karlok:
I don't understand why I should check the batteries with load.
I want to check is whether the batteries are still able to work as a power source if they are needed.
I understand the confusion, and I am partly the cause of it.
What you really want to know, is "are these batteries able to run my project, if I need them to?". The best way to know for sure is to put a load on them about the same level as your project would, and see if they can handle it. If you go buy a "battery tester" this is exactly what it does.
If you do not put load on the battery, you will be reading "open circuit" voltage. For some battery chemistries, open circuit voltage is a reasonable measure. For Alkaline, not so much. It is possible for Alkalines to look okay at open circuit, but really have no juice in them when you load them. So the test-under-load procedure is preferable. The down side is that you can't leave the load on all the time, or the batteries run down, defeating the whole purpose. Thus some switching approach.
It's really up to you which way you want to go in your project.
To complicate things even more, if you want to hear about zener diodes, I can think of a way to make the output-pin-as-ground trick work, and keep the A1 voltage within limits.