Handheld devices and battery charge

[Solipse]

Hello,

It's my first time on this forum, I'm an hobbyist repair guy and I'm mostly dealing with common household devices people are bringing me (and mines too when I've free time ).
One thing that I encounter quite often is that common battery powered stuff like cameras, cordless phones, and so on (and more specifically those on AA or AAA batteries) stop to work with their normal operating voltage. They would do well when siting on their charge base, being powered through a converter attached to the battery terminals for instance, but don't light up or die very fast when using even brand new fully charged batteries.
I was wondering, as it is so common with very similar behaviour, if a specific component is mostly always responsible for this (like a voltage regulator maybe)?
But I'm still pretty bad at reading circuits, even more when it's cms. And lacking time to spend on these devices that are fairly low value, but apart from that still working and useful !

I'm sure I can't be the only one wondering about this issue, but still I can't find much on the subject so far...