If we are talking about the battery in the battery back up unit (BBU), it is a 12 volt DC 7.2 amp-hour battery that is common to many alarm systems as wel as our BBU. The estimated life of the battery actually varies with usage; a fully charged battery is supposed to provide up to 8 hours of outgoing telephone service. If you receive many incoming cals or use the phone a lot while on battery power, it will by default last a shorter period of time.
Also, be aware that the battery has a reserve capacity to it after the ONT shuts down when on battery power. To power the ONT back up, you just press the 'Battery Emergency Use' button, which will provide power for one or two more calls.
Written by Kristen Bechdolt - John L
Last updated on 02.19.08