Digital external Power Monitoring

Yes this is really interesting. I have a 28V 40A power unit (I built it in 2002 using an off the shelf Lambda switcher, with an added overvoltage protection cct which cuts in at 30V) and one can see the current on that.

The whole aircraft draws 25A in flight, of which the landing and taxi lights are 3A (1.5A each in LED form, 4A each originally), and the pitot is 5A.

Additionally one has a few amps for the flap motor, plus 12A for the landing gear pump.

So if I fitted the backup alternator, which does 20A, I would need to shed a bit of load e.g. all lights and maybe the pitot heater. I know a guy who has a legal one of these on his TIO-550 aircraft and he says it’s brilliant. I will definitely fit one when my engine has to come out.

If your plane draws those currents at 12V (well, 14V) that’s pretty good going, and shows how much less stuff draws than it used to, supposedly.

From above site

Voltage stabilisation +/- 5 %

That is not very good at all.

Also, ask yourself if you want to risk blowing up your whole plane for a €110 chinese power supply. It may of course never happen but I wouldn’t risk it. At that price, you are looking at an absolute rock bottom in quality.

I do not much like that charger – it looks over cheapish generally, and more particularly it seems to produce a constant 13,8 volts – which means it will not charge most batteries fully, or only after a very long while. I think your expensive battery merits something better. Frankly, couldn’t you have a battery similar or even identical to your plane’s, kept in optima forma with a trickle charger in the hangar? Or even a cable with the required plug on the plane side and alligator clips to go on your car battery? I should think it is not impossible to bring your car airside at your home field?

How long does your testing take? If less than a couple of hours, your battery plus your current power supply should do reasonably well, and as long as you avoid deep discharge the battery will be back to good charge overnight. If more than half a day (and that’s a rough indication, of course) I should agree with Peter and judge the risk too great – if that cheapo thing burns a single semiconductor it might well fry your whole avionics stack. Even worse, it could trigger a beginning of damage in some component that will only fail much later. Flaps motor? Trim? Unless you have a good crowbar circuit there, but these seem to be almost universally absent, to my lasting surprise.

PS I do not agree with Peter’s headwagging at the 5 % regulation – even the cheapest of car batteries will take worse abuse with a smile.

Alternator/battery needs to be in good shape to keep all this stuff going.

I see no reason why the capacity of the alternator would deteriorate over time, so the question of the “shape” of the alternator is whether it works or not.

The battery is a different matter.

That €110 power supply is 100% definitely chinese, and low quality too.

To get an idea of how much a good quality switching power supply costs, go e.g. here It’s a few hundred quid just for the chassis mount module, no box etc. I have just paid 600 quid for a 28V 40A one, for a 40A aircraft ground power unit which will be switchable to 14V or 28V, which I am building for a friend of mine. On top of that it will have 2V-over-the-top overvoltage cutout, and various other stuff (digital meters for voltage and current e.g.).

Commercial power units of that power are about 1500 quid, and they are not IP68 sealed like this one will be, and they don’t have the extra overvoltage protection.

If the power supply blows up and pushes out say 50V it will blow up most of your avionics and possibly subtly damage the rest.

The other approach, as mentioned, is a battery. If you go around a few maintenance shops, they will have ones which failed the load test but these are usually perfect for everything else. At work we use the “failed” Concorde ones for UPSs (externally wired up) and they last for ever and are 100x better quality than the crap batteries you get inside even a 1000 quid UPS.

Unfortunately a battery will not run for very long… well under an hour usually, and if you can get 30 mins out of a good one during flight, following an alternator failure, you are doing very well.

I do not agree with Peter’s headwagging at the 5 % regulation

What I was getting at is that it is hard to build a power unit with regulation as bad as that. You really have to work at it It probably has no feedback from the output and instead takes feedback from a winding on the transformer. This is a cunning scheme which avoids optoisolated feedback but (I use it too) works only for a fairly constant load.

Also you can’t charge a lead-acid battery from such a crappy voltage source. 5% is the difference between 14V and 14.7V which is a massive difference in the charging current. A charged battery of good quality (Concorde; not a Gill which has a 1M resistor in series with the + terminal ) might draw 1A at 14V and at a wild guess 10-20A at 14.7V.

It’s a very good point to have electrical load analyse. It becomes more and more mandatory (with upgrades) to make one. It’s a good point in my opinion.

During flight it is very important to get informed quickly on a low voltage situation. I have seen quite often that pilot notice an electrical failure when avionics start to fail. At the point you have lost quite some time and thus capacity. A good low voltage warning based on voltage is IMHO very important.
Most older Cessna’s and Piper for example have a very poor, read useless voltage warning system. On Socata (TB’s at least this system works fine, just like on new Cessna’s). UK CAA has a specific requirement for a voltage controlled warning light for single engine IFR flights.

I see no reason why the capacity of the alternator would deteriorate over time, so the question of the “shape” of the alternator is whether it works or not.

This might seems a logic conclusion. It is not correct though. An alternator can partially fail. For example when one of the stator windings short or go open circuit, other stator windows will keep supplying their part. Therefore the maximum output reduces. The same is true for a situation where one of the rectifier diodes fails, it will produce less output. Without a voltage warning based on voltage you won’t be able to tell when this happens, until avionics switches off due to low voltage.