Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If your handset phone has a red light showing on it, and is also reading 'Install battery' on its display, then it is possible that the battery in it has become defective.
All rechargeable batteries for electronic devices, of whatever technology, do fail to deliver voltage or power or both, in time.
So I would advise you to open the battery compartment, check whether the battery is still plugged in to the phone properly - it most likely has a two-wire connector coming from the battery to the phone - re-seat that connection, try recharging the battery for 6+ hours continuously (if you haven't already, as esp. if the phone handset has had a period of disuse recently), and then conclude the battery needs replacing.
And of course, if you have another KX-TGA740 handset that works OK, with a battery in it, you could try swapping that battery into the problem handset, just to check that the battery not the handset is the problem, before spending money on a replacement battery.
Be sure to charge the replacement battery up for 6+ hours on the charger, before attempting to use it much. Batteries that have been sitting on a shelf need a prolonged recharge before their first use.
Let me know what you think, and what you observe, and how it goes. Thanks.