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Solar Power for Dataloggers ? - Few Commercial Choices

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Mr. B1ack
Guest

Tue Jan 07, 2014 8:30 am   



OK ... this has been driving me crazy for awhile now.

Solar battery-charger products generally assume you
need hundreds of watts - enough to light a house or
RV or something. Dataloggers need maybe 20 mA,
perhaps a brief surge to 250 mA when peripherial
devices switch on for the measurements.

Dataloggers also need tightly regulated voltage OUT
of the batteries - WHILE they are being charged. Ards
generally want 5.05v, Pi's I think are 3.3v ... not any
old thing between 3 and 7 but dead-on perfectly
regulated high-conversion-efficiency voltage.

Now for the Ards, two products sold by Seeed - the
"Li-Po Rider Pro" and a similar shield-incorporated
setup do in fact handle both functions pretty well,
for Li-ion cells anyway. Your solar can provide most
anything between 3.0 - 6.5 volts and it'll charge the
battery pack, and a seperate output provides rock
steady 5.05v whether the sun is bright or not or
you're on pure battery power.

AdaSoft also sells their version ... but it's NOT very
well regulated at all - indeed it could likely roast an
Arduino if the sun got really bright. They've got a
good cheap outdoor solar PV panel that's decent
however.

Note that said panel (1-watt, allegedly 6v), coupled
with the Li-Po Rider could be a problem. In very
bright sun the panel surges up to a little over 7v,
and the Li-Po Rider's specs say 6.5v max. As
the PV panel produces so little power, I just
bridged a 6v/3w zener across the outputs to
nail down any voltage peaks without wasting
power during off-peak output (could probably
have gone to a 6.25v, but I already had those
others in the parts drawer).

Li-ion batteries do have their attractions ... but
they're also nefarious for having a rather short
shelf-life (three, maybe four years) and then
they just die. You likely won't know when the
one YOU bought was actually manufactured.
If it's cheap, it's likely near its EOL.

Alternatives are, of course, Ni-Cad (charge
memory effect and sudden death if crystals
bridge the plates), NiMH (<500 recharge cycles,
sometimes much less) and lead-acid 6v or 12v.

Lead-acid batteries are, well, "availible" and if
you use common ones - like for UPS's or
emergency exit lights or home alarms - you
can score plenty of amp-hours for not too
much money. If not strained to the max they
can hold up for a long time - 5+ years is not
uncommon.

HOWEVER ... whip out yer Google and try to
find something like the "Li-Po Rider", but made
to work with lead-acid batteries that doesn't
cost a couple of proverbial body parts. Look
for something in a nice factory box that you
can show off to the boss as "See ! Off the
shelf !" and, well, I don't think it can be had
for even several body parts.

Now there are some semi-homebrew ways of
dealing with this. You'll need a TI battery manager
chip (they make many that will serve) to handle
keeping the battery topped off but not over-charged.
Then you'll need a seperate ~12v --> 5.05v switched
regulator to feed your datalogger.

I bought a TI UC3906N lead-acid battery manager chip
(digikey has a lot of 'em - about $6) which starts doing
something when the input reaches about 4.5v and does
more all the way up to 40v ... a nice wide range). It only
charges at about 1.5 amps max ... but even a fairly
ambitious datalogger project isn't likely to actually drain
a battery that's getting an amp or so charge for 6+ hours
a day. (oh yea, the UC3906n is availible in through-hole DIP
packaging :-)

For the regulated output I found a neat little DC-->DC package
from Recom Power (digikey 945-1690-1-nd) that can deliver
a steady 5.05v up to 500 mA from a source anywhere between
9v (a tad less actually) and 36v at (claimed) 87% efficiency.

The whole regulator fits onto a roughly 1/2" square tiny board
with the contacts at the corners. I just put another protoboard
under it and pushed some pins through at the corners so they
could be easily soldered to the regulator ... instant pluggable
power module. These things cost about $4 ... buy half a dozen
on a tape just to have 'em around. There exist higher-output
DC-->DC's in case you have really large transient loads.

Using this, I can port directly to an Arduinos +5v IN pin and
bypass the wasteful onboard 5v regulator (still need the
little 3.3v reg alas ... dammit !)

Now it's a pity that I can't find any commercial source for
what's essentially both kinds of chips integrated into a
single off-the-shelf style device. If you KNOW of one ...
I'd really like to hear about it because the Powers That
Be are fond of off-the-shelf solutions because they SEEM
cheaper, come in a pretty box and, of course, you can
blame failures on someone else.


Guest

Tue Jan 07, 2014 11:34 pm   



Followups set to sci.electronics.design.

In sci.electronics.design Mr. B1ack <nowhere_at_nada.net> wrote:
> Alternatives are, of course, Ni-Cad (charge memory effect

Much less of a problem in the real world than the vendors of all other
chemistries would have you believe. There will now be 500 follow-up
posts arguing this point to death.

> NiMH (<500 recharge cycles, sometimes much less)

The NiMH AAs I've used seem to give me more than that. YMMV.

> and lead-acid 6v or 12v.

You can also get PbA in 2 V increments - Gates/Hakwer/Genesis/Enersys
"Cyclon" cells. Sometimes being able to get a 4 V or 10 V battery
makes the rest of the power supply design easier.

Also, for really low-drain stuff like this, don't discount a plain old
alkaline battery. Energizer says their alkaline D cell will give you
about 20 Ah, when discharged at 25 mA from new to 0.8 V. That's a
little over a month of continuous runtime at 25 mA. About $1 each when
you buy several at once. Okay, maybe your device is at the top of a
radio tower, or down in a well, or stuck on a bobcat, or in some other
situation that makes it impractical to change the batteries.
http://data.energizer.com/PDFs/e95.pdf

Quote:
HOWEVER ... whip out yer Google and try to find something like the
"Li-Po Rider", but made to work with lead-acid batteries that doesn't
cost a couple of proverbial body parts. Look for something in a nice
factory box that you can show off to the boss as "See ! Off the
shelf !" and, well, I don't think it can be had for even several body
parts.


Powerstream sells a couple that might do what you want. They have this
board http://www.powerstream.com/solar-charge-alternate.htm that they
say they will program how you want; they have off-the-shelf versions
for PbA http://www.powerstream.com/pb-acid-charge-board.htm and NiMH/
NiCD http://www.powerstream.com/NiMH-NiCad-solar-charge-controller.htm
for $80 qty 1. They also have a smaller one, but it's designed for
48 V telecom stuff http://www.powerstream.com/48-pv-control.htm , for
$38 qty 1. I don't know if these chargers suck or not; I just know that
they exist.

Standard disclaimers apply: I don't get money or other consideration
from any companies mentioned.

Matt Roberds

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