The battery voltage will normally drop when a load is put on them and the furnace fan is one of the worst current draws of them all, it will suck a battery down in a hurry. Batteries will normally recover some after a high load is cut off, this is normal and all lead-acid types seem to have this trait. You can think of it this way: each device draws a certain power and that's calculated by the formula P = I X E (where I =s current in amps and E =s Voltage), if the voltage goes down the current must increase because the device will maintain the power in watts no matter what, you can burn up a motor if the voltage drops low enough. I'm not sure what the technical reason is for the battery kick back in voltage but it does happen. Hope my rambling explanation is of help
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2005 Springdale 249BH FW
2008 Toyota Tundra Crewmax 5.7L
Tow package
Timbrens
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