Someone Help A Newbie On Batteries Please?

fetam

I have a 200sqft cabin I stay in 10 weeks out of the year. All during the summer. The sun I receive is 5.8 Kwh/days average for the 10 weeks that I'm there. All I need to run is a 500 watt window Air Conditioner and 1 60 watt bulb. Both those need to run about 5 hours a day max. I've read a bunch but it seems my calculations are off somewhere. Can someone help me with equipment selection?

Will 2 - 250 watt panels be good enough for this?

I also will need to stay with regular marine or car 12v batteries for this.

I sure would appreciate some help. Thanks a bunch!

rich

Re: Someone Help A Newbie On Batteries Please?

Hi ,Im far from the resident expert (I assure You there here)..but You will not get to far with automotive or marine deepcycle starting batteries, You may want to rethink this or do some battery research...good luck.

fetam

Re: Someone Help A Newbie On Batteries Please?

I have to keep this economically as possible and I have a wholesale account with an automotive supplier is all. If they wont work then I guess I'll have to look at other options. But I just can't spend $4k on batteries for 10 weeks of A/C at the cabin. My wife would strangle me!

BB.

Re: Someone Help A Newbie On Batteries Please?

Welcome to the forum Fetam.

using Marine or Car batteries are going to be difficult--If you could get 6 volt "golf cart" batteries, the probably would last much longer (3-5 years vs 1-2 years as a guess for a car/marine battery).

We usually recommend 1-3 days of battery storage and 50% maximum discharge. We generally find a 2 day + 50% maximum discharge a good mix for long battery life and reduce generator usage:

• 550 Watt load * 5 hours per day * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 539 AH @ 24 volt battery bank

Recommend 5% to 13% rate of charge for the battery bank:

• 539 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 1,015 watt array minimum
• 539 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 2,030 watt array nominal
• 539 AH * 29 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 2,639 watt array "cost effective maximum"

And based on the amount of sun you get:

• 550 Watts * 5 hours usage * 1/0.52 system efficiency * 1/5.8 hours of sun per day = 912 watt array minimum

So, based on the above, you are looking at ~1,015 watt array minimum to meet your needs.

You will need to closely monitor your battery bank (hydrometer and possibly a Battery Monitor) to ensure you do not "deficit" charge the battery bank (start out at 100%, discharge to 75% recharge to 95%, discharge to 70%, recharge to 90%, etc..).

A 1,015 watt solar array would be cutting it pretty close. You probably need a good quality 1,500 watt AC inverter minimum. The starting loads for A/C compressors can be as high as 3-5x running loads. You also need a good battery bank and heavy DC wiring too.

-Bill

fetam

Re: Someone Help A Newbie On Batteries Please?

So that was a lot of numbers and I'm a little slow.

So would a setup of 4 - 250 watt panels and 8 - 6v 250amp/hr batteries work ok then? Doesn't have to be perfect, just has to be better than zero cooling and a lantern.

BB.

Re: Someone Help A Newbie On Batteries Please?

And while this is a "solar forum"--It is very difficult to beat a Honda eu2000i genset or Yamaha ~2kW inverter generator...

The Honda eu2000i (1,600 watt) will run 400 watts for around 9 hours on a gallon of gasoline.

For seasonal use, a small/efficient (and quiet) genset for ~$1,000 plus 35 to 70 gallons of gasoline for the season is usually hard to beat.

One compromise is a small system with 2-4 6 volt 220 AH golf cart batteries that you can charge from a smaller solar array+genset+ac battery charger. Use a 300 watt TSW 120 VAC inverter for lights, computer, cell phone charging, etc.. You run the genset when needed (run A/C) and use the battery system for evening/night/quiet time power.

-Bill

Cariboocoot

Re: Someone Help A Newbie On Batteries Please?

Welcome to the forum fetam.

"All I need to run is a 500 Watt window A/C".

Oh dear. Better read this first: http://forum.solar-electric.com/showthread.php?16201-Fedders-5000-BTU-A-C-test

A/C units are not very small in power consumption. They have large start-up demands and a healthy Watt hour appetite. 500 Watt running 1/3 of the time (duty cycle; it will vary with demand) for 5 hours is 834 Watt hours. On a 12 Volt system that would demand 70 Amp hours, meaning an absolute minimum of 140 Amp hours. In terms of using the less-than-ideal Marine/RV batteries you'd need two of the big ones. If I were you I'd check for prices on GC2 6 Volt 220 Amp hour batteries instead; they'll have more capacity (in case the duty cycle runs long) and will withstand deep cycling much better because they really are made for it. Some of the "warehouse" stores sell them under $100 apiece sometimes.

Two 250 Watt panels would be enough to recharge even the GC2 option, no doubt. They would supply the necessary Watt hours for the day too. The down side is that they inevitably would be "GT panels" with a Vmp around 30 and thus would require the use of a more expensive MPPT type controller to utilize all the power available. The numbers would look like this:

500 Watts * 0.77 derating / 12 = 32 Amps peak charge current (14.5% on the GC2's, too high for the RV batteries but current can be limited).
500 Watts * 4 hours good sun * 0.52 over-all efficiency = 1kW hour AC 'harvest'.

You could shrink the array size a bit to around 350 to 400 Watts and it would still work.

A couple of other things: you would need a good pure sine inverter around 2kW size to handle the start-up surge of the A/C and the wires from it to the batteries have to be very large (4/0) and very short to reduce problems with Voltage drop on high power demand.

BB.

Re: Someone Help A Newbie On Batteries Please?

fetam wrote: »

So that was a lot of numbers and I'm a little slow.

So would a setup of 4 - 250 watt panels and 8 - 6v 250amp/hr batteries work ok then? Doesn't have to be perfect, just has to be better than zero cooling and a lantern.

Yes, that would be the "minimum" I would recommend based on the numbers you have given us so far...

The A/C system is a bit of a wild card... There are some that are pretty efficient and don't have huge starting loads--And others that are "not so nice". I don't use A/C, so I am not a good person to suggest a small window unit (and vendors are changing A/C systems pretty quickly right now--so what was true a copule years ago may not be true today).

A smaller solar+battery+inverter to power lights/electronics plus a genset may be a better choice... I always worry about theft/damage with seasonal cabins (may not be an issue in your region).

Less expensive stuff left behind (smaller array, fewer batteries) and you can take a 50lb genset home with you at the end of the season.

-Bill

Panamretiree

Re: Someone Help A Newbie On Batteries Please?

fetam wrote: »

I have to keep this economically as possible and I have a wholesale account with an automotive supplier is all. If they wont work then I guess I'll have to look at other options. But I just can't spend $4k on batteries for 10 weeks of A/C at the cabin. My wife would strangle me!

Interesting thoughts on SWMBO. We bought our first boat, a 31 foot Uniflite, in '05. Did not have a generator so I thought this was my inroad into getting a good generator for boat and house; however, SWMBO stated that she was not having a generator operating on the boat no matter how quiet or non-smelly, but she did let me redo the electrical system in its entirety complete with 2-100W solar panels, 4-L16 house batteries, 1500W inverter charger, CC, new 12V and 120V Blue sea power panels, galvanic isolator, Balmar regulator - center fielder - duo charge units, Link battery monitor, and the likes. Price was more than the generator but SWMBO was happy. Never know, peace and quiet, eco friendly. Just some thoughts.

Cheers

Ernest

YehoshuaAgapao

Re: Someone Help A Newbie On Batteries Please?

No refrigerator to power? Even a small one? No cell phones, smart phones, tablets, portable game consoles, NIMH batteries (AA, AAA, C, D), or laptops to charge up?

Port-A-Cool evaporative coolers is such a wonderful technology. If you aren't in the rain forest (or Florida), these things will cool your place down good. Port-A-Cools are industrial units, so they put up with moderate amounts of humidity but they are loud. A single 16" unit will keep a whole 1600 square foot house house 80-85 in 105-110 degree weather at 15% or less humidity.

I think the minimum for any long term away site is 2 strings of full size 60-cell solar panels (6x 225-260W), combiner box (good to have for expansion), 60A MPPT charge controller (full @ 24V with 6 fullsize panels), 2000 watt pure sine wave inverter (probably won't be able to get less than 3000W with 48V DC), and a two 24V strings of 8 GC2 (golf cart) batteries. Build it into a cart on wheels and it will probably fit in the back of your truck (but probably take up most of the space). This kit will run around $5K probably. A 2000 watt Honda Inverter-Generator will give you peace of mind and these are quiet (no louder than a Port-A-Cool, 50-70DB) and portable. Extra $$$ for that.