battery issues

hi all i have a 1988 f-150 with 5.0 4speed auto air,electric windows,locks,etc just changed my battery cable ends old ones were bad. truck was hard to start for a few days after like a drained battery, i got to noticing that when i turned on my defroster fan motor the volt gauge went way down turned it off and it came back up, does this mean my blower motor is going out. looks like the blower motor is draining my battery, had the alt checked out is charging good, and had the battery checked out it is good. any ideas
thanks
jenn

The volt gauge on my 88 F150 basically acts the same way....theres really nothing wrong. If you just turn the key on without cranking, observe where the volt needle stays at. This will be a your reference point as to actual battery voltage. You can check the battery with a seperate volt meter to see where you stand.After you fire up the engine, the needle must go above this point.When you place loads on the charging system...it is common for the needle to go down. As long as it stays somewhat above that reference point...with a load on the alternator....you are fine. Now, what caused the truck to crank "heavy" the other day may be the extreme cold weather, the alternator may have a bad diode allowing discharge will not running, etc,etc.
Phil

i replaced the battery and it is starting now they checked the battery said it was bad. but it still seems to be dragging a little on start up,now it has been in the teens to single didgets here on the california/oregon border