Date: Sat Feb 04, 2006 1:59 pm. By: Harry Chickpea
BlueEyes wrote:
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.
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.)
I am assuming that the batteries ratting will allow me to run about 30 minutes a day for a month before recharging.
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.
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.
Slightly more, and if you have a long wire run to the lights, even a little more than that.
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.
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.
Am I correct that a 625 Amp battery should be able to supply approx 30 amps for 20 hours?
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.
Also anyone know of an inexpensive way to monitor the battery condition?
Specific gravity meter. Go to an auto parts store or bigbox store and pick one up for less than $5. 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.
Would a volt meter show when it is time to recharge?
Yes, see above.
At what voltage is it time?
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.
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?
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
All and any help would be appreciated. Even a site where I can just get information would be appreciated. Thanks in advance. Steve
Totalling it up,another $120 for a couple more batteries to get through a cloudy period, $700 for solar panels, $100 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.