I installed an electric gate opener which came with a transformer to be
operated from house current but was specifically made to allow a 12v
battery for power outages. Because of the gate's location, I removed
the transformer and put in a 7 amp hour gel cell with a "1.8 watt"
trickle solar charger. The battery ran down over a couple of weeks,
even without using the gate opener. I suspect the the constant battery
drain from the receiver may have something to do with this, along with
the fact that I live in cloudy Oregon. I bought a 5 watt solar panel
to replace the smaller one. It too is indicated to be a trickle
charger, although this is probably meant for an auto battery, not a
much smaller gell cell. I noticed the battery is at about 14volts when
not operating the opener, and I am concerned about over charging. I
thought about getting a charge controller, but with such a small
battery, could the charging current still be too high? I believe the
charging current curve for the controller is likely meant for larger
batteries. I also thought about connecting a power zener diode of
about 13.5 volts across the battery. Is this a bad idea? My last idea
is to get an auto battery, put it in a plastic case, and connect it to
the solar panel without any charge controller. Are any of these
solutions good or are there other ideas? Thanks
Now where did you get 7 WATTS from?
He said 7 AMP HOURS 'yknow, hardly the same units of measure, are they?
That "1.8 watt" solar charger probably puts out its rated 1.8 watts in
full sunlight, and of course nothing at night. Assuming it puts out say
1.0 watt average for 8 hours every day says it would put (1/12)*8 =2/3
of an amp hour per day into the battery if the charging were 100%
efficient, which it isn't, so lets guess at maybe 1/2 amp hour into the
battery per day.
Now, let's forget about the gate motor for the moment and assume all
that's if all that's draining the battery is the receiver. To pull 1/2
amp hour out of the battery in 24 hours takes a drain of 1/48 amp, or
about 21 milliamps.
So take a meter and see how much the unit draws on standby. If it's more
than 21 milliamps, then you've proven why the "1.8 watt" solar charger
wouldn't keep the battery up.
If you followed the math, you should be able to reverse the process and
see how large a solar panel you'll need to replace what the receiver is
constantly drawing from the battery, then go for a charger that puts out
a bit more than that so there's some energy in the battery to run the
gate motor for a few seconds a day.
If it was me, I'd go for a nicad pack and size the charger so it put a
current of 1/10 of the pack's amp hour rating (i.e. 1 amp for a 10 amp
hour pack) into the pack in full sunlight. Nicads will take that much
charging current forever without damage.
That's just what I did when I added a small solar panel to the top of
our "Yankee Flipper" squirrel flinging bird feeder several years ago. I
just added a series resistor between the panel and the pack sized to
give a charging current of 1/10 of the nicad's amp hour rating in full
sunlight. It's still working fine, but the squirrels catch on quick, so
there's not much to look at besides birdies these days.
Measure the current from the cell to the battery after it
has reached "full charge" condition. (In full sun, etc) if
that current is low enough for your battery, you should be
fine. One reference I found suggested trickle charging at
1/100 the battery capacity, which would be .07 amps for your
battery. You might find a spec for your particular battery
with proper searching. Or, you could adapt someone elses
charging circuit perhaps.
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