# Car Battery Boosting by PHONE Source: https://www.youtube.com/watch?v=MwTaGjsdxU4 Recap page: https://rapidrecap.app/video/MwTaGjsdxU4 Generated: 2025-12-07T17:36:25.825+00:00 --- ## Quick Overview It is possible to jump-start a car with a dead battery using only a smartphone, as demonstrated by successfully starting a Toyota Matrix after charging the battery for 30 minutes using a custom boost converter circuit powered by the phone's 5V USB output, which provided 2,700 Joules of energy, significantly more than the 2,400 Joules required for one successful crank. **Key Points:** - The experiment successfully used a phone's 5V USB output to charge a dead car battery enough to start the engine after 30 minutes of charging. - The car starter required approximately 2,400 Joules of energy per crank, calculated from the 12V battery capacity and 200A average crank current. - The phone's charging circuit, running at a constant 0.3 Amps for 15 minutes (900 seconds), delivered 2,700 Joules of energy, which was sufficient for one successful crank (2700J > 2400J). - The charging circuit employed a custom boost DC-to-DC switching converter design, which takes the 5V input and boosts it to charge the 12V car battery. - The host performed multiple charging attempts, with one 30-minute charge resulting in the battery voltage rising from 9.49V (no load) to 10.75V (under load), which allowed the engine to crank twice before failing again. - After a total charge time of 41 minutes and 2 seconds, the battery voltage reached 10.73V, and the phone's charging current dropped, suggesting the phone stopped supplying power. - The final successful crank occurred after the host gave the battery an extended charge, indicating that while challenging, providing enough cumulative energy from a phone can revive a dead car battery. ![Screenshot at 00:38: The host connects a heating element across the car battery terminals while the car ignition is on, setting up the initial test to show the rapid current draw from the car battery when accessories are left on, leading to a dead battery scenario.](https://ss.rapidrecap.app/screens/MwTaGjsdxU4/00-00-38.png) **Context:** The video explores the feasibility of using a modern smartphone's battery to provide enough energy to jump-start a standard gasoline car with a dead 12V lead-acid battery. The host, Mehdi Sadaghdar of ElectroBOOM, designs and tests a custom boost converter circuit that steps up the phone's 5V USB output to a voltage high enough to push a small charging current into the 12V battery. This demonstration aims to quantify the energy stored in a typical smartphone battery relative to the high energy required to crank an engine. ## Detailed Analysis The video proves that a smartphone can generate enough energy to jump-start a car with a dead battery, though it requires a significant charging time and specialized circuitry. The host first establishes the energy deficit of a dead car battery by showing that leaving the dome light on drains the battery quickly. He measures the required energy for one successful engine crank to be approximately 2,400 Joules, based on the 12V battery's 55 Ah capacity and an average crank current of 200 Amps over one second. To transfer energy from the phone (rated at 3.7V, 3.9Ah, or about 52,000 Joules total energy), the host constructs a boost DC-to-DC switching converter circuit. This circuit takes the phone's 5V USB output and steps it up to charge the 12V battery. After 30 minutes of charging at a constant 0.3 Amps, the phone delivered 2,700 Joules, enough for one crank, but this only raised the battery voltage slightly under load (from 9.49V to 10.75V), resulting in only two failed cranks. After running the charge for 41 minutes, the battery voltage barely increased, and the phone stopped charging. The host then disconnects the phone circuit and attempts to crank the car; it turns twice but fails to start. After further charging, the host successfully starts the car with one crank, concluding that while possible, it takes a substantial amount of time (around 15 minutes of charging for one crank, or much longer for sustained starting power) due to the low current output of the phone compared to the massive energy demands of the starter motor. ### Dead Battery Demonstration - Leaving the dome light on drains the 12V Energizer battery (55 Ah) below 11.8V in about 10 minutes, reaching 9.46A draw initially. ### Energy Calculation for Cranking - One car crank requires approximately 2,400 Joules (12V * 200A * 1s), calculated based on the engine's high current draw. ### Phone Charging Test Setup - A custom boost converter circuit steps up the phone's 5V USB output to charge the 12V battery at a steady 0.3 Amps. ### Charging Results - After 30 minutes, the phone delivered 2,700 Joules (enough for one crank), but the battery voltage only reached 10.75V under load, resulting in two failed cranks. ### Phone Battery Energy Calculation - A typical phone battery (3.7V, 3.9Ah) holds about 52,000 Joules, which translates to roughly 19.5 cranks if the charging circuit were 90% efficient. ### Final Test and Conclusion - After extended charging (over 41 minutes), the host successfully starts the car with one crank, confirming the concept but highlighting the impracticality due to time constraints. ### Circuit Explanation - The custom circuit uses a boost converter driven by a 100kHz oscillator/Schmitt trigger circuit to regulate the charging current based on the battery voltage feedback. ![Screenshot at 00:09: The host sets up the test with the white Toyota Matrix, driver's door open, and a clearly visible sign reading 'CAR' taped to the front bumper.](https://ss.rapidrecap.app/screens/MwTaGjsdxU4/00-00-09.png) ![Screenshot at 00:51: A close-up of the clamp meter showing a parasitic current draw of 0.364 Amps when all car accessories are off, indicating the dead battery issue.](https://ss.rapidrecap.app/screens/MwTaGjsdxU4/00-00-51.png) ![Screenshot at 01:00: Multiple multimeters are connected to the battery terminals: one measuring current \(0.0032A initially\), and another measuring voltage \(11.816V under load from the charging circuit\).](https://ss.rapidrecap.app/screens/MwTaGjsdxU4/00-01-00.png) ![Screenshot at 02:27: The host reacts dramatically after the phone's charging circuit causes a small explosion or smoke release in the engine bay when attempting to charge the battery.](https://ss.rapidrecap.app/screens/MwTaGjsdxU4/00-02-27.png) ![Screenshot at 03:40: The host proudly displays the small custom circuit board designed to trick the phone into supplying charging current to the car battery system.](https://ss.rapidrecap.app/screens/MwTaGjsdxU4/00-03-40.png)