Battery - Alternator Fusible Link

When things go wrong.

Purpose of the Fusible Link.

If the battery shorts, all of the battery power will be trying to get to ground. 
Fusible link (or links on some models) will protect the rest of the wiring from damage if there is a short before any fuses or circuit breakers.
Unlike fuses and circuit breakers, fusible can handle normal high current loads, such as running ignition and headlights and other equipment for short periods of time. 
When subject to high current for over time periods, the link should overheat and fail before the attached wires and connectors do.
If there is a dead short, the battery current is high enough to melt the fusible link quickly.

Examples:

Damaged Insulation on a Wire Close to Battery

Bare Conductor touches chassis.  Battery current goes to ground at contact. Ammeter shows nothing; Or, if alternator is running, may briefly show maximum charging.

Link Melted. Evidence of overheating likely at connectors.

Example 2: Internal Short in Alternator

Shown here the short is in the output circuit of the alternator. Ammeter will show maximum discharge!

Fusible link should have protected against major damage.  Expect evidence of overheating at all connections.  Any places that were loose or oxididized will be worse.

Problems

Fusible Link and Continuous Equipment Current

Example of Power flow with new circuits added at starter solenoid.

Circuit Amperage

In this example, 15 amp could be from:   Lights on relays,  9 amps   Electric Fans, 6 amp minimum  20 amps assumes 5 amps for charging.  25 amps from alternator includes:      2.5 amps for field circuit      2.5 amps for ignition coil   Some modern amperage loads at AA1car.com Electric Winches are special consideration.   They can draw 200 to 400 amps in use.   An alternator can not supply that much current. It will have to come from the battery.  Then the alternator will have to recharge the battery.

Solutions

• Add new circuits to alternator side.  (special exception for winches)
  
• Use an alternator that can power all equipment at idle so the battery doesn't need continual recharging. (For vehicles that idle frequently in traffic, etc)
  
• Replace or add wires with less or better connections to reduce resistance.
• Check ammeter for internal resistance (indicating damage such as loose shunt connection).  If present, repair, replace or bypass. 
  
• If adding significant new loads, consider an appropriate redesign. 
  
• Battery dead or low?   If charging with alternator - watch ammeter, keep rpms low, and if needed, turn on lights to reduce the current through the charging circuit. Shut off, allow electrics to cool. Batteries recharge better when cool.  Resistance in wiring will also go down when allowed to cool.   Best choice is to use a battery charger.