Hey Texas Skooliemom!
First let me just say you are a goddess divine for including the link!
From the datasheet your refrigerator is a DC refrigerator, 4.1A on 12V & 2.1A on 24V.
So, you won't hook it up to the inverter, instead it hooks up to the battery and you don't need to worry about inrush current as that's the one thing batteries are really good at.
Now, your actual question is
would 2 AGM deep cycle battery at 120ah@20hrs be safe?
If it's physical safety, as long as nothing metal can get across the terminals and the system fused properly. You can also put caps over the terminals to shield them or wrap electrical tape around them.
But I suspect you want to know if two batteries are sufficient for your needs. The answer to that depends on the total amount of power you need. But it sounds like you've some experience with this and are only concerned with the fridge so let's talk about that.
The
Battery FAQ can explain most of this in better detail, but a 120 Ah lead acid battery is measured at the 20 hour rate, what that means is it'll last 120 hours as long as you don't pull more than 120 / 20 = 6 amps out of it. So far so good, the fridge only pulls 4.1 amps!
Next, all batteries have a limit on many times they can recharged before they need to be replaced. Most people only pull 50% power out of their lead acid batteries in order to increase the number of times they can be recharged The more they are discharged each time, the fewer recharge cycles they can support. As you can see from the
Battery FAQ, some battery vendors supply datasheets that allow you to understand what depth of discharge (DoD) works best for you.
So, if you go with the 50% depth of discharge, a 120 Ah lead acid battery only has 60 Ah available to you. Next is temperature. At 40 degrees F a lead acid battery has roughly 70% the power (an actual value would come from the battery's datasheet). So, your 60 Ah drops to 60 x .7 = 42 Ah.
Sorry for the long post, but only you know the conditions you'll be operating in and hopefully this explains how to figure it out. So, if you had two 120 Ah batteries that you operated at 50% DoD at 40 degrees F you'd have a total of 120 Ah x 2 batteries x 50% DoD x 70% temperature correction factor = 84 Ah. That's at 12 V, so 84 amp hours x 12 V = 1008 watt hours.
The refrigeration draws 4.1 amps x 12 volts = 50 watts while it's
running. But usually refrigerators turn on, then turn off until the insides warms up a bit. That's called the
duty cycle, and it depends on the refrigerator's insulation, how often the door is open, etc. It's really hard to guess this number, but let's say the compressor in the refrigerator runs 15 minutes for each hour, that's a 25% duty cycle.
So, each day the refrigerator will consume 24 hours x 25% x 50 watts = 300 watts per day. So, if your two batteries have a total of 1008 watt hours, you should have 1008/300 = 3.36 days on them.
Hope that helps!
