How do I know that the battery needs recharging vs. failing?

Look for a date code, stamped or printed on the battery. But frankly, given the application - your phone and networking! - I'd say replace it every three-four years with a fresh battery. And keep one around on-charge for backup. You can trickle charge old batteries with a few-dollar 12V trickle charger - anything that puts out 10% of rated current at voltage (720mA at 14 volts max) will not cook your battery. Safety compels me to say: don't charge batteries unattended. There's a small chance of overcharge or shorting and excessive current, to lead to a fire. As to where to get these batteries. Many places that sell electroniics and car batteries locally sell them, at higher prices. Alarm companies use them, emergency lighting uses them, so those vendors may source them. Or you can mail-order them, and pay several dollars shipping. Jeff, without further comment - this is a once every three-four year purchase, so you decide how much shopping you want to do. This is a $15-$30 purchase. check the date of any battery you buy off the shelf. If it's more than a year old, I'd not buy it at retail prices. The size and placement of the terminals matters, as noted previously. So carry the old one to compare, or measure and photograph it, so you don't "ooops, the terminals are backwards, the wires won't reach, the connectors won't fit". Rule of thumb for lead type batteries: the heavier the battery for the same "ratings", the more capacity you'll actually have. The lead carries and produces the current; more lead means more current. battery makers won't waste lead but a more expensive brand won't cut lead. Herb PS: this is vaguely on-topic, for the robot builders. Gel batteries are often used in hobby robots, they are common and cheap as used/discards. -- Herbert R. Johnson, New Jersey USA http://www.retrotechnology.com OR .net