Does this imply that the Cirrus battery is not charged by the external power source?
If so, then the only way you can charge the Cirrus batteries is either by running the engine or removing the cowl and directly connecting to the Cirrus battery, right?
Paul,
No… in most normal situations, the internal battery will successfully power the relay that connects it to the External Power, and once it does, it will happily accept a charge. However, if it’s very nearly (or completely) dead, then it might not be able to “throw” that relay. In that case, the only way to get going again would be to remove the cowl, as you say.
In practice, if the internal battery is drained from, say, an unsuccessful engine starting session, then waiting 10 minutes or so before connecting the outside source might give a successful result - lead-acid batteries rebound somewhat after heavy discharge. On the other hand, if the internal battery is dead because last week, when you put the airplane away, you left the Master Battery switch on, then it’s not likely to have any “rebound” no matter how long you wait.
By the way, that internal relay is wired through the Battery Master switch, too… so that switch must be ON for external power to work at all. In the Dead/Nearly-Dead Battery scenario, it makes sense to pull all the breakers you don’t need before you switch on the Master, so minimize unnecessary load, and maximize the chances of success.
All of the above is for the SR20 - I don’t know the situation on the SR22.
Mike,
Is your concern the 28 Volts? From posts back in September, it sounded like you and others were using either one 24 Volt or two 12 Volt lead acid batteries in series connected directly into the ground power receptacle.
Lou,
My setup has been a current- and voltage-regulated external supply, that can deliver 28 Volts at up to 25 amps. In itself, there’s nothing wrong with 28 volts as a source. And from a voltage-regulation point of view, nothing beats batteries - they simply CAN’T provide too much voltage. The idea of having 28 volts of lead-acid batteries as a supply is going to be just fine MOST of the time, but in certain circumstances, it can be bad.
Let’s say you’ve just drained your primary battery following an unsuccessful engine-starting session. You now have an on-board battery that is very “hungry”, and your external 28 Volt supply is eager to “feed” it. Lead-acid batteries will provide huge currents (100’s of amps) if you let them, because of their very low internal resistance. The schematic in the POH (which may not be 100% accurate) shows no limiting resistor or circuit of any sort between the battery and the External Power Receptacle, so an 28V external battery souce will want to provide more current than is desirable. In practice, the actual wiring between the external battery and the internal battery will provide some limiting resistance - and it will get hot in the process of limiting the current. In the extreme cases, if your external wiring isn’t of a heavy enough gauge, it will melt; and if it’s of a sufficiently heavy gauge, it will survive but the battery might not (plates might buckle).
There’s a really easy and effective insurance policy that I would recommend if you’re using the external 28V lead-acid battery: Put a heavy fuse (e.g. 30 amp) in series with the external power plug. For normal use (like preventing the primary battery from running down while you work on the avionics), 30 amps is more than enough; but the fuse will protect both internal and external batteries if for any reason the current wants to run away. And use heavy wire, so that you CAN get the full 30 amps when you need it, without the danger of overheated wires.
I used 4 gage wire from the batteries to the plug which should be plenty to handle 30 amps. However, the red lexan plug from Aircraft Spruce requires some faily tight bends in the wire so that it will fit the grooves inside the plug. The bends are not easy with 4 gage. Anyone trying this will probably find it easier to bend the wire before soldering the terminals.
Have used the upgraded 24v Aero-Tow E-200 for 2 weeks. Key feature was the variable speed control, which starts off pretty slowly, accelerates smoothly and has a reasonably moderate top speed. With 2 Cirrus SR2x in a 46-foot hangar, we do way too much jockeying of planes, but at least we have hangar space in San Diego!
So the maneuverability of the Aero-Tow is getting a workout. Lots of power to move an SR22. Lots of grip, even with blown sand on the hangar apron. Good maneuverability. Getting Cirrus tow-bar on/off is reasonable (added a bungee cord to spring-load the 2 arms).
Steering backwards and up a slight incline into the hangar is fabulous. Gentle turns are easy; occasional need for sharper turns requires lifting tug handle and pivoting whole thing. Controlling my tail position is critical so the variable speed (really, an initial slow speed) really works. Still have to compensate for the tail swing when the nose wheel is turned, as the nose wheel pivot point is off center from the wheel contact point. And pulling plane out is a piece of cake.
The variable-speed Aero-Tow definitely makes it possible to jockey an SR22 in tight quarters with just 1 person.
The Aero-Tow E-200 makes it easy to steer the SR22 backwards. For gentle turns, I keep both wheels on the ground. For sharper turns, I lean the E-200 so only one wheel is on the ground, and for really sharp turns I lift the E-200 and use it to turn the nose wheel in place. I have an asphalt ramp that rises 8 inches in 10 feet at the entrance to the hangar. I think that the E-200 is close to its limit in power and traction in pushing the SR22 bckwards up this ramp. In other words, the E-200 has suffcient power for my purpose but I doubt that it could handle a steeper grade.