Having the battery on line helps stabalize the bus against voltage
spikes of various kinds.

So things that put resistance between the bus and the battery
defeat this purpose. Corrsosion on connections is a suspect.
If this has the master solenoid with big terminal bolts on it,
loosen each nut and tighten the stack of nuts starting at the insulator
block.
Often the pheonolic insulator block shrinks and that make the
path from the wires to the contacts flakey.

Low water level in the battery also dramatically increases its
internal impedance level, leading to the same result.

Bill Hale