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Old 09-15-2002, 09:07 PM   #1
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Solar battery charging

I have some experience here in that my island cabin uses solar: 2 golf cart 210 amp hr batteries, and a 100 wt panal @12v. The situation is comparable to the Airstream. But I also have a backup 5000wt generator, propane range and refrigerator, and a certain amount of wood heat. The solar is used mostly for the lights, radio, and a very small circulating fan in the bathroom.
I have had two interesting revelations; 1) the solar panal does not produce a whole lot of power (at least in Northern Wisconsin) and 2) it is amazing how little power you can get by with for lights and radio. I have been using 6wt halogen lights but, in reality, florescents are even more efficient. Don't even think of being reliant on solar for heat, a/c, or much use of a power tool.

To quantify what I am talking about, the solar panel produces about 1/2 kw of power on an average day. Compared to commercial electric rates that is about $.03 worth. The panal costs about $500 and with mounts, regulator, and wiring, the cost per 100wts is about $800. Potential payback period for the system is about 73 years. You really have to have a special need and interest, as I did, staying on an island where commercial electricity is not available. However, I could merely have run the generator for a short period each day to charge the battery.

By the way you shouldn't have to pay so much for a panal. At the energy show in Amherst WI last year it was possible to pick up a 120wt Kyocera for about $450. The best battery deals are Trojen 6v golf cart batteries for about $62. However, I noticed that "Sams" had what looked like a similar battery with the same amp hour rating for only $45. Does anybody know if this is a privite labled Trojen battery? Supposedly these Trojen batteries will sustain the greatest number of recharges. Hook them up in series for 12v.
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Old 09-16-2002, 09:41 AM   #2
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Wiring 6 volt batteries in series, to obtain 12 volts, is not a good idea.
Should one battery fail, then it's all over.
When batteries are wired in parallel, when one fails, it will take out the second one, that's true. However, if you disconnect the bad battery, you still have battery power available.
It's like series or parallel Christmas tree lights.


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Old 09-16-2002, 10:10 AM   #3
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Charles,
I know from previous posts that Inland RV Andy does not like the idea of 2-4 Trojan 6 v. batteries wired together but there are a number of websites that push the four 6 v. idea with the use of solar. One site is www.rvsolarelectric.com. They have a small pamphlet they mail out and it is great as a starter. They have other books on understanding solar and one for use in homes/cabins. Their prices on panels and system kits is not bad.
I can understand Andy's point of view about the 12 v. batteries to keep it simple but it seems that if you want to go all out to get the most amp hrs., you go with four of the 6 v. Trojans and back it up with with a 100-120 watt panel or two of the 75 watt panels. If I was limited to 2 batteries then I would stay with the 12 v. batteries but if I had the space (and the money) I would go with a slideout drawer with 4 Trojans. I would then add an inverter and supplement with a small generator if I had to. This is taking into consideration that I would not be running AC everywhere I went because I would have carry a larger generator. If someone were to give me a generator capable of running my AC, I sure would not turn it down because it sometimes gets near 100 in my trailer during the hottest time of the summer in TN.
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Old 09-16-2002, 12:25 PM   #4
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Charles, I have one 120 watt solar panel and two 6 volt Trojan batteries and I am able to run lights, fans, TV, VCR, satellite receiver, inverter,stereo, water pump, etc without running out of battery. I am located in Southern California so I get a bit more sun in the winter than the folks up North. I also have a generator but I only use it when I need to run power tools. Andy has a good point about a 6 volt battery failing and leaving you with nothing. However, Trojan claims you can cycle these batteries 500 times if you do not take them below 50%. If I cycle them once a week they will last almost 10 years!
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Old 09-16-2002, 12:39 PM   #5
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Craig. Ampere hours is another way of saying "watts." A 6 volt 200 ampere hour battery contains at full charge 1200 watts. Watts is equal to voltage times the current.
A 12 volt 100 ampere hour battery delivers the same 1200 watts.
Yet basically the two batteries are the same physical size.
The reason the automotive industry switched to 12 volts is that, amoung other things, it is more efficient, and the wiring size was reduced.
The only wire size considerations in an Airstream would be the wiring at the batteries.
There is "no" advantage to wiring 6 volt batteries in series, or series parallel for that matter, but there are disadvantages.
Wattage is wattage, pure and simple.
The misleading information comes from double the amperes from a 6 volt battery. But they didn't tell you that at double the amperes, the voltage was 1/2.
Again the comparison to a 12 volt battery is that it gives you twice the voltage but at half the current.
Additionally, 12 volt batteries today, can be found almost anywhere. Not true for the 6 volt.
Basic "ohms law" spells it out. E=IR, I= E over R, R= E over I, W=EI, W=I squared x R.
It's not me that disagree's, it's "ohms law."


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Old 09-16-2002, 01:11 PM   #6
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Andy,
Thanks for the refresher. I guess I need to go back through the solar material. I was thinking that with the four 6 v. batteries wired correctly in rvsolarelectric's diagram, you had the 12 volts but the amp hrs. was around 420 rather than the 210 of the two 12 v. batteries. I believe the diagram even listed the 420 amp hrs. I will look again this evening. I have been known to make a few mistakes in my lifetime.
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Old 09-16-2002, 01:47 PM   #7
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Craig, I believe you are correct: four 6-volt Trojan T-105 golf cart batteries wired in series/parallel will give you 12 volts and 450 AmpHours.
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Old 09-16-2002, 02:27 PM   #8
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Thanks Jerry,
I sometimes think I am losing my mind at 51. I understood that the 6 v. golfcart batteries have the thickest battery plates and therefore are heavier. Because of the construction they are better deep cycle batteries than RV/marine. If you have the money to build a bank of 4 of these as well as a few solar panels, you have the money to replace a battery if it starts to get weak. I know that it must be a pain to use a hydrometer? or whatever you call it to check the batteries and to make sure that your always have enough water in each cell but oh what a system. For the general RVer, Andy is right. You don't want to be caught without any power. Those with advanced experience who are able to build a system like this and maintain it so that they can occasionally run the microwave by use of an inverter when no 110 v. power are in a league all their own. I'll stay with the KISS principle, keep it simple stupid!
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Old 09-16-2002, 03:40 PM   #9
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Windsun.com great resource for info on solar panels, batteries, inverters, etc.

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Old 09-16-2002, 06:07 PM   #10
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If a Trojan T-105 means 105 amps then 4 of them in series would give you 24 volts at 105 amps.
However, if you put them in series parallel, you will have 12 volts at 210 amps.
If you put them all in parallel, you would have 6 volts at 420 amps.
Making a series parallel network would double the voltage and HALF the current.
One 6 volt battery at 105 amps is 630 watts. Four of them give you 2520 watts.
Four 6 volt batteries in series parallel at 105 amps each do not give you 5040 watts, they can only give you the same 2520 watts.
Therefore the series parallel provides 12 volts at 210 amps, which is 2520 watts.
Again, guys, basic "ohms law."



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Old 09-16-2002, 06:59 PM   #11
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Andy, they are each rated at 225 AmpHours. Two in series gives you 12 volts and 225 AmpHours. Parallel two more and you get 12 volts and 450 AmpHours?
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Old 09-16-2002, 07:58 PM   #12
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If I might... yes, a T-105 provides 6 volts for 225AH... at a 20 hour rate, meaning 225/20=11.25 amp continous draw. Two of them in series will provide 225AH at 11.25 amp continous draw with 12 volts. And if you parallel two sets of two in series, you get 450 amp hours at a continous 22.5 amp draw. Keep in mind they are significantly larger than Group 27 batteries and might not fit in RV battery boxes.

Now lets look at Trojan's 12 volt Group 27 batteries. They'll provide 115 amp-hours at a 20 hour rate or 115/20=5.75 amp continous draw. Two of them in parallel will provide 230 amp-hours at 11.5 amp continous draw... slightly little better than two T-105 batteries in series, and they'll fit in standard battery compartments. And yes, four of them in parallel will provide 460 amp-hours.

Where's the advantage to the larger T-105?

But remember... these amp-hour specs are at a 20 hour rate. As discharge current goes up, amp-hour capacity goes down. A series pair of T-105 with a 75A load on it doesn't quite last 2 hours, so at that discharge rate, it's capacity is less than 150 amp-hours. That's true of both golf cart and Marine/RV batteries. The same degradation in capacity will happen with the 12V Group 27 battery at a 37.5 amp discharge rate (but there are two in parallel for a total 75A discharge rate).

That's about the max I'd want to pull from a battery (80A from a golf cart or 40A from Group 27) continuously and I'd really prefer to keep it at near half that for increased capacity and battery life.

A 1500W Trace DR1512 inverter will suck a whopping 165 amps from your batteries at rated output (i.e. powering a 1000W microwave oven, which typically consumes 1500W when the magnetron is powered (for roughly 2/3 of the time on the high power setting). That's a lot, and it will probably take 5 hours of good sun on four 120 watt/7 amp panels to replace what the microwave takes through the inverter in 15 minutes because it's consuming higher-current amp-hours.
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Old 09-16-2002, 09:28 PM   #13
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Batteries in Series

Not a thing wrong with wiring batteries in series. They do it on farm tractors, construction equipment, semi tractors, (24 volt starting systems)you name it. I even have a UPS at work that has 24, yes count them, 24 12 volt batteries in series. Yes, if one opens up, the whole shooting match goes down. In fact it did, about 4 years ago. But such failures are very rare. You can go 4-5 years without a failure, and if you have a decent battery maintenance program, never a failure.
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Old 09-16-2002, 10:11 PM   #14
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Re: Batteries in Series

Quote:
Originally posted by Pick
I even have a UPS at work that has 24, yes count them, 24 12 volt batteries in series. Yes, if one opens up, the whole shooting match goes down. In fact it did, about 4 years ago. But such failures are very rare.
I do too, but I'd be hard pressed to used the 20 kVA, and 3 phase power, it produces in an Airstream. Our first battery set lasted 7 years with no failures, then they were replaced under the maintenance contract, so who knows how long they would've gone.

HOWEVER... we only lost commercial power and had to use them twice in those 7 years... so you can't compare these to batteries that are discharged regularly, especially at the rates some inverters tax them.
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