Converting to an Alternator

When upgrading to a bigger alternator there is only one wire that may need to be beefed up. It is the big red wire from the alternator to the solenoid.

The wires must be sized so that they can carry the required current without getting to hot. Taken to the extreme too small a wire and to high a current, the wire melts quickly and we call it a fuze. Most of the time we worry more about the wire staying cool enough so the insulation does not melt or breakdown. This problem is worse in a hot environment (engine near exhaust) or when many wires are bundled together. The use of modern teflon insulation allows much higher safe temperature rises than the more common PVC insulation.

For most car applications the required wire size is set by the load current. ie the current each load such as the headlights draw from the battery. We pick the right sized wire and place a fuze in series with it so if something goes wrong with the load and it draws too much current the fuze blows before the wire insulation melts. Because the stock Alpine /Tiger has only one fuze (35A) for all the ignition switched loads and a spare (generally not used )... for the interior lamp.. all the load wires should be sized to carry 35 A if Lucas had done it right. ;-) A separate smaller fuze sized to each load would be a much better idea. Since we are not changing any of the loads none of the wiring from the fuze panel needs changing.

This leaves the starter circuit. From the battery to the solenoid and on to the starter. It also includes the wiring from the battery to the engine block. This circuit is sized to carry the very high starter current (up to 300A). The wire is actually smaller than might be expected for such a high current, but big wires gets hot slowly and the starter runs for only short times. The alternator hooks into this circuit at the solenoid and these connections can easily carry any reasonable alternator output.

The Big Red Wire

Its the connection from the alternator to the solenoid (big red wire) that we must be concerned with. This wires current is set by the capacity of the alternator. The internal construction of the alternator will limit the maximum current available. When the engine is running normally all the load current is drawn from the alternator, and some current is used to charge the battery. The alternators output is controlled, by the voltage regulator, to keep the battery fully charged at about 14.3 volts. The maximum current is produced by the alternator just after starting when the battery is very drained and the full capacity of the alternator is not enough to raise the voltage over 14.3V.

If you have only stock loads at night you will draw about 30A total and the 35 A stock alternator will do. If you use a bigger alternator it will charge the battery faster but on average you will only draw 30A thru the big red wire and it will probably stay cool enough. If you have added loads.. A/C.. Driving lights.. big radios etc you probably should increase the wire size.

A 10 Gauge (AWG) copper wire in open air should be good for 55 A. I'd go for teflon insulation if it gets anywhere near the exhaust.

It is modern automotive practice to use a fuzable link for this connection. While under normal conditions the alternator cannot produce more current than its rating, under failure conditions it can draw big currents from the battery. A fuzeable link is just a piece of wire designed to open circuit if too much current is drawn through it.

A common problem often neglected with these older charging systems is the connections in these hi current circuits. It is important that they be clean, shiny and tight. A small layer of corrosion or oil can be deadly. Take them apart and use a wire brush until they shine. Be sure to disconnect the battery, (removal of the ground connection is sufficient), before working on the solenoid connections. I remember well losing a clutch master cylinder in a flash. A lose battery hot wire brushing against the metal Locheed cylinder will arc right through.

Don't forget the connections from the battery to the car frame and engine block. They are part of the 300 A starting circuit. Also the connection from the alternator body to the engine block. I have often added a heavy jumper to avoid relying on a ground connection through the alternator's mounting bolt.

Steve F

Steve Finberg                        W1GSL
PO Box 397082 MIT Br        Cambridge MA  02139-7082              617
258 3754

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