Battery gauge fluctuates while idling?

Ok.woke up one morning and my 96 Ford e150 would not start. Took the battery in to be tested and charged. Took the alternator in to be tested. Battery was on last quarter of life alternator tested out fine. Put both back in woke up next day. Would not start. Got a new battery and a remanufactured alternator. Vehicle started up fine the next day. But when I'm at a stop light ( idling) the battery guage shoots down. Need help please.

You may have a short in the electrical system which is draining the battery and causing low alternator output/ low readings from the gauge at low speeds/idle when alt output is lower… Have a shop do a "parasitic draw test" and to track down the short if needed… Or DIY if you have some skills and test equipment like a DVOM. Unless you are using a lot of accessories that draw high amperage like headlights, wipers, add-on radio speakers/ amps, etc. Which may cause low readings at idle but should still be above 12 volts… Normal gauge readings should be around 13.5-15.5 volts… If they drop below that or 11-12 volts and lower, then there's a problem with the charging system/alt or the related alt circuit wiring or a short in vehicle wiring/devices. In many cases in modern cars, a bad cigarette lighter or power port due to bent terminals from a device plugged into it can cause an excessive draw on the electrical system… Pull the fuse for the lighter or power port and see if the gauge readings improve… If so, then check/replace the lighter port.

Testing info…
http://www.aa1car.com/library/charging_checks.htm

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