Depending on the standby time you could have the geny on during daylight hours.
When work is complete recommend the batteries to be replaced.
I recommended this same thing for a customer that was having a three phase supply installed to replace a single phase supply.
The system worked overnight and recharged on the geny during the day, was over a weekend IIRC.
Worked well, replaced the 8x 7Ah in the ACU's the Monday after they had finished.
RE mags, should have a constant current however there is a higher inrush when energising.