If the digit 4 appear at a TF1 module when the unit has been powered on the battery is low and eventually the TF1 module will sound weirded/not at all. The battery is soldered on the board. It can be replaced with a battery holder that makes it easier next time the battery need to be replaced. The new battery for the holder if appliceble should be CR2032 (a common "clock battery", can be found about everywhere in regular electronic stores and store departments). The life time of a battery when installed can be anything between a year to plenty of years depending on the battery.
Here is a documented battery replacement a TX816 made:
The article is similar to the experience I had when I replaced soldered batteries to battery holders and CR2032 batteries. It worked out well except I made a couple mistakes:
- I ruined one TF1 as I don’t used solder sucking when desoldered the old battery. Eventually I bought a replacement TF1 and it already had a battery holder installed.
- I bought the cheapest batteries. Some of them lost the power to soon. Luckily its a piece of cake to replace them now when needed.