I found a great write up on lead acid batteries that brings together so many of the facts that have been stated on different threads here but thought I would share as it covers so much. The article was written by Dave Thacker from the web site
http://www.radicalrc.com/blog.
Life of Lead Acid Batteries
by Dave Thacker
I'm a battery and charging professional, I live in the Radio Control world and own Radical RC. I've got quite a bit of experience with batteries and thought perhaps I could help you and your readers over some common issues with 12V lead acid batteries used with inverters and other off grid purposes. This information is provided in general rule of thumb format. So, let us begin....
An important concept to remember about lead acid batteries is their life is shortened in direct per-portion to how deeply they are discharged and how long they are left discharged. Conversely, the greater a percentage of the time they are full, the longer they last. Every minute they are not 100% full, they are sulfating. The more discharged they are, the faster this occurs. If you want to get maximum life out of them, you want a very large bank so your typical discharge (over night or cloudy day) is something small like 10 to 15% maximum of the total capacity. The less percentage of discharge the better. In the morning, you want to fill them quickly so they are essentially full all day (full the maximum amount of time over their life). From a replacement battery cost and reliability sense, over time, it's going to be cheaper to own excess solar panel so you can punch them up quickly and hold them full all day.
The idea of having enough battery to get through the night, essentially running them down 1/2 way or more every night will mean you'll be replacing them on a rapid schedule. This will be very expensive. I think those cells your using are about 70 amp hours @6V each. With 4 of them, your probably sporting about 3360 watt hours (280amp hours) of battery. So, I'm guessing on a fridge with your other loads, they would degrade about 50% in performance (storage capacity) every month or so.
I've owned two houseboats and it is important for them to run off the grid for days at a time. The last boat, I had a 2000 watt inverter and 4 8D truck batteries. These were anomony type cells (have to service with water periodically) and are great for this purpose. Having an account with a battery distributor, I was able to obtain them all brand new as blemished batteries for about $80 each. A great bargain considering they are about 200 amp hour @ 12v each. So, I had about 800 amp hour of capacity or about 9600 watt hours of energy available. 8D truck batteries are large and intended to survive while slowly discharging in a slowly idling diesel tractor all night for many many nights.
Obviously I had lots of small incidental loads to service just like you, Laptops, cell phones, camera charging, occasionally a friend with a CPAP machine and etc...
Some interesting facts and examples. Your inverter is multiplying the 12v DC to 110V AC. To increase the voltage, the amps drawn must be multiplied by an equal amount. So, if we want to multiply the voltage by 10, it will take 10x as many amps at 12v as we are demanding at 110v. Simply, 1 amp 110v appliance running on an inverter will draw about 10 amps (10 x 1amp) from the 12v battery. So, simple low wattage/low current things like cell phone chargers are fine, heavy duty things like hair dryers, electric heaters and microwaves will draw much much more. Many 110v appliances have a label that tells you the watts but not the amps. If you know the 110v watts, a simple way to figure it is divide the watts by the 12volts of the battery to arrive at how many amps this costs are 12v. Example: 1500 watt heater to warm a bathroom. If you divide 12v into 1500watts, you get a 125amp load on your battery. Yikes! Most appliances with heating elements will be about 1500 watts. So, an RV Fridge running on 110v, hair dryer, larger microwaves, electric coffee pots and such. 125amp load is HUGE on your 12v battery bank. Putting a load like that on your battery bank for 15 minutes (1/4 hour) will liberate 1/4 of 125 amps or about 31 amp-hours out of your bank. So, if you want to make a pot of coffee, and don't want to use more than 10% of your bank, you need a bank of at least 310 amp hours in size. AND you have to start with the bank full, AND you need to be able to recharge that bank with your energy source in as short a time as possible.
My main concern on the boat was being able to run the refrigerator 24 hours and not listen to a generator all day and to have the convenience of incidental 110v for toys like phones and cameras. It's also nice to run a drill or circular saw once in a while. Always a project underway on a houseboat. The off grid camper has a very similar problem, wanting to run a fridge and incidentals on limited solar input. With my system, I ran the generator only when we ran large loads like coffee pot or baking potatoes in the microwave. The boat could successfully run for a week with only 1 to 2 hours of generator time per day. And, I never took much out of the battery bank.
I looked up a similar fridge to what I had, about 10 cu ft like Sanyo model SR-1031W/S. If you click the downloads tab and load up the image of the Energy Guide, you'll see it uses 350 kw per year. A KW Hour is 1000 watts for one hour. So, in one year, this unit will consume 350,000 watt hours or 350 Kilo Watt Hours. In reality, the boat and the off grid camper are not air conditioned in the summer, the Energy Guide rating is probably assuming it's installed in an air conditioned space. So, energy use will be higher in summer, probably lower in winter. To keep it real simple, lets say energy use is 365kw hours in the real world. 1000 watts per day or 1kw per day. That means it will take at least 250 watts of panels 4 hours per day at full tilt to replenish what this unit used from the battery bank. 1000 watts divided by 12v means it will use about 84 amps out of our battery bank each day or 3.5 amps (84/24hrs) of that bank on average each hour. So, there we go, in 12 hours of darkness the bank will be drained by 12 hours x 3.5 amps or about 42 amps. If you only want to pull 10% off your bank overnight, it needs to be at least 420 amp hours in size just to service this refrigerator. AND, that's not considering other loads like a 12v mini fan to sleep under and the ipod you left plugged in and all the other incidentals. I installed mine in a wooden cabinet and it was lined with foam insulation. I unbolted the evaporator coil on the back and put a layer of Reflextix on the back. Pennys spent here meant less generator run time and longer battery bank life.
Using the above logic and understanding of how things work leads you to using a very large battery bank, My batteries lasted at least shelf life, I never replaced them in 8 years. Pretty shortly, this becomes the least expensive way to get the job done.
Dave Thacker
Radicalrc.com, Owner
Radicalrc.com/blog Blogsite