Gameman -
Look closely at the various models with identical LED configurations available and note that most of them specify 'equal to' when referring to light output, even though this one for some reason doesn't. Actual power input is much less; forget anything about 15-20 watts - it's irrelevant for this problem.
That being said, hopefully this little 'worksheet' can help you find your answer:
Typical 'regular' white LEDs are driven somewhere around 20mA each, sometimes as high as 30mA if the designer of the fixture was feeling froggy. A little math reveals that with 20 LEDs @ 20mA, the total is 400mA; @30, 600mA.
Unfortunately, just saying '8 NiMH batteries' doesnt describe how they are configured, so we have no way of knowing what the actual losses are to calculate your usable runtime.
At best, I can surmise a set of raw, ideal calculations based on general knowledge:
8 x 1000mAH =
8AH @ 1.2V =
9.6WH Maximum theoretical capacity.
And, I'm going to take a stab and assume that they are configured in series-parallel to provide 4.8V @ 2000mAH, based on the fact that there are 16 total solar cells: 4 in each 'panel', total of ~8V less the drop across the blocking diode = 7.3V in full sunlight. Plausible configuration to charge a 4.8V battery.
White LEDs of this nature hit saturation at about 3.2V. Power input to the LEDs @ 3.2V @ 400mA = 1.28W
Assuming they are using a dropping resistor to limit current (rather than a buck converter), the resistor will be dissipating (for argument's sake) an average of .8V @ 400mA = .32W (I base this on the idea that at full charge, it drops 1.6V, at 'past-dead' it is dropping negligible voltage as the LED array begins to fall out of saturation. Ignorantly using a linear discharge curve, the average over the cycle is .
.
Total notable power consumption = 1.28W + .32W = 1.6W
9.6WH / 1.6W = 6 Hours
This falls about into place for the 12 hour charge for 6 hour runtime, given the likely high charge inefficiencies and all.
I am curious however, to see exactly how you got
'most of the batts up to 1700 mah' from 1000mAH. Not gonna happen, I don't care what the charger manufacturer claims.
While your runtime increased, it is likely that you just got a very thorough charge in them, not something that 'plop-em-on-a-pole' solar lights are famous for. Also, as I kind of hinted to above, it depends on what you call 'dead'... When the light begins to dim (falling below saturation) or no longer visible at all... ?
It seems to me that you're going to need not only more solar input, but a higher capacity battery layout to store it in. I'm guessing something like 3 times the solar and twice the AH capacity should get you close.
Too many factors to speculate on here, but there's the general idea. Hope it helped.
Steve
