wrote:

On Thu, 26 Feb 2009 17:55:56 -0600, "Pete C."
wrote:


David Nebenzahl wrote:

On 2/26/2009 1:13 PM Pete C. spake thus:

David Nebenzahl wrote:

Told a guy I'd do some research for him into a small simple solar system
for use in maintaining the charge on a 12 volt automotive battery. Found
this at Harbor Freight:

http://www.harborfreight.com/cpi/cta...emnumber=96418

It's a 12 volt, 15 watt panel for cheap ($70).

The 45W three panel package that Harbor Freight sells for ~$200 is the
best value $/W and the panels are decent. You have to replace the junk
charge controller they provide with a decent unit though.

Any suggestions? I'm not familiar with these units (charge controllers),
could use a little guidance here.

45W = 3.75A or so so you don't need a big one.

How do you come up with this Amp figure?

P/V=I

45W/12V=3.75A

It won't be exactly that since the voltage won't be exactly 12V, it
could be more like 3.21A @ 14V. It's also unlikely that it will get
quite up to the 45W rating unless you have a MPPT charge controller on
it.

I was looking at the 5 watt model that Harbor Freight has for $37
http://www.harborfreight.com/cpi/cta...emnumber=41144
which is made to clip right to the car battery. I was trying to
determine what the Amp output would be?

~0.4A or so.

And what kind of amperage does a standard car battery actually put
out?

Entirely variable. You usually don't find a proper Ah rating on a car
battery either, just a "group" and a "CCA" rating.

My truck battery is rated at 525 Amps, but I know thats the
rated starting amps, which I know is way overrated peak power.

Not over rated, it is probably "CCA" or Cold Cranking Amps, a rating of
the amperage the battery can supply for a short time in cold conditions.
It doesn't really translate into an total capacity figure like Ampere
Hours.


So, lets use the figure that you used above for a 45W solar panel
putting out 3.75 A. If I can charge my drained car battery in two
hours with a 10A plug in charger, will this solar charger charge the
same battery in 5.4 hours? 10A divided by 3.75 is 2.7 (approx), times
2 hours equals 5.4. (of course under full sunlight and proper aiming).

Roughly. Battery charging is a complex thing and as you may have noticed
with a battery charger that has an amp meter on it, charging isn't
constant current and the battery will only draw 10A at the start and
then slowly drop off over time as the battery charges.


With this in mind (if it's accurate), that 5 watt solar panel would
take 9 times as much time, or about 49 hours.

Of course I know these are not really intended to charge a drained
battery, but to add power and keep them at full charge.

Exactly. They are intended mainly to compensate for the battery's self
discharge and small loads like engine computers and radio memory.


Speaking about charging batteries. I recently had a really dead
battery (left the dome light on all night in real cold weather). I
needed to get the car started ASAP. I hooked TWO battery chargers up
at the same time.(a 10A and a 6A). It worked and charged pretty fast.

LM