How to tell which is red and black on a car battery?

Question

HeidiLee · Accepted Answer

To safely identify car battery terminals, rely on these four reliable methods in order: look for the “+” (positive/red) and “–” (negative/black) symbols stamped on the case, check for red plastic covers or cables on the positive side, note that the positive terminal post is physically larger on most standard batteries, and use a multimeter for definitive verification. Incorrect connection can cause severe electrical damage, so accurate identification is crucial.

The most fail-safe identifier is the permanent symbol marking. Near each terminal, the battery casing is molded with a "+" sign for positive and a "-" sign for negative. These symbols are often accompanied by the abbreviations "POS" or "P" and "NEG" or "N". Even with dirt or light corrosion, these indentations are usually discoverable upon close inspection and are the manufacturer's primary guide.

Color-coding is the universal visual standard. The positive terminal, its protective cover (if present), and the cable attached to it are almost universally red. Conversely, the negative side uses black for its terminal, cover, and cable. This color scheme extends to jumper cables and most automotive electrical diagrams. However, be cautious as aftermarket repairs or heavy corrosion can sometimes compromise this color code.

A physical characteristic provides a strong secondary check. On common top-post batteries (Group Size 48, 24F, etc.), the positive terminal post has a larger diameter than the negative post. This is a deliberate design to prevent accidental reversal. The positive post typically measures approximately 19.7 mm (0.78 inches) in diameter, while the negative post is about 17.5 mm (0.69 inches). The difference is subtle but noticeable by sight or touch.

Identification Method	What to Look For	Reliability & Notes
Symbol Markings	“+” and “POS” near positive; “-” and “NEG” near negative.	Most reliable. Permanent, stamped into the case.
Color Coding	Red components (cable, cover) on positive; Black on negative.	High reliability (standard practice). Can be degraded by dirt/repairs.
Terminal Post Size	Positive post is visibly and tangibly larger in diameter.	High reliability on standard top-post batteries.
Multimeter Test	DC voltage reading shows ~12.6V with red probe on positive.	100% definitive. Essential if other markers are unreadable.

When visual clues are obscured, a digital multimeter provides absolute certainty. Set it to DC voltage (20V range). Touch the red probe to one terminal and the black probe to the other. A normal reading of approximately +12.6 volts indicates the red probe is on the positive terminal. If the display shows -12.6 volts, your probes are reversed; the terminal touched by the black probe is positive.

Safety is paramount during any battery work. Before disconnecting or performing maintenance, always disconnect the negative (black) terminal first and reconnect it last. This sequence minimizes the risk of a dangerous short circuit if a tool accidentally contacts the vehicle's metal frame while touching the positive terminal.

JesseRose · Answer

As a mechanic for over 20 years, I’ve seen every dirty, corroded battery out there. My first move is always to feel for the “+” and “-” stamps on the plastic. They’re almost always there, even under gunk. If I can’t find them, I look for any trace of red color on a cable or boot. The post size check is my final visual confirm—the positive one is fatter. No guesswork needed. Just remember: disconnect the black cable first, every single time. It’s a habit that keeps you safe.

SanEmerson · Answer

I’m not a pro, just a careful DIYer. My main advice is to never rely on color alone , especially on an older car. I once helped a neighbor whose battery was so dirty everything looked gray. What saved us was the multimeter trick . We got a positive 12.7-volt reading, which instantly told us which post was which. It’s simple: if the number on the screen is positive (like 12.6), the red probe you’re holding is touching the positive terminal. This method leaves zero room for doubt, which is what I need for peace of mind before touching anything.

DiAlex · Answer

Here’s the quick guide I follow in my garage:

Look for the signs: Scan for the engraved ‘+’ and ‘-’.
Spot the red: Find the red cable or cap.
Compare the posts: See which metal post looks thicker. That’s positive.
Test if unsure: Multimeter set to DC voltage (20V). Red probe to a terminal. If the reading is a normal positive voltage (~12.6V), you’ve found the positive post.

Why does the size matter? It’s a foolproof design to stop you from connecting jumper cables backwards. That physical difference is a built-in safety feature you can trust with your eyes closed.

TuckerRose · Answer

From an engineering and standards perspective, the identification methods are systematic. The International Electrotechnical Commission (IEC) and Japanese Industrial Standards (JIS) codify the use of red for positive and black or blue for negative in DC systems. The diameter difference between posts is not arbitrary; it’s a specified dimensional standard to ensure polarized compatibility and prevent misconnection, which can lead to catastrophic failures like ECU damage or electrical fires. The permanent symbol markings are part of the battery’s UL or SAE certification. When a multimeter shows a positive potential difference , it is measuring the fundamental electrical polarity. Thus, these methods are layered redundancies: color for quick reference, symbols for permanent identification, physical design for error prevention, and instrumentation for empirical verification.