So I was at Harbor Freight and bought: 1.5 Watt Solar Battery Charger Back in the office, the 'blue' LED flashed indicating it was making power. Know the problem of patching solar panel generation to the load, I bought a buck-boost converter. Wired everything up and: Input Solar Panel: 18V open circuit from solar panel 1.7V into buck-boost converter 0.75 ma into meter Output: 0V Taking the open circuit voltage times meter read current, 13.5 mW, just enough to flash the blue LED. Now there is a DC blocking diode but it tests good. Nothing that should limit the current. Bottom line, this Harbor Freight part is useless. So I've ordered something else, a 5-7.5W polycrystalline panel, from Amazon. I'll test it when it arrives and return it if it doesn't generate enough power. Wished I'd done this simple test with the Harbor Freight POS. Bob Wilson
* Was the 0.75 mA into the converter (1.7V), into a short circuit (the meter only), or into some other load? It certainly was not with that 18V open circuit measurement, so you were not getting 13.5mW. With any PV, maximum output power is significantly less than Voc * Isc. Under lower light, especially with common amorphous cells, power output is far short of that figure. * Under what sort of light did you make your measurements? To get the full rating, PV needs to be perpendicular to full sunlight (no clouds, overcast, or even window glass). Ordinary overhead office lights are not even close, by roughly two orders of magnitude. * What is the minimum unloaded operating current (Iq) of the converter? Few will even start up with such low current availability. * What sort of bulk storage (capacitor, battery) was on the converter input? Whether internal or external, some storage is essential. The PV is current limited, and has no storage of its own, so cannot supply the input current pulses inherent to most such converters. Testing a similar car charger a couple decades ago, I found that in common local overcast light inside a car, it was providing enough voltage and current to illuminate the indicator LED (red at that time, blue wasn't yet available), but was way short of the 12V needed to put anything into the car battery. A reverse protection diode prevented the LED from drawing current from the car. In low-moderate light, the indicator LED consumed the entire output. Removing that LED allowed some micro-trickle charging under those conditions, but not enough to offset ordinary battery self-discharge. Nevermind the indicator LED, the device really needed direct full (or nearly so) sun.
i have one of those in my boat and i checked it gives some current back to the 12V battery, but only in full sun (~50 mA IIRC). how much goes to the LED alone?
I bought via Amazon a 7.5W rated solar panel, about the same size. Direct sunlight, ~18V and 224 ma into the meter (dead short.) But as pointed out and tested, not enough to even start the buck-boost converter. Although I've ordered a buck converter that might work, I'm not that hopeful. So I've also picked up a set of 1W 13.1V Zener diodes. Based on the solar panel metrics, I'll put 150 ohm current limiting resistors in series with three Zeners and epoxy the zeners to a heat sink. All three zener+resistors strings will be in parallel. In effect, over 3W, shunt regulator. Per specs, half of the required regulation BUT the panel will be behind the 2010 windshield. My reason is any voltage over 12.85V first puts the lead acid battery into resistor mode until the over voltage reaches electrolysis potential, ~1.23V. Then you get gas generation, a bad thing I want to avoid. Now if I knew the resistance of the lead acid battery over 12.85V, the current limiting and even zeners might not be needed. Bob Wilson
Bob, IMHO, you're overthinking this by a huge margin. There is no frigging way that small solar battery is going to overcharge/dry your battery.
