12v LED lights flickering when furnace turned on...

[ArmyAV8R]

Pardon the basic nature of my question.
2010 Flagstaff, no idea the age of battery.
30a shore power, all well.
Turned furnace on for first cool weather this season and overhead 12v start flickering, going dimmer and dimmer.
Voltmeter with no furnace or other load: 13.5v
Turn furnace on, flickering starts, and volts drop to 8.6ish....
Turn furnace off, goes through cool down, fan stops....lights stay dim for maybe a minute and then are back on, normally.
Bad battery?

[TitanMike]

If you have power and converter is switched on (must be if you have 13.5 volts) there may be a loose connection. Check connections at battery and also to the Converter's main 12 Volt wire (usually a Red #8 ga wire.

Also possible is a loose negative wire where they all are connected behind the power center. Ground connections to frame are also suspect.

All the possible connection issues should be checked and eliminated before replacing the battery. Otherwise you'll have a new battery that doesn't cure anything.

[ArmyAV8R]

Thank you Titan Mike.
Auto parts store near by; battery was bad.
New one installed, all is well again.