MakeShift Challenge: Dead Battery: “Most Creative” Winning Entry

MakeShift 01: Jim Gasbarro’s “Most Creative” Winning Entry
by William Lidwell
May 13, 2005

Frictional losses in Jim Gasbarro’s assembly (wheel to pulley) will almost certainly require more than an hour of spinning, but that’s a quibble. His analysis is dead on, and his approach is the soundest of the alternator bunch. Definitely a guy you want in the car when your battery dies in the middle of nowhere. Congratulations Jim!

Here’s Jim’s winning entry in the MakeShift Master – Creative category:

Remove the car’s battery and alternator using your tools. You will also need a small light bulb, socket, and some wire. Possible sources for these are the trunk light, the engine compartment light, or an interior courtesy light.

Warm the battery. Use the Sterno, flares, and/or whatever fuel that you have nearby to begin to gently warm the battery. Use the sleeping bags and/or tent to contain the heat and to preserve it once the battery is warmed.

Identify one wheel on the car that will spin freely. If the car is front wheel drive, use a back wheel, or vice versa. If the car is four wheel drive, disconnect a hub.

Jack up the free wheel.

Presumably you have the car’s jack with you. If not, go to step 4b. Get the wheel up high enough that you can dig a hole under the tire with your tools (screwdrivers, hammer claw, etc). It may help to temporarily remove the tire. It also may help to temporarily remove the brake pads to reduce friction (you’ll see why later).

You idiot. You’re 50 miles from nowhere and you don’t have a jack. Use local materials (rocks, branches, etc) to build some cribbing under the axle of interest. Excavate under the tire until it spins freely and then dig the hole.

Connect wires to the alternator. Connect the red jumper wire to the battery terminal of the alternator. Connect the black jumper wire to the frame. Connect the lamp wire to the “field coil” terminal.

Bury the alternator in the hole with the pulley facing up. First dump the potato chips and wrap the alternator with the bag to prevent small particles of dirt from getting inside. Line the hole with small rocks to make it more stable and then tamp in dirt until the alternator is secure. Put the tire back on the axle and lower the car until the tire just touches the rim of the pulley. If you didn’t have a jack, you’ll have to carefully excavate under your cribbing to lower the vehicle.

Connect the jumper cables to the battery.

Apply field current. Connect the free wire from the lamp to the battery. If you’re lucky, the lamp will light, indicating that you have current in the field coil of the alternator. Lead acid batteries will often recover some of their charge after they have been deeply discharged if they are just left to sit for a while. Warming them helps. If the lamp does not light, then you have to go to your backup field current source, which is the depleted cellphone battery. The cell phone is smart enough that it will not discharge the battery below about 3V. So there should be enough charge left to power the alternator field current for a few seconds (see About the Alternator section below). Use the adhesive bandages to tape wires onto the cell phone battery as necessary.

Start turning the tire. Turning the tire turns the pulley of the alternator, but at a much higher rate than you could achieve by hand turning the alternator. The tire also provides a flywheel effect to keep the alternator going between pushes. Most alternators will produce an output current if you achieve a minimum of 1000 rpm on the alternator shaft. Depending on your wheel size, the tire diameter is between 24 and 30 inches. The pulley diameter is about 2.5 inches. This gives a 10-12 speedup between the rotation rate of the tire and the alternator. 1000 rpm is 16 rps, so you have to turn the tire a little faster than one revolution per second. Not unreasonable.

Charge the battery. So how long do you have to turn the tire to charge the battery? In order to start the vehicle, let’s say that you need 50A for 5 seconds. That’s 3000 W-s or about 1W-hr. Let’s say that your arms can supply about 5W of power. A lot of that is going to be lost to friction and powering the field winding. If you’re lucky, you’ll get 1W of power into the battery, so you better keep up the turning for an hour or so to be safe.

Reinstall the battery and alternator.

Cut up the limes and squeeze them into the water bottles. Scrape some of the snow that has just started falling and put that into the water bottles as well.

Start the engine and drive out, while enjoying your cool limeade. It may not be as good as a frosty beer, but you can have that when you return to civilization in an hour or so.
About the Alternator

Alternators are like AC motors in that they have no permanent magnets in them to set up a magnetic field for the armature winding to spin in (which in turn generates the electric current). That is why you have to supply a field current in order to get the alternator to work. The field winding is very low resistance, so the lamp acts as a ballast to limit the flow of current.

The externally supplied field current is not needed for long. There is a set of internal diodes in the alternator that automatically shunt some of the power from the output to the field winding. When the alternator gets up to speed, the lamp will dim or even go out as the alternator begins to supply its own field current. This is why the cellphone battery could actually be enough to get the alternator working: you only need external field current long enough to bootstrap the system.

Another feature of the alternator that is helpful is the “soft start” feature. The alternator has an internal regulator on the field current that limits the current when the battery voltage is low. When the battery is low, it appears as a near short circuit across the output of the alternator. If normal levels of field current were applied, the amount of torque required to turn the alternator would be very high and the tire would probably just slip. The soft start feature limits the field current so that the alternator produces less output current and the torque is kept to a reasonable level. As the battery voltage increases, more current is allowed to flow in the field winding, and the effective charging rate improves.

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