Mark Houlahan
Tech Editor, Mustang Monthly
May 22, 2012

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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