12V.
( http://fieldlines.com/board/index.php/topic,127848.0.html )

The good thing about charging slow like that is (I believe) the charging efficiency is much better.
G-

[DamonHD]

I hear what you say, but the 470R feels like belt and braces to me and for example makes me feel comfortable using a signal diode if I want to for lower leakage, because I know that peak current is clearly limited.  I can't really seeing the resistor doing any harm since it will dissipate negligible amounts when only mW are emerging from the PV.

Rgds

Damon

[DamonHD]

OK, I gonandun it.  And today swapped out the plain NiMH cells (2500mAh) for hybrids (2100mAh) that I have to hand on the hunch that the lower self-discharge will eke out more life given the very small charge and discharge currents...

Rgds

Damon