System voltage drop during start

Discussion in 'Instruments / Avionics / Electrical System' started by rmoehle, Jul 6, 2017.

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  1. Sep 13, 2017 #21

    TFF

    TFF

    TFF

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    I would come up with some big lead jumpers and run a second ground to the engine from the battery and the same to the starter. You might need to hot wire the master closed and turn stuff on with the circuit breakers for testing. Is the master a solenoid or a mechanical switch?
     
  2. Sep 18, 2017 #22

    Pilotgil

    Pilotgil

    Pilotgil

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    It also would be good to know what wire gauge you have in your system. Battery and starter wires need to be heavy gauge (i.e. bigger gauge than 4 AWG) with terminal lugs properly crimped onto the wire. I like using high strand count welding cable. Most hand crimpers cannot generate the required force to mash the wire and lug together. Harbor Freight sells a very good hydraulic crimper. Wires/lugs should also be properly sealed/treated (Detoxit DN5) and then sealed with a Medium Wall Adhesive Heat Shrink Tubing. While the schematic helps, how the circuit is physically wired is the important factor. Load paths and voltage drops must be considered. Also those darn cheap battery and starter relays should be a preventative maintenance item replaced before they become a high resistance problem. The relays also need a flyback diode to clamp the coil - 1N4006 or 1N4007 - it's very common to have several hundred volts generated when the field collapses. If your buss is dropping significantly, I would check the battery under load (auto parts store), wire/lug/strap conditions and gauge, relays (if they look old, replace them) and if all that fails (unlikely), in racing engine ECUs, we would install a big capacitor right near the ECU. The only way to determine the value is a bit trial and error. A storage digital oscilloscope with good bandwidth and some patience.
     
  3. Oct 9, 2017 #23

    Arthur Brown

    Arthur Brown

    Arthur Brown

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    First check the voltage across the starter battery during cranking. If the battery is down to 6 - 8v under load it's probably past it's best. If the battery is OK then look for volts lost at a connection or component in the system -corrosion is the enemy here! Remove every contact and polish with Brasso or metal polish.

    Jury rig solutions include a two battery system were the starter battery spins the engine and another battery supports all other loads during cranking only, -BUT every complication has to be properly thought through to ensure that it really does as intended, and complies with airworthyness regs.

    Start by making sure that everything that's not needed is turned OFF when you need cranking power - an automotive starter switch turns off lots of things as you turn it to the starter position -lights, audio, heater, demisters etc all go off to save amps for starting.
     

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