ACK ELT failed self-test: no beeps from audio unit (which annunciate error codes, if any). Tested with EAA 663's ELT tester--all seemed OK, but no beeps meant pulling the audio unit. Found leaking CR2 Li battery, developing < 1.5 V vs. nominal 3 V. Images show the damage. Corrosion on positive contact was scraped off, sanded w/220 paper, and re-tinned with 37/63 solder. (The positive contact solder joint (one of two) to the board was also cracked--repaired it while I was at it.). Also checked the PX28L 6V Li battery in the panel control unit: no leakage or corrosion, but it put out less than 3 V: replaced both batteries, reinstalled, and ELT self-test beeps returned no error conditions.