Iridium Plugs and Gas Mileage

I had bad contact on battery, got lots OBD errors and gauges were acting crazy.
Battery protect from alternator spikes.
normal alternators give a constant voltage and variable amps afaik. I've driven some old cars without a battery, i jumped it with another car and drove it with the positive cable stuffed in a rag. worked for me, not recommended.

a bad contact will make the battery connected intermittently and so it switching between alternator power only and buffered battery power will wreak havock with your gauges. and smart alternators are too dumb to make electricity when you need it and will make the battery work harder