Many motorsport ECUs are fitted with an internal battery that is not rechargeable. Depending on the ECU, these typically need changing every 1-3 years. Many are also soldered to the electronic circuit board and old batteries need to be de-soldered and new ones soldered back in. There is a limited number of times this can be done before the electronic circuit board becomes damaged. The battery change also involves returning the ECU somewhere for the work to be done, the shipping carrying its own risk of loss.
Depending on the ECU, it’s functions will be reduced when the battery is not present, such as, but not limited to:
OLDtronics have built a small box, in a similar style to an ECU that contains the battery. The ECU needs to be adapted to have a flying lead bringing out the battery connections. The battery is of a larger capacity than is typically fitted inside the ECU, therefore should last between 5-10 years.