My experience with batteries in the Edge 705:
I bought it new in 2008.
The original battery lasted 7 years.

I bought a replacement on eBay.
It lasted one year.

I bought another replacement from New Power.
It lasted one year.

I installed another replacement, which was still working until a few months later when the USB port went bad, and I quit using it.

From what I can tell, all the batteries available are the same one with the "Cameron Sino" label.
And once installed, the battery gauge in the display was never again accurate;
It might show fully charged when it really had only a partial charge,
or it might show nearly empty, but then run for several hours more.

So the good new is the replacement batteries might only cost a fraction of what Garmin charges.
The bad news is they have only a small fraction of the life of the OEM Garmin battery, and you never know how much charge you really have remaining.