(google ULTRALIFE High RATE)
Pilot Shop sells them for about $25 each and you will need two. Also other battery vendors stock them. I'm getting ready to replace mine.
Of coarse you will need to remove the battery tray from the ELT. You'll need a very small screw driver to slide the blue plastic catch latch that hold the ribbon cable connector, and slide it off. Be sure to take a few pictures of how it is assembled, and they did glue the battery to the tray with white bathroom shower RTV, so you'll need to clean it up.
It should be a one hour job to replace just the batteries.
The batteries are ULTRALIFE High RATE. The Cell dates on mine are June 2010, with an expiration date of November 2016 (by Artex). The Ultralife website indicates they have up to 10 years shelf life.
Here is a picture after cleaning up the RTV used to hold the batteries. I've found them on line for as little as $15 each. I'll be ordering mine later tonight online.
Part No U10013, U10014, U10015, U10016
NSN 6135-01-554-3803 (U10014 only)
Voltage Range 1.5 to 3.3V
Average Voltage 3.0V
Nominal Capacity 11.1Ah @ 250mA to 2.0V @ 23°C
Max. Discharge 3.3A continuous
Pulse Capability Up to 7.0A
Varies according to pulse characteristics, temperature, cell history and the application. Consult Ultralife.
PTC U10014 and U10016 have PTC (re-settable fuse)
U10013 and U10015 do NOT
Operating Temperature -40°C to 72°C
Storage Temperature -40°C to 95°C
Exterior/Housing Hermetic Ni-plated steel can with Mylar sleeve
Terminals/Connector Flat Ni-plated +/- End Caps: U10013 and U10014
Flat Ni-plated +/- End Caps with Nickel Tabs: U10015 and U10016
Safety UL component recognized
Material Safety Datasheet - MSDS023
Safety Guide UBM-5135
Transportation Class 9 - U.S. and International (see note)
Note A complete description of transportation regulations, lithium weights and transportation classifications is available on the Ultralife website.