Can the battery be charged directly in the car?

[VonLilliana]

It is possible to charge the battery directly in the car. When starting the car, the starter requires a large current, which a regular charger cannot provide. If the battery is dead, the car won't start. The principle of charging is that the voltage must be about 20% higher than the battery's voltage to charge it effectively. Too high a voltage can damage the battery, while too low a voltage won't charge it. Uses of car batteries: 1. For starting and lighting in cars, motorcycles, tractors, diesel engines, etc.; 2. As backup power for protection and automatic control in communications, power plants, and computer systems; 3. As power sources for various battery-powered vehicles, forklifts, and loaders; 4. For starting and lighting power in diesel locomotives, electric locomotives, and passenger trains.

[AngelinaLee]

I've been driving trucks for over a decade, and charging directly in the vehicle depends on the situation. If the engine is still running, the alternator can automatically recharge the battery, effectively charging while driving. However, if the battery is completely dead, idling alone won't cut it—you need to rev the engine to at least 1500 RPM for effective charging. In neutral gear, the alternator basically doesn't work, and you won't get any charge in P or N gear. I once tried charging in neutral for two hours during a delivery run, only to waste gas. For emergency charging, remember to keep the engine running at medium to high speed for at least half an hour, and turn off power-hungry accessories like AC and headlights. If a battery over 3-4 years old won't hold a charge, it's probably time for a replacement.

[Ora]

Just helped my wife deal with a dead battery issue last week. Straight to the point: it works but inefficiently. The engine does charge the battery while running, but at a frustratingly slow pace. Actual testing with a multimeter showed only about 5 amps charging current at idle. To fully charge a standard 20 amp-hour battery would take roughly four hours. This might work as an emergency solution for new cars, but older vehicles with aging wiring can be risky—I once smelled burning and had to shut it off immediately. The safest approach is either removing the battery for a proper charger or driving continuously for over half an hour. Pro tip: when charging overnight, monitor brightness changes by keeping the headlights on.

[DeJuniper]

From the perspective of automotive electrical circuit principles, the alternator, battery, and electrical appliances are connected in parallel. After the engine starts, the belt drives the alternator to rotate, generating alternating current (AC), which is then converted into direct current (DC) by the rectifier to charge the battery. However, the charging current is affected by the engine speed, with the alternator outputting at most 20% of its power at idle. Our repair shop's actual measurements show that at -10°C in winter, the charging current at idle is only one-third of the normal level. If you notice the charging indicator light stays on, there's an 80% chance the alternator's carbon brushes are worn out. Here's a lesser-known fact: when the air conditioning is turned on, the alternator automatically increases its speed, resulting in higher charging efficiency. However, if the battery is severely depleted, it's recommended to replace it directly.

[SanColeman]

Newbies, take note! Never make the same mistake I did when charging in the car: Last time I watched a video saying idling to charge saves money, but when I tried it in my own garage, I nearly got carbon monoxide poisoning. Absolutely never do it in an enclosed space! It's okay in a ventilated environment, but you have to put it in D gear, engage the handbrake to secure the wheels, or have a friend press the brake pedal. Honestly, the charging efficiency is terrible—my SUV only gained 5% battery after idling for an hour, and the gas money spent could have paid for two roadside assistance calls. Now I’ve learned my lesson and always keep a power bank-sized emergency battery in the trunk. Veteran drivers say it’s easiest to just replace the battery if it’s over four years old, to avoid breaking down halfway.