Using a Laptop Charger to Start a Car with a Dead Battery in 30 Minutes

Using a Laptop Charger to Start a Car? The title says it all. The steps are provided below. Things are not as easy as they seem, because a laptop battery is essentially a lithium polymer one and can be quite a capricious component. Here is how you can carry this process out effectively not to mention safely.

Essentially an item that can be ‘found around the house’, laptop batteries if used wrongly can irreparably damage your car battery. Know that it is a risky fire-hazard procedure that could harm you in the process; stop grinning, this is not a thrill-moment.

Here is a list of all you will need to put this together:

  • Metal wire with 1.5 Ohm resistance
  • Metal wire to connect inside the (+) power plug
  • To attach wire to the outside of (-) plug, you will require a tie wrap
  • 12-volt automotive plug
  • 14-19 V (volt) DC power supply
  • Soldering iron and solder to go with it
  • Multimeter
  • Electricity terminal block
  • Electricity wire to help make connections
  • Either duct or electrical tape
  • To wind the wire on, you will need a plastic or wooden stick

Get yourself a 14-volt power suppy that can handle 0.5 A (amps) maximum current. This is you playing it safe and maintaining the battery’s charging rate at a goodly 13.2 – 14.4 volts.

  • Take your car’s cigarette lighter nodule. It contains a 0.5-ohm resistance coil spring, which you will need.
  • Consider the power supply and the pole from the list above. Place the 1.5-ohm resistor between these two. You should also have the resistor between the positive (red) cigarette lighter plug’s wire.
  • What pole is this? Well you can use the handle of a hammer and wind a metal wire of 1.5-ohm total resistance all around the stem. This is to prevent heat from dissipating through the wood during the charging process. Either end of the pole will later see you using tape to affix the connecting (cold) wires.
  • Bear in mind that you are using the resistor as a ‘shunt’ (i.e., a conductor having low resistance in parallel with another device to divert a fraction of the current).
  • This is the equation to better understand what you are doing… Current (I) = Voltage (U) passed via the shunt divided by the 1.5 ohm resistance value (R) [by extension, U = I x R]
  • Performing the calculation based on what you have, we get 0.52 V / 1.5 Ohm = 0.35 A. At least some of this current is going into the battery.

Charging will take all of two or so hours. However, there is a high chance that the engine will come on but lacks sufficient juice to start. You can charge the battery overnight (about 7-8 hours) but there is a faster way over this obstacle.

  • Cautionary Note: If you are using a power supply that goes in excess of 14.4 volts and are charging your car’s battery for too long with it, the battery will over-charge and result in either a fire or full-fledged explosion. Your battery charger’s makeshift circuit needs you to tell it when to stop.
  • To speed things along, start by measuring the charging current. You need, say, 2.9 V. Do the math… 2.9 V / 1.5 Ohm = 1.9 A, which is a good rate of charge. It will not overload your car’s circuits or your laptop’s power supply.
  • Leave things be for 15 minutes when you are done setting it all up.
  • When you return you will notice the resistance wire has grown hot. The pole and its environs will be warm as well; positive signs. The entire ensemble has to be exposed to the air or things can get extremely hot. Taping around all apt spots takes care of heat dissipation.
  • Even the 12-volt plug with its 0.5-Ohm resistance (the cigarette lighter) will show similar signs of heat accumulation. The coil spring resistance has heated the metal tip. If your lighter is in a closed space, now will be a good time to take it out and up for a little bit of cooling (prevents melting).

After only half an hour of this, give the car battery another try. It is wiser to discharge the battery than over-charge it. Take the lighter out of its plug and turn the ignition. There is a high chance your car will roar to life. Keep it running for five minutes.

Side Note 1: The cigarette lighter is generally fused at 10 A, so it is best to keep the current running below 4 A.

Side Note 2: When dealing with a 19 V power supply that can work with 4 A (and you have a 12 V car battery), the minimum needed resistance R = voltage supply – 12 / limit of current. The equation goes like this… (19-12) / 4 = 1.75 Ohm.

Side Note 3: All wires to be used need to have good connections between them. You do not want things over-heating and becoming a fire hazard. Sandpaper will help you remove dirt or oxidation at the connections.

Side Note 4: Circuits should never directly be linked to the car battery. A fuse should be present between them.


This entire method need only apply to help get the engine going. Drive the car and let its own alternator do the rest of the charging. If you see the battery exceeding 14.4 volts, cease charging altogether. This could happen with old or damaged batteries. If you are still unsure, best call a professional or jumpstart your car using the battery power from another vehicle.

One Response

  1. Lyon Bassey

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