Originally Posted by
dogle
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Have someone check carefully that all connections in the onboard battery cranking circuit are, not just good, but perfect (that includes the contacts in any breaker/solenoid dedicated to that circuit, and is not quite as easy as it sounds).
Here's why: suppose f'rinstance that you have a 28.5 V starting motor which is drawing 50 A at full chat; if somewhere in the circuit there has appeared a tiny spurious resistance of 0.1 Ohm (that's way too small to measure with an ordinary multimeter) in some connection, it will 'drop' five of your available volts and the motor will only be seeing 23.5 V. Because a square law is applicable, that's over 30% of your cranking power gone.
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Good call.
This is a problem for all low voltage systems. It doesn't matter whether it's aircraft, cars, trucks or segways.
On road vehicles, the usual way of checking is to measure the volt drop while cranking. This examines your worst case conditions and gives a useful measurable voltage that means something.
You have to examine both the negative line and the positive line to the starter since both sides of the circuit have resistance that is costing you voltage. Learn where every millivolt is going. (Check the generator lines during high charging demand while you are at this!)
It is typical to find connections and switches costing several hundred millivolts each if they have been regularly exposed to cold and damp. Obviously, that soon adds up to a significant voltage across the entire system. If cleaning connections can drive down volt drops to tens of mV at each one then you are getting somewhere. There may be a volt drop table available for your aircraft.
Where you draw the line between cleaning and replacement depends on what the maker and regulator have to say on the subject.
Wire size and crimp faults are other things to think about. If a section of wiring has been replaced then it is possible that an error has occurred that will increase voltage drop.
Always suspect any earthing braids or wires between units and check volt drops across them.