Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: How will I know when the batteries need to be replaced?
Document: He loves it, he has only needed to use it once up to now but I know he will need it more! I got tired of him misplacing the keys and practically turning the house upside down looking for them, so I saw this on Amazon and knew it was what he needed.
I think. Maybe I'll get another one for my library card. I might need one for all my things.Anyway, kidding aside, this thing works great, now that I know ow to use it - and the seller is, I think, the nicest on all Amazon if not the whole world. She writes follow up emails to ask how you like it. The truth is the first set I ordered I became frustrated with it because it didn't seem to be working right so I just threw it away.
The one "glitch" that I encountered, which is also the subject of several negative reviews, is that after the first couple of weeks the blue transmitter would start beeping for no reason. I know that the transmitters are supposed to beep when the batteries are low, but I tested the batteries with a battery tester, and they were fine. I then contacted the seller by email to explain the problem.5.
Within 10 seconds she homed in on the beeping receiver hanging amongst a cluster of leaves in one of our bushes, 40 feet away. I was really impressed. Even though this wasn't a real "lost key" situation, it boosted my confidence. My only concern now is "how long will the batteries last?". For now, I've decided to keep the Transmitter in my possession with the battery out of it until I need it (hopefully this will prolong battery life).
Service was very fast and came to my house well packaged. These key finders are the best that I have ever found! They are small and easily placed on my TV remotes or key chains. I was very impressed with receiving extra batteries! This will save me time and frustration when I do need a new battery. ONE IMPORTANT NOTE...READ and follow the directions and your device will work wonderfully! These would make GREAT Christmas gifts!
This was a terrific "make my life easier" gadget!The price was perfect, and I called the seller with a question which was answered right on the spot. I could not be happier with the product or cheerful service!One tip, keep a new set of batteries on hand for when the ones that are shipped with it go. They make a little chirp to let you know the old battery is failing and you will want to take care of it right away.
Since my wife often misplaces her car keys I thought this gadget might be the answer. But after the beeper woke us up in the middle of the night once, and went off in a movie theater on another occasion, we've decided to deep six it. The beeper is supposed to sound off when the battery gets weak. I tested the batteries in the sender and receiver, and they were still good. So, a good idea, but the manufacturers of this device need to go back to the drawing board.
Didn't matter which button you pushed, the blue or black, but both remotes would go off at once. But the did work great since you didn't need a direct line of site. Helped me find my keys and wallet quickly several time. But alas, after about two months they started beeping on its own. Tried switching batteries, but it didn't work. Had to throw them out.
Dead from the start. Total junk. I opened and the units did not work. I replaced the battery and the units did not work. Avoid
Got these because I kept losing my keys. These things started beeping all the time without me using them.They only worked for a week or two and it wasn't the battery because it kept doing it after the battery was replaced. Not recommended
Answer: I don't believe there is a way to tell until the transmitter and/or receivers stop working. The batteries should last a long time though. We have devices that use the same battery and they lasted at least two years, in most cases more than that long. Hope this helps.