Why do I need to modify the module ?
The RTC module as supplied has a potential issue. The battery supplied is a simple CR2032 none rechargeable battery and the RTC module charges the battery via a very simple charging circuit. Without modification and powered from 5v the battery will overheat and hence may explode and damage other components. The very best outcome of this is that the battery will simply die leaving the controller without a battery backup.
How to fix it ?
The charge circuit comprises of a resistor and diode in series between the power input and the battery positive terminal. The best method to use to disable the circuit is to remove the resistor. In order to remove the resistor you will need to heat up both tags simultaneously with a soldering iron. Moving the iron from one end to the other will provide enough heat but you will need to move quickly and accurately. Once the resistor is removed the charge circuit is open and the RTC module will be safe to use.
Alternatively, if desoldering isn’t an option, cutting the track will also work. Cut the track just above the diode. Cut the track marked in the image above. Use a sharp blade, a scalpel of modelling knife, and care must be taken not to damage surrounding areas. Once this track is cut the charge circuit is disabled and the RTC module is safe to use.
Changing the battery for a rechargeable ?
Changing the battery for a rechargeable one would, theoretically, fix the problem. Allowing the charge circuit to trickle charge the battery removing the need to replace the battery every few years. The design of the charge circuit is very simple though and using such a simple circuit to charge a small battery is risky. this is particularly the case with a battery that is hidden away somewhere in your vehicle. The risk of fire with a suitable Lithium Iron battery is well documented and the circuit has no over voltage protection.
Utilising the rechargeable battery would require, at the very least, that the module be run at 3.5 volts instead of 5 volts. As a result it is probably not a good idea, creating more problems than it resolves.
Just remove the battery ?
Removing the battery altogether is an easy option and would resolve the issue. Removing it would mean that all settings stored in the RTC including the current date/time, would be reset if the controller was disconnected from the power. That aside, there would be no other problems caused by removing the battery.