The SmartServer internal battery has a shelf life (un-powered) of 2 years and while powered should last 10 years.
All i.LONs (at least as far back as i.LON 100 e2) have an internal battery that is used to keep the real time clock up to date. The battery is fairly small and isn't rechargeable or replaceable. It is used to give the i.LON a shelf life of 2 years. The battery can only be used internally to the i.LON and thus can't be used to power up the GPRS modem. Whiled powered the battery should last 10 years.
A time server can be added to the SmartServer in order to correct the SmartServer time when the battery no longer has power.