- Battery
- Cables and terminals
- Starter solenoid
- Starter motor
- Engine runs
- Alternator
- Recharges battery
Batteries, starting, and charging
A conventional 12-volt battery stores energy for starting and for the electronics when the engine is off. Many newer vehicles with start-stop systems use an absorbent glass mat or enhanced flooded battery, and some use a sensor on the battery so the vehicle can manage charging. The starter is a small motor that engages the flywheel. The alternator makes electricity once the engine runs.
Common parts
- BS·112-volt batteries in flooded, enhanced flooded, and AGM designs
- BS·2Alternators, voltage regulators, and alternator pulleys
- BS·3Starters and starter solenoids
- BS·4Battery cables, terminals, and hold-downs
- BS·5Battery sensors and current sensors
- BS·6Battery chargers, maintainers, and jump starters
Signs of trouble
A slow crank, clicking with no crank, or dim lights at startup usually point at the battery or its connections. Corroded or loose terminals are a common and cheap cause. A battery warning light, dimming lights while driving, or a whine that rises with engine speed can point at the alternator. A single loud click with no crank, while the lights stay bright, often points at the starter or its circuit. Most parts stores will test a battery, starter, and alternator for free.
Finding the right fit
Batteries are sold by group size, which sets the case size and terminal layout, and by cold cranking amps and reserve capacity. Use a battery that meets or beats the ratings the vehicle calls for, and the same type: a vehicle built with an AGM battery should get an AGM battery. Some vehicles need the new battery registered with a scan tool so the charging system treats it correctly. Starters and alternators vary by engine and by electrical package, and many are sold remanufactured with a core charge refunded when you return the old unit.
Symptom to part
| What you notice | Usually points at | Check first |
|---|---|---|
| Slow crank, dim lights | Weak battery or corroded terminals | Clean the terminals and have the battery load-tested |
| One click, no crank, lights stay bright | Starter or starter circuit | Check voltage at the starter while cranking |
| Battery light while driving | Alternator or belt | Measure voltage with the engine running |
12-volt battery types
Flooded
Conventional design on many older and base vehicles
Enhanced flooded (EFB)
Built for more charge cycles; common with entry start-stop systems
AGM
Absorbent glass mat; common with start-stop and heavy electrical loads. Replace like with like
Before you order
Start with the year, make, model, and engine, then narrow it down with the trim, drive type, and any option the part depends on. The VIN is the most reliable way to do that, because many parts catalogs and dealer counters decode it to the exact build. If the old part is in hand, its original-equipment number is the best cross-reference. Aftermarket brands publish their own numbers and usually list the original number they replace. Two listings that both say they fit can still differ, so compare the photos, connectors, mounting points, and the notes in the listing. The find the right part guide covers this step, and the all makes reference lists each maker’s official parts catalog.
Once you know the part number, compare the same number across stores. Look past the first price at the core charge, shipping, the return window, and what the warranty covers and who honors it. A remanufactured part, a new aftermarket part, and an original-equipment part can each be the right choice, but they are not the same purchase. The compare prices and warranties guide walks through what to read. Prices on PartIndex carry the date they were recorded, so confirm the current price and fitment with the retailer before you buy.
Batteries contain acid and can release hydrogen gas. Wear eye protection, disconnect the negative cable first and reconnect it last, and keep sparks away. When jump starting, follow the order in the owner’s manual. Hybrid and electric vehicles also have a separate high-voltage battery covered in its own guide.