I’m looking for input. What should be a “typical” voltage level that you measure in the avionics panel after start with engine idling? My system has one battery, it’s a Concorde AGM, so it’s a “12V” system, but any input from a 24V system is welcomed, too. I’ve read that open circuit voltage “full” should be around 12.9 Volts, that correct?
Of course, when RPM is increased the regulator “takes over” and you get quite quickly to the voltage that is output by the alternator. But if the engine is left in idle, the typical alternator will not deliver much, so the voltage indicated in the panel will drop to the battery voltage.
I see 12.7 to 12.8 Volts after start, engine idle, instruments on, and am asking myself whether this is o.k. or not.
UdoR wrote:
I see 12.7 to 12.8 Volts after start, engine idle, instruments on, and am asking myself whether this is o.k. or not.
Before getting into the 0.1V differences I would measure the bus with a voltmeter. Those instruments are not very precise. Ours has got a very basic adjustment screw so better make sure it is adjusted properly.
@Sebastian_G thank you for your reply!
It’s the voltage electronically indicated in my PFD, and that’s what I am into at the moment. Could a Voltage of 12.7 Volts be too low for it? That’s what I am chasing. I don’t think that an electronics device from 2020ies will be far off, like an old analog gauge, but yes, maybe I’ll double-check this with the voltmeter.
The measurement will of course vary depending on where it is taken. I have a JPI EDM on my avionics bus and an EI VA-1A which is on the main/master bus. There is a difference of about 0.3-0.4V (24V system) between the two. I’ve queried my avionics shop on this, and they said it varies a bit but that difference is about right, although I’m not entirely clear why (beyond due to separate buses in series).
Here is a excerpt from the Concorde RG CMM (p 1501). While it relates to storage, it does give some insight into what should be expected from a fully charged battery, particularly points C and D. Keep in mind that there may be parasite loads when measuring a battery connected to the aircraft circuit (e.g. clock).
1. StorageConditions
A. Batteries are serviced and charged at the factory prior to shipment.
B. To minimize self-discharge, batteries should be stored in a cool location,
ideally below 20EC (68EF).
C. The open circuit voltage (OCV) of a fully charged battery is approximately
26.0 volts (13.0 volts for a12-volt battery). As the battery state of charge drops
due to self-discharge, its OCV also declines.
D. Batteries should be boost charged per Section 6 when the OCV declines to
25.0 volts (12.5 volts for a 12-volt battery).
E. Batteries with an OCV below 25.0 volts (12.5 volts for a 12-volt battery) must
be capacity tested per Section 7 before being placed in service.
F. Batteries in storage for longer than 24 months must be capacity tested per
Section 7 before being placed in service.
Thank you @chflyer for your reply.
I see I have to ask more precisely.
If the voltage with engine running in idle drops to 12.7 Volts, does this mean I have a broken alternator regulator? Or a battery problem?
In normal operation (e.g. cruise) I see more than 14 Volts and charging and I don’t have any issues with my electrical system, despite that I need to know whether the Voltage is normal in idle engine operation…
If the voltage with engine running in idle drops to 12.7 Volts, does this mean I have a broken alternator regulator?
No, I don’t think there’s a problem. The VR will limit the exciting current to about four amps and that’s probably not enough for more than 12.7V at idle. If you see 14V at say 1700 RPM or more you’re good.
What’s the voltage of the battery while nothing is on? It should be about 12.7 volts, and with the engine running and an alternator on, it should about 14.3 volts.
Thanks @terbang and @eurogaguest1980 that’s what I do think, too.
PFD manufacturer tells me that my idle voltage was too low (read: for the PFD to operate well) and I do not agree.
Assuming the battery is OK, it generally doesn’t take much increase in idle speed to increase bus voltage when excitation current is maxed-out at idle.
BTW if the PFD is that sensitive to voltage well above 12 V it sounds like a poor PFD design feature. I suspect that this is not the case.
