What they said. But also, an engine starter can throw 600-2000 volt spikes as it disengages, and those spikes can blow electronics
very easily. You might want to consider throwing the breaker and turning off the electronics (or switching them to a battery that is not involved in starting) in order to protect them from that. Unless you have some spike protection installed in the instrument circuits.
Yes, in theory the battery swamps out those spikes. In practice, several component makers say that's just not good enough and one top LED maker will not warranty their automotive products if they are allowed to see starter spikes, i.e. unprotected by anything except the battery.