I own a Yaege FC-1 frequency counter. It is around 4 years old, and recently I noticed its internal battery is dead: No more than 10 minutes of autonomy. The battery was a 18350 / CR123A, so it should be easy to find a new one.
But recently I have had bad experiences buying batteries at eBay: All the batteries I purchased were dead, with almost no capacity and very high internal resistance (in other words, useless), so I looked for a different solution.
I noticed there is a lot of room inside the frequency meter case. There is so plenty space that some small batteries from mobile phones fit inside. I took a spare Samsung battery I had around from an old phone and wired it to the frequency meter. The exact model is AB653039CE
It works fine! In fact I never got more than 2 or 3 hours of autonomy with the supplied battery and now, with this AB653039CE battery, rated at 900 mAh, I can get more than 12 hours of continuous operation.
It's unbelievable what a in good shape battery can do with your gadgets.