A 12V battery, freshly charged, will be around 12.5V. This voltage is also temperature dependent. When it is on a charger, unless it is a very slow (trickle) charger, it will be 14V.
How much RPM an alternator needs to produce 14V depends on the system design. On the TB20, it is 1100-1200 RPM. There are alternators one could fit which achieve it at 900-1000 RPM.
chflyer wrote:
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).
The wiring and switches downstream of the voltmeter on the main bus to the voltmeter on the avionics bus all have a small but definite resistance. Looking at a load analysis from Garmin I found online, the avionics bus on a 24v system can be drawing as much as 17 amps. A drop of 0.3v would suggest the resistance of the wiring and switches etc. between the two voltmeters would be around 18 milliohms, which sounds reasonable.
UdoR wrote:
Could a Voltage of 12.7 Volts be too low for it?
Assuming you would like the PFD to continue working if the alternator fails, then a voltage from 12.8 fully charged to 11 volts is the range with no load (11volts is just about dead and probably damaged). Its about 2.1 volts per cell with lead acid which can very with temperature for fully charged. Certainly a lead acid battery at 12Volts with a significant load could still have a considerable capacity.
