More electrical woes
08-13-2006, 07:56 PM
#16
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Alan, I have to disagree somewhat with the first para. I've seen quite a difference from various points within the B+ circuit on the 928. As I mentionied, the voltmeter can read 12.5 and the battery is charging at well over 13 with 2100RPM. I certainly don't have an answer, but the voltmeter is clearly wrong. I guess if you consider 1.2 volts close to reality, then I would accept that.
What bugs me is that it definantly increases as I drive, while the batt recovers from the starting load. I haven't seen that on any other car, just as our first poster mentioned. Which leads me to believe that the volmeter is quite close to the car load, and not near the well sunk battery. That means a lot of line loss from the load area near the front of the car, vs the battery at the back.
If I remember my HS physics correctly, a 35% solution of H2SO4 emulsed in Pb/PbO2 will yield about 12.8V with clean plates. Typical charging is going to be always above 13.2 volts to overcome the electrolytic process of the plate diffusion. Over about 14 volts seems to warp plates, particulary on narrow plate batts. The batt spec for the 928 is quite robust for short duration, large discharge rates, so it doesn't surprise me that the voltage is up to 13.8 regulated.
I still wish I had an ammeter. It's required equip in all aircraft, and is sure nice to just glance down and see that the meter is centered, and not waiting for the voltmeter to begin to sag if the charging system fails. The ammeter gives instant gratification when things bad happen.
Doc
What bugs me is that it definantly increases as I drive, while the batt recovers from the starting load. I haven't seen that on any other car, just as our first poster mentioned. Which leads me to believe that the volmeter is quite close to the car load, and not near the well sunk battery. That means a lot of line loss from the load area near the front of the car, vs the battery at the back.
If I remember my HS physics correctly, a 35% solution of H2SO4 emulsed in Pb/PbO2 will yield about 12.8V with clean plates. Typical charging is going to be always above 13.2 volts to overcome the electrolytic process of the plate diffusion. Over about 14 volts seems to warp plates, particulary on narrow plate batts. The batt spec for the 928 is quite robust for short duration, large discharge rates, so it doesn't surprise me that the voltage is up to 13.8 regulated.
I still wish I had an ammeter. It's required equip in all aircraft, and is sure nice to just glance down and see that the meter is centered, and not waiting for the voltmeter to begin to sag if the charging system fails. The ammeter gives instant gratification when things bad happen.
Doc
08-13-2006, 10:23 PM
#17
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Doc - re-read what I posted...
Agreed there is an absolute difference with many dashboard voltmeters - maybe on some of 1.2v as you say... but if you subsequently see a change of a volt on the voltmeter - probably a change of about a volt actually happened on the car supply (so I'm saying its relatively correct).
Using a calibrated voltmeter there should be very little difference between the battery and alternator since they should connect by the longish 35mm^2 (~1AWG) wire from the starter to the battery (~10') and a shorter 16mm^2 (~6AWG) from the starter to the alternator, (say ~3').
Even assuming rapid initial charging @ 50A we'd still only see a voltage drop from the alternator to the battery of around 1/10th of a volt.
(35mm^2 wiring has a resistance of approx 0.41mOhms/meter
and 16mm^2 wiring has a resistance of approx 1.3mOhms/meter
so approx 50 x 0.00041 * ~3 + 50 x 0.0013 = 0.13v)
Almost all the non-charging loads on the system sit at the alternator end and are in fact supplied in parallel to the Alternator -> Starter -> Battery connections via the CE panel.
The dash voltmeter is located on this seperate path from CE so likely is also influenced independently by shared loads on the CE routed consumption side rather than the battery charging current. The CE supply is via 16mm^2 wires from the alternator then splitting at the jump post to 2x 10mm^2 wires to the CE panel top. It is not the ideal configuration to see the alternator output voltage.
I do agree that a battery ammeter would be a nice addition. Many modern high volume cars have much more sophisticated battery monitoring that can alert you to battery issues before a catastrophic failure occurs... nice to have!
Alan
Agreed there is an absolute difference with many dashboard voltmeters - maybe on some of 1.2v as you say... but if you subsequently see a change of a volt on the voltmeter - probably a change of about a volt actually happened on the car supply (so I'm saying its relatively correct).
Using a calibrated voltmeter there should be very little difference between the battery and alternator since they should connect by the longish 35mm^2 (~1AWG) wire from the starter to the battery (~10') and a shorter 16mm^2 (~6AWG) from the starter to the alternator, (say ~3').
Even assuming rapid initial charging @ 50A we'd still only see a voltage drop from the alternator to the battery of around 1/10th of a volt.
(35mm^2 wiring has a resistance of approx 0.41mOhms/meter
and 16mm^2 wiring has a resistance of approx 1.3mOhms/meter
so approx 50 x 0.00041 * ~3 + 50 x 0.0013 = 0.13v)
Almost all the non-charging loads on the system sit at the alternator end and are in fact supplied in parallel to the Alternator -> Starter -> Battery connections via the CE panel.
The dash voltmeter is located on this seperate path from CE so likely is also influenced independently by shared loads on the CE routed consumption side rather than the battery charging current. The CE supply is via 16mm^2 wires from the alternator then splitting at the jump post to 2x 10mm^2 wires to the CE panel top. It is not the ideal configuration to see the alternator output voltage.
I do agree that a battery ammeter would be a nice addition. Many modern high volume cars have much more sophisticated battery monitoring that can alert you to battery issues before a catastrophic failure occurs... nice to have!
Alan