I have noticed recently for the first few minutes after starting the battery will discharge and within a few seconds will start charging again. All lights dim during this period. This will continue for several minutes before returning to normal.

My truck is a 2006 with the original batteries with 98,000 miles.

What is happening?

 

Your grid heater is cycling cause it's cold out. Nothing wrong, just the way it is.

 

What he said...

 

Thanks for educating me on this.

 

From the owners manual...

• Voltmeter Operation:
• The voltmeter may show a gauge fluctuation if
certain engine temperatures are below 66°F (19°C).
This cycling operation is caused by the post-heat
cycle of the intake manifold heater system. The
number of cycles and the length of the cycling
operation is controlled by the engine control module,
this time will not exceed 150 seconds. The
needle should then stabilize at the approximate
operation point.
• The cycling action will cause temporary dimming of
the headlamps, interior lamps, and also a noticeable
reduction in blower motor speed.