There's no way to know if this will be adequate without more information about your application. What is the current draw that you need? How long do you need the batteries to last?

What is the highest and lowest voltage that your system can run properly on? Is that what the 3.5 V to 5 V that is mentioned in your first post refers to?

The effective capacity of alkaline batteries is pretty sensitive to the actual current draw. In general, the higher the current, the less the available capacity is.

Thank you for the input. This is ALL helpful. It seems the first thing I should do is a test like I did for the 12v refrigerator in my van because they are similar in the sense that they run and they sleep. so for the fridge I hooked up a multi meter, turned on the fridge and it drew 5 amps for 2 minutes (the compressor) when the compressor shut off it only drew .08 amps for the following 5 minutes. then repeated. I did this with a stop watch for 1 hour then tallied up the whole thing and got a pretty good estimate of how many amps it would draw in (1) hour. There might be an easier way but I couldn't think of one. I'll do this with my new project and then I will be able to answer your first question accurately. Thanks ALL, have a great weekend, Fred

 

You still have not stated what the load current is (or will be). For example, the Duracell charts (in the PDF I posted earlier) show various values for "service life". Assuming you want to keep 3 cells each at 1.2V as a minimum (3.6V total for 3 cells), you get various "life" of the battery (at room temperature).
At 150mA load current (approx. 10 ohm load), you can expect about 76-78 hours before the battery reachs 1.2V from 1.5V
At 750mA load current, you can expect about 3.5 hours before the cell drain down to1.2V
At 1A load current, expect around 1.5 hours to reach 1.2V left in the battery.
The relationship is not linear. You have to determine the load you will be putting on the batteries before anyone can estimate how long they will last.

Thank you for the input. This is ALL helpful. It seems the first thing I should do is a test like I did for the 12v refrigerator in my van because they are similar in the sense that they run and they sleep. so for the fridge I hooked up a multi meter, turned on the fridge and it drew 5 amps for 2 minutes (the compressor) when the compressor shut off it only drew .08 amps for the following 5 minutes. then repeated. I did this with a stop watch for 1 hour then tallied up the whole thing and got a pretty good estimate of how many amps it would draw in (1) hour. There might be an easier way but I couldn't think of one. I'll do this with my new project and then I will be able to answer your first question accurately. Thanks ALL, have a great weekend, Fred

 

There might be an easier way but I couldn't think of one

The easier way is to use a Kill A Watt

You plug it into the wall and plug the fridge in to it and leave it for as long as you like and it will show you the average current and wattage.

 

The easier way is to use a Kill A Watt

You plug it into the wall and plug the fridge in to it and leave it for as long as you like and it will show you the average current and wattage.

if my fridge was running off my 3000w inverter at 120v it seems it would work no doubt. however, it is a "12v fridge" that runs off a 600Ah lithium iron phosphate smart battery bank in my van. it is DC, not AC. could you give an explanation as to how to use the KILL A WATT device in my situation? Thank you. Fred