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sizing solar panel to recharge 1 12 volt battery

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Posted by BlueEyes on February 4, 2006, 12:04 pm
 



Hi,
I am hoping that someone in this group can
give me a web site where I can find solar
panels  and information that will be used to
charge a marine battery.

I have just rented a barn that no longer has electricity in it.
I purchased a 12 volt marine battery with 625 CCA's
and installed 3ea  50 watt 12 volt light bulbs.

I am assuming that the batteries ratting will allow me to
run about 30 minutes a day for a month before recharging.
I doubt that I use all three lights at one time. More like one
or two.

I figured that each bulb would consume 4 amps while on.
I know quite a load for a battery. I have read that the rating
on a battery is usually calculated for a current drain over
approx 20 hours.

Am I correct that a 625 Amp battery should be able to supply
approx 30 amps for 20 hours?

Also anyone know of an inexpensive way to monitor the battery
condition?  Would a volt meter show when it is time to recharge?
At what voltage is it time? Also, how big of a solar panel
(output watts) would I require? Is there a diode that is required
to prevent the battery from discharging on cloudy days?

All and any help would be appreciated. Even a site where I can just
get information would be appreciated. Thanks in advance.
Steve



Posted by Duane C. Johnson on February 4, 2006, 12:47 pm
 


Hi Steve;


 > I am hoping that someone in this group can
 > give me a web site where I can find solar
 > panels  and information that will be used to
 > charge a marine battery.

 > I have just rented a barn that no longer has electricity
 > in it.I purchased a 12 volt marine battery with 625 CCA's
 > and installed 3ea  50 watt 12 volt light bulbs.

 > I am assuming that the batteries ratting will allow me to
 > run about 30 minutes a day for a month before recharging.
 > I doubt that I use all three lights at one time. More
 > like one or two.

 > I figured that each bulb would consume 4 amps while on.
 > I know quite a load for a battery. I have read that the
 > rating on a battery is usually calculated for a current
 > drain over approx 20 hours.

 > Am I correct that a 625 Amp battery should be able to
 > supply approx 30 amps for 20 hours?

No, CCA or Cold Cranking Amps is a rating for the
ability to start an engine.

What you need to know is the Ahr or amp hour rating.
Generally the Ahr rating is measured at the 20 hour
rate.

Your battery, complete guess here, probably in the
100 Ahr to 200 Ahr range.

 > Also anyone know of an inexpensive way to monitor the
 > battery condition?  Would a volt meter show when it is
 > time to recharge? At what voltage is it time? Also, how
 > big of a solar panel (output watts) would I require?
 > Is there a diode that is required to prevent the
 > battery from discharging on cloudy days?

 > All and any help would be appreciated. Even a site
 > where I can just get information would be appreciated.
 > Thanks in advance.
 > Steve

Duane

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Posted by Harry Chickpea on February 4, 2006, 2:59 pm
 



Marine battery, as used for trolling motors?  Or marine battery as
built with stonger plates to take the pounding of a marine
environment?  The reason I ask is that trolling motor batteries
typically have an AmpHour rating over a number of hours, like 90 AH
175 minutes (read that as - the dreaming capacity of the battery is a
TOTAL of 90 amp hours IF you discharge it evenly over 175 minutes.)


You assume wrong, for a couple of reasons.  Lead-acid wet cell
batteries do not like to be left in a discharged state for any length
of time.  There are chemical reactions that occur that will reduce the
capacity of the battery permanently.  A battery is ideally used and
then immediately recharged.  In practice, a battery that is discharged
up to about 20% is fine for a week (this is very rough rule-of-thumb
and will undoubtedly spark discussion.)

Secondly, a group 27 marine battery typically has a store rating of
about 100 amphours.  (You won't find one rated more than 120 amphours)
De-rate that by at least 10% to get a realistic rating, then de-rate
it another 20% so that you don't design your system with a greater
than 80% discharge, which will kill a battery in short order.  That
gives you a MAX of 70 amphours that you should ever take out of the
battery.  If you want the battery to last for the rated 6 years or
more, only take out a maximum of 50 amphours.

150 watts / 12 volts = 12.5 amps   / 30 minutes  > 25 Amphours.  You
have 50 amphours usable capacity.  That gives two days of use if all
three bulbs run 30 minutes each day.  You can go longer than that, as
long as three or four days, and it will seem fine for a while, but
within a month, you'll have damaged the battery beyond saving.


Slightly more, and if you have a long wire run to the lights, even a
little more than that.


Sort of.  Battery manufacturers who want to make their batteries seem
more powerful use a C20 rating, which provides more juice out of a
battery than a C10 or C5 rating, due to the Peukert effect.


If you had a battery that large, and wanted to destroy it with
continued usage, yes.  If you wanted to do it with marine batteries,
and make them last, 10 or 12 of them in parallel should do the job.


Specific gravity meter.  Go to an auto parts store or bigbox store and
pick one up for less than $.  Battery state-of-charge can be measured
with a voltmeter, but unless you know how to compensate for a few
factors, the readings can be more decptive than helpful.


Yes, see above.


It primarily depends on a combination of the load, any charging
current present, temperature, battery capacity, whether the battery
has been used recently, and time since the last charge.


The diode is usually built-in.  The size solar panel you would need is
dependent on the site.  You also need a solar charge controller.  Off
the cuff, about 200 watts of output would probably let you do on a
daily basis what you first described


Totalling it up,another $20 for a couple more batteries to get
through a cloudy period, $00 for solar panels, $00 for a charge
controller, amazing how fast dreams of cheap solar power get crushed,
huh?  Put your single battery on a handtruck, pull it to the barn at
night, pull it back with you and plug it into a smart charger
connected to grid power and charge it for the next night, and you
should be fine.  Put the lights on the handtruck and you have a really
big flashlight.  :-)

Cut down the amount of amphours you use and pray for sun, and you
_might_ get away with a single 75 to 100 watt panel.  Remember that
during the winter there is less sun and you need more lights.  In the
summer there is more sun and you need less lights.

For your situation, a better solution might be a rechargable camping
lantern with a florescent lamp. NiMH batteries take much more abuse
and are lighter.


Posted by Vaughn on February 4, 2006, 6:33 pm
 



     Or quadruple the efficiency of your lighting and greatly reduce the size of
the system you need to power it.

Buy these lamps:  http://tinyurl.com/cw5mx  and you will have the same amount of
light as before, but only draw one amp each. (be sure to hook them up with + to
the tip)

   Now you can get by (at least for a while) with the battery you already have
and perhaps 50 to 75 watts of solar panel.

   You will still need a controller.  Here is a good one:
http://www.affordable-solar.com/80.html

Good Luck;
Vaughn




Posted by Harry Chickpea on February 4, 2006, 7:18 pm
 

hchickpeaREMOVEME@hotmail.com (Harry Chickpea) wrote:


<doH!> mybad.  I just re-read what I wrote.  If you are using 12.5
amps for 30 minutes, you are using 6.25 amphours, not 25 amphours.  I
_thought_ the capacity seemed way smaller than it should be.  That is
what I get for responding before coffee.

With 50 amphours available, that would mean pulling the battery and
recharging once a week.  A single panel and charge controller would be
more than adequate.  Carry on.

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