can two 9v batteries start a car

[DiWilliam]

No, two 9V batteries cannot start a car. While connecting them in series would theoretically provide 18 volts, which is close to a car battery's 12 volts, they completely lack the necessary cranking amps (CA). A typical car starter motor requires hundreds of amps to turn the engine over. A standard 9V battery can only deliver a tiny fraction of that current, making it physically impossible. Attempting this could damage the batteries, cause them to overheat, and will not result in a started engine.

The core issue is the immense difference in power delivery. A car battery is designed for high-current, short-duration bursts. Its cold cranking amps (CCA) rating, which can range from 400 to 800 amps for most passenger vehicles, represents its ability to deliver power at low temperatures. In contrast, a common 9V alkaline battery has a current delivery capability of about 500 milliamps (0.5 amps). This difference is over a thousandfold.

Connecting two 9V batteries in series increases voltage but does not increase the available current. The starter motor, when engaged, presents an extremely low resistance load. The small 9V batteries cannot supply the demanded current, causing their voltage to instantly plummet to near zero—a phenomenon known as "brownout." The internal resistance of the 9V batteries is too high to support the starter motor's needs.

For a successful jump-start, you need a power source with sufficient voltage and amperage. The only safe and reliable solutions are:

• A healthy 12V car battery from another vehicle, using jumper cables.
• A dedicated portable jump starter pack, which contains a special high-discharge lithium battery designed for this exact purpose.
• In some manual transmission cars, a push-start (or roll-start) can be an effective mechanical alternative.

Power Source	Typical Voltage	Typical Cranking Amps	Can it Start a Car?
Car Battery	12V	400 - 800 A	Yes
Portable Jump Starter	12V	200 - 1000 A	Yes
Two 9V Batteries (in series)	18V	~0.5 A	No
AA Battery (for comparison)	1.5V	~2 A	No

[Clark]

Forget it. It’s like trying to fill a swimming pool with a squirt gun. The little rectangular 9-volt batteries you use in smoke detectors don't have the muscle. Your car's starter needs a massive, instant jolt of power that only a proper 12-volt car battery can deliver. You'd be wasting your time and probably just kill those small batteries. Your best bet is to call for a jump-start or use a portable booster pack.

[JaxsonFitz]

From an electrical standpoint, this is not feasible. The critical missing component is current, measured in amperes. A vehicle's starter motor requires a surge of several hundred amps. Alkaline 9V batteries have a high internal resistance, severely limiting their current output to about half an amp. When subjected to the starter's load, their voltage would collapse immediately. You need a low-impedance power source like a lead-acid battery designed for high discharge rates.

[VanElisa]

I tried this once out of sheer desperation when my truck died in the driveway. I hooked up a bunch of 9-volts with some wires, and absolutely nothing happened. No click, no groan from the engine—just dead silence. It was a complete non-starter. I ended up walking to the store to buy a real jump starter pack, which fired up the engine in seconds. Save yourself the hassle and skip the science experiment.

[Taylor]

It's important to understand why this won't work, so you can avoid potential hazards. While the voltage might seem close, the power capacity is the problem. A car battery is built with thick lead plates and liquid electrolyte to release energy violently for a few seconds. A 9V battery has a small, dry cell designed for low-power devices over a long period. Forcing it to act like a car battery can cause it to leak, get very hot, or even rupture. Always use the correct, safe equipment for your vehicle.