I have a Charter-supplied phone modem that works off of the TV cable in my home. It has a battery backup, and the battery is a 8.4v, 2.2Ah module, although the modem has space for one about twice that size. I have 2 identical battery units that I swap about every 2 months, so the one not installed is no more than 2 months from a full charge. The modem/charger is in the basement, at a year-round temp range of 50-65F.
So the batteries have been cared for pretty well. Power outages happen about 3 times per year, and one unit is typically discharged each time.
While I can test the voltage if needed – I have a good voltmeter, but the probes won’t reach into the contacts, so I’d have to rig up some wire-type probe – but I have no way of putting the batteries on a load for a valid test of remaining capacity. Both batts are almost 7 years old.
Should I assume they are at the end of their useful life and replace them? I’d hate to find out they were shot after a major winter snowstorm.
Lithium ion batteries have a lot of good features, but a long life isn’t one of them. They are compact and lightweight, and have a good energy density for their size, but even if you treat them kindly they still tend to die an early death compared to other battery types. After seven years I would expect them to be pretty well shot. It’s time to replace them.
I guess it’s time for a test. Perhaps the most representative load would be the phone system itself. I have just disconnected the AC power to the modem after installing a fully-charged battery.
I was told the nominal standby time for one of these batteries is 6 hrs; talk time, 2 hours. Although I have never confirmed these numbers, they seem reasonable in my experience.
I started my stopwatch, and I’ll check to see if the phone still works every hour from now until it fails. Unless someone calls (not likely today), this will be a test of the standby time of one battery. Then I’ll try the other unit, since they are the same age.