Mark Houlahan
Brand Manager, Mustang Monthly
May 22, 2012

One of the most feared aspects of car repair, maintenance, or even a restoration is the vehicle's electrical system. Watts, volts, ohms, and other terms swirl around most people's heads like sharks circling a bleeding surfer, and at the heart of that electrical system is one of the most misunderstood parts--the battery. In our Mustangs, be it vintage or late-model, the starting and charging system is based on a 12-volt negative ground service, so we're not going to bother with six-volt or positive ground systems for this story.

As the heart of your electrical system, your battery, must not only be able to start your Mustang by supplying enough power to the starter, but it also must be able to provide auxiliary power for key on and key off accessories (when the engine is not running and therefore the alternator not supplying the vehicle's power, for things like listening to the radio or the interior lights illuminating when the doors are opened). Lastly, the battery must be able to store this power between vehicle use (but not long-term storage) so that the car will start the next time you use it.

So why is it that this most important part of your electrical system is so often just left to fend for itself and never given maintenance until that fateful day when you get up early to head out to a car show and you find your Mustang in the garage with a dead battery? Your club members are meeting for the drive to the show in 20 minutes and you're standing there with the hood up wondering what you should do.

Taking care of your battery is really a simple process. It is something you need to add to your regular maintenance schedule. You check your engine's oil level on a regular basis right? You need to check your battery's level on a regular basis too. Keeping your battery properly charged not only means you'll be able to hop in and twist the key when you need it most, but the battery will last longer and save you money. Of course, there are a few preventive measures you can perform on your battery as well, and we'll go over those as well.

Battery Condition

Batteries do not last forever. They should be considered a consumable item. That being said, sometimes it is best to start off with a new battery if your battery is of a questionable history. Just bought a classic Mustang and there's no real indication of the battery's age? You might want to play it safe and simply buy a new battery. Depending upon location, you can often guestimate how close you are to needing a replacement battery. In hot climates like Florida, Arizona, and Texas, you're probably going to see three years max out of a high-quality battery.

Today's batteries are typically a sealed, maintenance-free design, so there's little you can do to service the battery except for keeping the terminals clean. Often an "eye" on the battery will tell you its condition. If your battery does have removable service caps to allow checking the battery's level, then you should check the level at regular intervals. Be sure to remove all jewelry before servicing your battery and wear eye and hand protection. Remove the caps and check the fluid level. The fluid level should allow for the battery's lead plates to be completely submersed. If not, you can carefully add distilled water to bring the level back up.

If you own and know how to use a hydrometer, you can also check the specific gravity of your cells if the caps are removable. The same can be said for the battery's open circuit voltage (measured across the terminals). If you own a volt-meter, you can check the voltage too. Use the above chart to determine your battery's state of charge.

Finally, external condition is just as important as the internal condition. Case damage, terminal damage, and more should all be things to look for on a regular basis. Of course, if your battery is properly secured, there's little chance of external damage, but we've seen cases nicked when being dropped into place, so be careful and don't rush things when installing your battery.

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Battery Fitment/Detailing

Having the properly sized battery for your Mustang is just as important as having the proper capacity. Generally speaking, using the battery group as originally specified by Ford is the easiest way to ensure things go smoothly. If your Mustang uses a Group 24F, don't use a Group 58, and so forth. Problems like terminals on the wrong end of the case, cases too wide or too tall (hitting the hood of the car or the inner fender), or simply the footprint of the case being too large for the battery tray are all issues that can be solved by simply using the proper battery group for your application. There are still companies that make the proper battery cases for our vintage Mustangs; you just have to look for them. However, they are getting harder to find.

One solution for '65-'66 owners is to update the battery tray to a conversion tray that uses the '67-'70 style hold-down assembly. While not correct for concours use, it is a solid and safe solution that bolts right in and cost less than $30. For those who can find the correct battery group, try to purchase a flat top, maintenance-free battery and then you'll be able to use an Autolite replica battery cover that slips over the battery posts and makes your universal battery look like a correct Autolite.

Battery Maintenance

Maintaining your battery means ensuring it is properly charged as well as taking care of the battery's fluid level, connections, and exterior condition during the months you use your Mustang. All you need is a few simple tools to keep your battery's connections in proper working order. As we said before, add your battery to your list of regular maintenance items or get in the habit of checking the condition monthly, or perhaps at each oil change (especially for those daily drivers out there).

Charging Basics

Charging a discharged or sulfated battery is a simple process, but it is imperative that the proper charging equipment and safety procedures be followed. Standard flooded batteries are much more susceptible to sulfation of the lead plates. This limits the battery's performance and can cause a no-start or lack of charging. Modern battery chargers and maintainers have a de-sulfation mode that helps break down these sulfates and put new life into older batteries. Look for a quality charger/maintainer that has multiple battery type settings so you'll only need to purchase one product no matter what type of battery you are charging.

Lastly, never consider your car's alternator as a battery charger. If your battery has died from leaving the lights on or other roadside issue, you should really charge the battery fully before starting the car, versus jump starting it and then driving the car to charge the battery. We've all done it, especially in an emergency, but the best case scenario is to have a friend or family member bring out another battery and charge the dead one properly at home.

AGM batteries can be a bit tricky, as they have different considerations for charging. Older 12-volt chargers will usually only charge an AGM type battery to approximately 80 percent, which is why we again recommend a newer charger/maintainer that has an AGM setting. AGM batteries, like flooded batteries, do well with a lower amperage charge (1-10amps). Charging at higher rates generates a lot of heat, which reduces battery life. AGM batteries are most known for not "taking" a charge if they are deeply discharged, often meaning below 10.5-volts at rest. If your AGM battery is drained below 10.5-volts, you'll need to charge it either by using an AGM specific charger or by connecting the discharged AGM battery in parallel with a good battery so that the traditional charger "sees" the good battery and charges appropriately.

Voltage Approximate State-Of-Charge at 80° Average Specific Gravity
12.65 100 Percent 1.265
12.45 75 Percent 1.225
12.24 50 Percent 1.190
12.06 25 Percent 1.155
11.89 or less Discharged 1.120 or less

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