I’ve been using the LiPo-Rider-Pro solar harvesting boards for environmental
dataloggers in the field based on the Arduino Mega boards.
On the whole they work quite well - the solar cells keep the battery charged and,
with judicious use of sleep/powerdown modes, generally keep working with just
a two or three watt solar panel.
However … solar cells have a “rated” voltage and then there’s the “open-circuit”
voltage that appears when there’s little or no load on the cell. This can be nearly
double the rated voltage. This means a five or six volt rated panel can totally fry
a LiPo-Rider once the battery is full and there’s little load on the solar cells. The
spec says no more than about 6 vdc for extended periods.
So, the question is how best to deal with the open-circuit voltage. I’ve used a
couple of 5-watt zeners to “bleed off” excess voltage, and while it works the
zeners get HOT - even with a 2w solar panel in bright sun - enough to start
melting electrical tape. They eventually die. Clearly, also, they’re wasting a LOT
of electrical energy.
The most recent trick is just an ordinary 6v LDO 3-terminal voltage regulator.
These seem “better”. They don’t seem to get hot and, like the zener, continue
to pass through power even when the solar voltage drops below their operating
threshold … ie 4v in = about 3.2v out which still lights up the indicator on the
LiPo-Rider. So, that early/late sunlight isn’t getting wasted.
Still though, that “extra” voltage around noontime has to be getting wasted
somewhere - and even if the regulator doesn’t feel hot it’s still sure to be
dissipating the excess as heat.
Both the zeners and 3-terminal regulators are CHEAP and EASY. My question
is whether anybody’s found any solution that’s better, yet still fairly cheap
and easy. I’ve looked at DC-DC converters but the conversion efficiency
varies considerably and ones with a really wide input voltage range run
almost $20. Of course there MAY be “perfect” converters that haven’t shown
up in my searches yet …
Any good suggestions ? I’ve already deployed five of these units and need to
make about ten or fifteen more. They’re WAY out “in the field” so they HAVE
to be solar powered and reliable. Only by using bigger (more expensive) than
strictly desired solar panels and just wasting x-percent of the power have
I been able to meet all my needs.
Any info/experiences/dire-warnings appreciated.