Engine failed to get onto backup system because of control batteries

Service engineers attended MTU 4000 standby generator engine, which provides emergency power for a data center with another 4 sets.

Whilst carrying out the service routine, engineers came across an unusual problem getting the engine onto the control board.
The engine was able to start, but on an attempt to put it onto the backup system, the control panel wouldn’t let this happen. Instead, the control panel flashed up an error message pointing to a battery problem.
As the engine started fine, it wasn’t likely to be the main batteries, but they were re-checked and found to be in new condition, and fully charged.
Then the 24V control batteries were checked, which are separate to the start batteries.
It was found these were only able to hold 60% charge, even after they'd been left on trickle charge.
Once they were replaced and the panel reset, the generator was again able to add power to the backup system.
These batteries are not part of the routine service checks, but they should be checked and maintained.