RFA-188 short life
It has been our experience that these batteries only last about 3 weeks or so. It is probably underspec'ed for the little dog collar, more than likely to save space and weight.
A little bigger battery with more capacity would be good...

Great fence, awful battery design
THe Petsafe Invisible fence works fantastic, and I bought a standard size collar for my lab. The battery life, and overall performance is terrific. We got a second much smaller dog, and I made the mistake of buying the Petsafe Ultralight collar. It cost twice as much as the full size collar to start with, and as other reviewers have stated, the $10-$14 battery only lasts about 3 weeks, which makes the ongoing cost unacceptable.

Petsafe Battery Review
The Petsafe Battery does not live up to the specs that indicate that it will be approximately 3 months life. One of the recent batteries I purchased was completely dead in 2 weeks. Typical is 2 months.

dies in about a month
I've run through 3 of these.. .I keep meaning to break it apart and stick in a generic $3 battery, but haven't gotten around to it.

I have a tiny little dog that doesn't get anywhere near the fence, so the collar doesn't go off and yet the batteries last less than a month??!!

Petsafe.. come on.. . get with it.. make a rechargeable version, make it so we can put less expensive batteries in or something. 8 or 10 bucks a month is a bit too much.

Don't buy the "little dog collar".

Buy replacement batteries yourself.
The Petsafe RFA-188 can be DIY replaced by opening up the bottom cap. It is glued so you'll have to use a plier. Underneath it you'll find a Sanyo CR-1/3N battery that can be purchased online for 3 dollars. I plan on buying a bunch in bulk to make shipping worth it, slap it in and use some glue or tape to put it back together. To find the inner Battery, just google it.

This is what it looks like but you can find it cheaper elsewhere on the internet.
Sanyo CR-1/3N CR-1/3N Photo Lithium Battery