Following on from the other threads about losing one's electrics etc...
Most SE planes have only one alternator, or they have two but some limitations on what can be powered from what source. The only SE I know of which has a "proper" dual system is the $750k Cessna 400 which has 2 alternators, 2 batteries, and crossover switches.
Yet installing a 2nd alternator is technically easy. There is a number of units which screw onto a vacuum pump mounting point, and most engines have one of these spare. Or they have a vacuum pump which drives only one instrument that actually needs the "non-electrical" redundancy - a horizon - and those can be electric which is a far better solution anyway since vacuum horizons don't last very long, and those which act as pitch/roll source to an autopilot (KI256) are eye wateringly expensive.
B&C do what they call "standby alternators" which are STCd for certain planes. This one can do 20A which is probably enough to light up your whole panel and some - not the pitot heat though.
For planes not on the STC, one can use the contents of the STC (not the STC itself) for a field approval, as this project shows (using an EHSI STC for a 421C to install the EHSI in a TB20).
GAMI have been talking about this 44A unit for a long time. It uses a clever voltage regulator to deliver a stable DC without needing a battery in the circuit. No idea if it is approved in any way though.
A second alternator enables an all-electric plane, with no vacuum pump, which is a much better solution to redundancy.
Under the new EASA-FAA treaty, test data used to obtain an FAA STC is in some way allowable towards an EASA STC, so there may be a way ahead for EASA-reg planes.
those can be electric which is a far better solution anyway since vacuum horizons don't last very long
I couldn't disagree more! All data I have gathered and my personal experience says that vacuum driven gyros tend to be more durable than electric. This might not be true for new electric gyros but the motors used some 20/30 years ago are not very durable. I have inherited a collection of 20/30 year old gyros and had them tested by an avionics shop. All electrical ones were either impossible or very expensive to refurbish and all vacuum ones were either within limits or rather inexpensive to fix.
A second alternator enables an all-electric plane, with no vacuum pump, which is a much better solution to redundancy.
Agree that for new airplanes, there is no reason to have a vacuum system. However, the vast majority of aircraft are old. There I don't see much value in removing the vacuum system. Currently I have one pneumatic instrument left, my old AI for backup. My gut feeling tells me that even the cleverest dual electric system can fail due to the way the two systems are connected whereas the vacuum system has absolutely nothing to do with the rest.
Would a 2nd electrical system make my airplane safer? Yes but not much. In case of an electrical failure, I keep my pneumatic AI and two altimeters. My Aspen will continue to run on its 30 minute internal battery (unless lightning killed everything). A handheld radio with an external VHF antenna will allow me to contact ATC and my yoke mounted Garmin 695 is good enough to perform a VOR/DME approach. Depending on where I am, a military base should be able to give me a GCA approach.
Therefore I'd say there is limited value in adding a 2nd alternator.
My own experience is a 5:1 MTBF difference in favour of electric, and this is across several instruments.
However there are some crap electric horizons out there (in the €1000 price range) because the dirt cheap end of that market is electric.
My Aspen will continue to run on its 30 minute internal battery
30 mins (if the battery is perfect) is not a lot in bad wx or if it happens halfway across the Alps.
And in bad wx, the best option is an ILS. I know there is a handheld radio now which can display the ILS but if I was flying an ILS in conditions that actually need it (OVC002) and a lot of things have gone wrong I would prefer to use the autopilot.
Having to hand fly in IMC, partial panel, degrades a lot of things all at the same time, because cockpit workload goes up some 10x.
The main issue I see however is that dry vac pumps do pack up. Almost none of them make thousands of hours, but most electric gyros, and most alternators (if they have actually been serviced at the Annual) do make thousands of hours. One has a "process" for making electrics last, but there is no "process" for making a vac pump last, or for getting around its infant mortality issue.
There are no vacuum instruments in my plane. Standby AI is electric. There are two vacuum pumps however.
There are no vacuum instruments in my plane.
There are two vacuum pumps however.
I think I can guess the answer to the puzzle but maybe not everybody else can 
Encourages discussion!
how about reducing the amount of draw on the battery. i read on some American forum where a guy had an o-300 powered 172 and hw wanted to upgrade the avionics. due increased current draw he was going to have to replace his alternator but in the end he swapped all his lighting ti led which reduced the current draw sufficiently to allow an upgrade without having to upgrade the alternator.
wouldn't something like this give an increase in time before the battery went flat.
another option would be to swap out the battery for a li po one like i have seen in micolights that way you could have a much higher capacity battery.
Well yes but lower current draw wouldn't create any redundancy. Also you can't just change the battery for anything on a certified aircraft. It needs to be approved.
The last thing one would be doing is flying with all lights on if the alternator failed.
One might want the pitot heat however, especially in the later stages of the descent, and that is the biggest draw - best part of 20A potentially.
In 2007 I changed the Gill battery for a Concorde one, which really does work very much better. Life is similar; mine did 5 years before failing the load test though there was not the slightest evidence of any deteoriation in engine starting, whereas the previous Gill one just suddenly died at the 5 year point and left me stuck in France (where I got ripped off €1500 for a replacement starter motor 
). The battery change is a Minor Mod (N-reg) so just a logbook entry. A lot of people put them in in G-regs too.
Can a LIPO be charged with a standard constant voltage system? The consumer LIPOs certainly cannot. I used to design battery powered products and the charging circuits and lithium cells needed a very different end-charge detection from lead-acid (which basically don't need any).
LED landing/taxi lights are routinely fitted in N-regs as a logbook entry. Same size, etc. But their current draw is much lower which can render defunct cockpit indicators which use current sensing (the TB20 has this issue, though it is marginal).
mine did 5 years before failing the load test
How do you do that load test? My Concorde is ca. 6 years old (the replacement Gill has been sitting on my shelf for some time) and while it appears to be strong, I would like to determine its capacity. Maybe you have a clever setup?
LED landing/taxi lights are routinely fitted in N-regs as a logbook entry.
Installed 2 Alphabeam LED lights after looking at all options on the market. Absolutely fantastic. They are PMA approved so neither my IA nor myself had any concerns. I keep them on all the time when flying although I get routinely asked by the tower to turn them off during approach
