Charging a Car Battery


Having decided that selecting the proper resistance to be in series with the alternator's field coil could be the key to success, we purchased a handful of 22 Ohm, 10W resistors and returned - yet again - to the ghatta.

Alternator Setup

Alternator Setup

  

Initially, we connected five 22 Ohm resistors in parallel to the alternator's field coil. This was enough current to start the alternator, but not so much as to cause the ghatta stone to grind to a halt. We were charging a car battery! The voltage, however, never reached the optimal 14.4V charging voltage, and hovered varied between 12.6V and 12.8V instead. We repeated our tests with a 12V motorcycle battery and a 12V sealed lead-acid battery. The current drawn by these batteries was lower, but the voltage remained at about 12.8V. We also tried using three 22 Ohm resistors in parallel, and this worked as well.

During these trials, the alternator was mounted upside-down to a wooden box. All alternators turn clockwise and, as far as we're aware, all ghattas turn counterclockwise. We used small blocks of wood to prop the alternator up, and relied mostly on previous measurements to choose the right height for the box. Due to the lack of an adequate mounting system, we had to hold the box back ourselves to achieve tension, which grew to become rather tiresome. The belt required a fair amount of tension to turn the alternator, and there was a lot of wobble in our bike tire. This meant that to keep the belt in tension, the box had to be pulled back and forth with the wobble. We determined that the wobble was mostly coming from the looseness of the connection between the shaft and the bicycle rim.

In an attempt to reach higher rpms, we also put together a second set of pulleys with a 3:1 ratio. These were connected by a vertical shaft to a set of sealed bearings and mounted to the edge of our wooden box. We intended this to be more of a "proof-of-concept" test, as sealed bearings are expensive, and we hoped to arrive eventually at a simpler and less expensive solution. Unfortunately, the belt we had bought to connect the second set of pulleys to the alternator was too big. Although the unloaded alternator turned much faster than before, we were unable to begin producing current. We thought the size of the belt might have been a factor.

Key Lessons from this Test:


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