Number of batteries with solar?

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fire-99

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Mar 14, 2006
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Right now we are staying in a place where we don't have the ability to plug in.  We can run the generator but it is noisy and can get expensive with gas around $3/gallon.  We have an 85 watt solar panel which we are using to keep our batteries charged.  My question is:  will the two group 27 deep cycle batteries do me or should I think about adding a couple?  We are pretty stingy with the power, never that much on, and then usually 12 volt.  If we get a couple more batteries will our 85 watt panel charge them okay or am I going overboard?

thanks,

Adam
 

John From Detroit

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27's are kind of wimpy now days, but depending on what you are doing with them should work ok,  Problem is, you did not say.  My motorhome came with a pair of Work-A-Holic U-220 6V Deep Cycle Golf Cart type, they run most of what I need till about 3am, then I need to recharge, I have a Xantrex Prosine 2000 inverter, and one of my passangers liked to watch TV till the wee hours (Sattalite TV usually)

I added a couple of, I think 8D AGM batteries and transfered just the Xantrex to them.  The Work-A-Holics (And the AGM's) lasted all night and into the next day

Now, I use a LOT of power here, Microwave (Xantrex) two televisions (Xantrex) and all the motor systems (non-xantrex) we had left the isle lights on all night (We don't now)

I would say try this, Monitor the voltage for a couple of night, if you are at home make it like you were camping, See if the batteries hold, if not, add batteries, and panels
 

Jeff

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John:

Won't the real issue be whether or not the 85 watt panel is capable of creating more power than the batteries Adam has can store. If so he needs more, if not then the two are sufficient.

Adam:

I think your 85 watt panel is creating 60-65 effective watts an hour at full charging rate and depending where you are can put out 600-800 watts a day. (I'm sure someone here has much more accurate numbers) That would work out to 50-66 amp/hours at 12 volts which your 2 group 27's should be able to store if they are in good shape.

What I think John is suggesting is to work the other way, determine what you need for power to live the way you want and then install enough battery storage to provide that amount of power and a means to charge them.
 

John From Detroit

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Yes, that is PART of the issue, There are two issues

1: Are the batteries big enough to provide the power he needs and
2: Is the panel big enough to recharge the batteries

In both cases, I suspect not

But a test, while parked in his driveway or wherever he parks when at home, might just find out
 

fire-99

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Mar 14, 2006
Posts
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Well so far I have made it since Tuesday on just my batteries leaving the panel out during the day.  It's been quite cloudy the last couple of days.  We don't watch TV so that's not a problem.  Mostly we use a couple of the cabin lights during the night and of course the water pump.  But I'm not here during the day so right now nothing is being used during daylight hours.  I think my question really is; would I be better off with more batteries and would my panel keep them charged.  I figure 4 days on batteries so far is about double what I've ever gotten before so I'm already ahead.

Adam
 

Jeff

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Adam:

If your batteries can be fully charged by your panel each day then you could use more batteries. You might try intentionally using them up some night and see if a sunny day can recharge them back to full charge in one day. If it does then you could use more storage.
 

Karl

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Adam,

It really doesn't matter how many batteries you have (2, 4, 6, 8, etc.) as long as you can put back at least the same amount of power as you take out of them. For people without solar, the advantage of using multiple batteries is not having to run the generator as frequently. More full batteries - longer time between charges. Even people with solar may have to run a generator occasionally if there is a long period of cloudy weather, or higher than expected use of power. That's when having extra batteries will give you an additional 'cushion' of power for a few days. Will your panel keep the batteries charged? Not an easy question to answer. You have to know how much power you use in watt/hours per day, how healthy the batteries are (will they charge completely and hold the charge?), how much power can you reasonably expect the solar panel to put out on a daily basis, and maybe a few other variables I may have missed. Bottom line is there is no "Yes" or "No" answer; it all depends. Maybe some of our solar experts can give you some rules of thumb to go by. 
 

Gary RV_Wizard

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There are two ways to come at this problem.

If you have enough batteriy capacity you can run longer between charges and perhaps can wait until you have a shore power connection available to re-charge.

If you have limited battery capacity, you need more charging capability so you can keep re-charging them. 

Obviously there is a range of solutions here, balancing battery capacity and re-charge capacity.  With the limited solar recharge capability (65-85 watts), more batteries merely means you have some more reserve for cloudy days or above-avergae usage. On good solar days, you may replace most of what you use.  Other times you will not, so more (or larger size) batteries will extend your camping time.  It sounds as though you are using more than your panels are re-charging (you wouldn't run out otherwise), so more batteries wold indeed extend your time.

A new Group 27 battery stores about 105 amp-hours when new (210 per pair). If you could replace them with Group 31's, that would increase to about 130 a-h (260/pair). Replacing the two 27's  with a pair of 6V golf cart batteries in series would yield 220a-h, probably not worth the cost (though golf cart batteries have a superior life span). Adding a third 27 would increase capacity 50% and give an extra day or two before recharging is needed. However, once discharged, your single panel cannot quickly recharge the extra capacity, so there is no real benefit beyond the initial cycle.

There is a small benefit in re-charging a larger battery bank in that it when discharged it accepts the full charging rate for a longer time. Once a battery bank reaches somewhere between 2/3 and 3/4 charge level, the rate of charge slows down dramtically and it takes a long time to reach 100%.  Typically the last 10-15% of charge takes 24 hours, regardless of the size of the charger.  A bigger battery bank has more total amp-hours available in that range where re-charge is fairly fast.
 

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