Telling when an Li battery in an old phone needs replacing

[Infraviolet]

I have an old phone with a removable battery for which  had been able to get a replacement copy. The phone's battery life today is much shorter than when new, and the battery must have eben through >1000 charge cycles in its life, I'm trying to work out when I should swap to the spare battery though. The battery is a dumb, or mostly dumb, type with 3 pins (+V, Gnd and one which I think is temperature sensing), the phone itself contains all the charging controlling chips. Are there any multimeter tests I could make of the battery which would determine when it is time to swap the old one for new? Or should I just keep using it until it won't charge at all (I've seen that happen to a laptop battery many years ago). Do phone batteries ever spontaneously ignite just from age *there's no swelling or mechanical damage or anything like that involved in this matter), would it therefore be wise to swap over to the new battery soon and dispose of the old (but semi-working) one ASAP? The new one appears genuine and seems to take charge fine turing some test runs, but I can't be 100% sure about it. I do not have any kind of specialised internal resistance meter or milliohm-meter, except to the extent that such could be formed by building up other circuits to feed signals/voltages to a mutlimeter or oscilloscope.
Thanks

[RoGeorge]

- A swollen battery is a good sign to replace it.
- Phone batteries won't stop taking charge like a laptop battery.
- Old phones batteries do not spontaneous combust or explode, never seen or heard of such failure mode.
- There is no test to make other then measuring the battery capacity and compare that with the original.
- As long as you are happy with the running time, and as long as the battery is not swollen, you can use it no matter the age and no matter how many charging cycles it took already.

Charge the new battery, leave it in the phone, and measure how long does the new battery last.  Do the same with the old battery and compare the results.  To measure correctly, avoid taking or making calls during these measurements (during calls the battery drains significantly faster than in standby).