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I have trouble understanding how the battery master switch activates the battery contactor in this schematic from the Cessna 152 manual. This is the model from 1979 or later with an Alternator Control Unit.

Cessna 152 wiring diagram Image source: Flight Literacy

I believe that the battery contactor is a relay that connects the top and right contacts (as in the image), when the left contact is powered.

Initially, all switches are off, the alternator is not turning, and there is no oil pressure:

  • The left contact of the battery contactor is connected to ground via the battery master switch in off position.
  • The battery contactor is not powered. It disconnects the battery from the connection through the Ammeter to the primary bus.
  • The battery voltage is distributed only to the clock and the oil pressure switch.
  • The two diodes close to the battery contactor are in reverse bias and therefore not conducting.

When I change the battery master switch to the on position, I expect that the battery contactor is closing the contact, and connects the battery via the Ammeter to the primary bus.

  • That requires power on the left contact of the battery contactor.
  • The battery master switch connects this contact (via "Alternator field circuit" breaker and "Pull off ALT" breaker) to the primary bus.
  • The primary bus is not powered yet.
  • In the way I read this schematic, the battery contactor cannot be activated. Nothing happens.

What am I missing?

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4 Answers 4

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See picture below of the C152 electrical system.

enter image description here

The battery contactor is a solenoid controlled relay. The battery is connected directly to the input terminal of the contactor and this terminal and the BATT switch terminal on the contactor remain electrically hot as long as the battery has charge. When the BATT MASTER switch is energized, this provides a path from the hot input terminal of the contactor to ground, causing current to flow from the battery, through the contactor solenoid and to ground (yellow path).

enter image description here

The energized solenoid makes contact between the input terminal and the and the main bus bar, energizing the airplane’s electrical system (blue path).

enter image description here

Current from the battery also passes through the ammeter enroute to the bus bar, indicating the rate of current either from battery discharge or battery charging provided by the alternator.

If the ALT switch is not energized, The overvoltage sensor is grounded, thereby preventing the regulator from supplying power to the alternator field coil and the alternator from producing electricity to be fed into the bus (purple path).

enter image description here

This causes the OVER VOLTAGE light to illuminate, indicating that the alternator is either not producing power to the aircraft electrical system, or the electrical system is operating at an abnormal voltage.

Current to the main bus bar from the alternator may also be interrupted by means of the ALT circuit breaker.

If the ALT switch is energized, the overvoltage sensor is not grounded, battery power is supplied to the regulator, which in turn energizes the alternator field coil (orange path), thereby allowing the alternator to generate electricity at a voltage greater than the battery (green path) when the engine is driving it.

enter image description here

The diode fitted between the two contacts on the battery contactor allows the alternator to charge the battery during engine operation.

Alternatively both the aircraft’s electrical bus and battery charging can be operated by connecting an external power plug into the A/C’s external power socket (red path).

enter image description here

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I think two of my premises were wrong.

  1. The battery contactor is activated by connecting the left contact to ground (instead of 24 V as I thought). It works different from the starter contactor (which has a permanent ground connection).
  2. The battery master switch toggles between ground [on] and floating [off] (instead of primary bus [on] and ground [off] as I thought).

Like this, the plane will be able to start!

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The left connection on the contactor is brought to ground on the right, BAT, side of the master switch. That allows current to flow between the center contactor terminal and the left contactor terminal.

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The battery contactor is a solenoid. Closing the BAT switch to ground activates the solenoid, which then physically closes the circuits providing power to the bus boards and starter switch from the battery.

The starter switch connects to yet another solenoid, which sends power to the starter motor. In exactly the same way as the battery contacter, the starter solenoid reopens when the key moves out of the "start" position, stopping the current flow required to keep its "switch" closed for the starter motor once the engine is running on its own.

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