Jim Smart
February 12, 2010

If anything keeps us befuddled about classic Mustangs, it is charging systems and storage batteries. You turn the key and nothing happens, or perhaps you get that annoying chatter of a solenoid barely energized by a weak battery. Regardless, it leaves you sitting there waiting for a jump-start. So here's what you need to know about vintage Mustang charging systems and how not to get stuck with a dead player. The charging system includes the generator ('64 1/2 only) or alternator, external voltage regulator, either a charge light or ammeter, cables, and the battery.

Your Mustang's battery stores electricity, averaging anywhere from 12-14.5 volts. Ideally, it will have 14.5 volts with the engine running and the necessary 9.5 volts of direct current necessary to get the engine started. To keep a storage battery at a proper state of charge, you have to return more electricity than you use. Think of your battery as an electrical reservoir. The charging system's job is to keep it full. Lead acid batteries don't like to sit inactive. The more they sit, the shorter they'll live. Batteries remain healthy when we keep them on a regular cycle of discharging and charging. If your Mustang sits a lot, the battery needs to stay on a trickle charge from a battery maintainer or a charger set at the lowest possible amperage. It is suggested you disconnect your Mustang's battery when it is being charged.

Classic Mustangs have an alternator with an external voltage regulator. Original equipment voltage regulators are mechanical with contact points. The voltage regulator is designed to protect the battery from overcharging, which is worse than not getting enough. Overcharging causes batteries to overheat to the point where acid boils out of the vents and creates dangerous hydrogen gas. This is why a new solid-state voltage regulator should accompany every alternator replacement or rebuild.

Understanding Lead-Acid Batteries
As incredible as it may seem, we're still starting Mustangs with lead-acid storage batteries in 2010. The lead-acid storage battery's origins go way back 150 years to Gaston Plante, who made the lead-acid storage battery practical for commercial use. For reasons of simple and practical economics, we still use such an archaic source to start and operate our Mustangs today. Ford continues to install lead-acid batteries in new Mustangs because they remain practical and affordable.

Lead-acid batteries yield a powerful wallop to spin starters and energize ignition systems, which is another reason why they remain popular. It is the chemical reaction between lead, lead dioxide, and an electrolyte (sulfuric acid) that creates electricity. When a lead-acid battery is overcharged, it creates hydrogen gas and oxygen, which vents from the battery along with the bubbling electrolyte. If the battery is allowed to discharge, the lead/lead oxide sandwich plates will sulfate and break down. When this happens enough, plates short out and become dead cells. This can happen when a lead-acid battery sits uncharged. For a time, a dead battery offers forgiveness with recharges. In due course, they die from deteriorated (sulfated) plates and cannot be jump started. This has probably happened to you when you tried to jump start a car and it would not turn over under any circumstances.

A battery's state of health can be determined by deep cycle charging (trickle charging) and a load test. If a battery will not support a load after eight hours of trickle charging, it must be replaced. Another time-proven method is checking the electrolyte's specific gravity. When specific gravity falls below 1.225 at 12.4 volts, it is a dying battery. At 1.220, a 12-volt battery is completely discharged.

The key to battery survival is a healthy charging system and that predictable pattern of discharging and charging. When your Mustang has to sit, keep the battery on a trickle charger. The best system is something like a Battery Tender that keeps the battery fully charged. If acid weeps from the vents, the battery is in trouble via overcharging or sulfating plates.

Here's a word about battery safety. Remember high school science class when you made hydrogen gas in a test tube? You were instructed to stick a lighted punk in the test tube and the hydrogen gas ignited with a startling noise. This is what hydrogen gas from a car battery does-only bigger and more destructively. When hydrogen gas is ignited, it explodes quickly with a sharp bang, blowing the battery case apart and showering everything close by with sulfuric acid. This is why you must be careful with any lead-acid battery. Sulfuric acid will blind and scar, and it will destroy paint. When jump-starting or connecting a battery to a battery charger, never make the negative cable connection at the battery because there's always a certain amount of sparking no matter how small. Even static electricity near a battery vent can cause an explosion. Always wear eye and face protection along with gloves.

Autolite Alternator Specifications
Stamp ColorAmp RatingMinimum Speed RPMAmp Rating RPM
Purple384002,000 Cold
2,900 Hot
Orange424002,000 Cold
2,900 Hot
Red554002,000 Cold
2,900 Hot

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