# External Power

I have a 24 vojlt Aero-tow (tug for moving my 22 in and out of the hangar), and keep the batteries charged with a solar panel and controller since the hangar does not have power. This works fine. Following the advice of this forum, I have connected the 24 volt battery to a plug that fits the 22’s external power socket. When I connect this external power source and turn bat 1 and avionics on, the Garmins, Sandel etc turn on as expected. I’d like to be able to power the instruments on the ground using only the external power source. Page 7-41 of the POH shows the external power source and bat 1 in parallel. I don’t fully understand what happens in this situation but I assume that if both batteries have the same voltage they both contribute to the load. Hence, bat 1 is discharged at half the rate if external power is connected - not what I had in mind. Raising the voltage of the external power source by a couple of volts would probably spare bat 1 completely but I have no practical way of doing this with the Aero-Tow battery. Any suggestions?

Roger Buchanan

I have a 24 vojlt Aero-tow (tug for moving my 22 in and out of the hangar), and keep the batteries charged with a solar panel and controller since the hangar does not have power. This works fine. Following the advice of this forum, I have connected the 24 volt battery to a plug that fits the 22’s external power socket. When I connect this external power source and turn bat 1 and avionics on, the Garmins, Sandel etc turn on as expected. I’d like to be able to power the instruments on the ground using only the external power source. Page 7-41 of the POH shows the external power source and bat 1 in parallel. I don’t fully understand what happens in this situation but I assume that if both batteries have the same voltage they both contribute to the load. Hence, bat 1 is discharged at half the rate if external power is connected - not what I had in mind. Raising the voltage of the external power source by a couple of volts would probably spare bat 1 completely but I have no practical way of doing this with the Aero-Tow battery. Any suggestions?

Roger,

You’re quite correct - doing it the way you describe, you would reduce the contribution of Bat 1. The reduction isn’t necessarily 50% - but that’s a whole other can of worms.

There is a relatively straightforward way for you to raise the voltage; it isn’t pretty, but it will work. The PRETTY way would be to find yourself a DC-DC converter that will take say 18-30 volts in, and provide you with say 27 volts (a good number for this) at the output. They do exist, but for a couple of reasons, I suggest you go with this ugly method:

Get a power inverter – the kind that will give you 110 VAC from your DC source. [They are a dime-a-dozen for 12V input, so hopefully you can find a 12V point to “tap” from on your Aero-tow; otherwise you’ll have to find one that takes 24V]. From that point on, you can run an AC powered 24V charger into your airplane. Of course, you’ll need to be sure that your inverter can handle the charger you’re using; but unless you go for a monster charger, that will not be difficult.

Even though Rube Goldberg might want a personal visit to see the creation, it will have the distinct advantage of giving you 110 VAC in your hangar for those times you might want it for other purposes.

Mike.

Roger, Be aware that your hobbs will run as soon as the Batt switch is turned on.

1. i should have waited for the 24 volt version
2. you can buy a 4 volt battery and attach it with a short wire and simple battery jumper clips…this will bring the load to 28 and it will pull from external power
3. if you are going to ‘do avionics’ for some time…take 5 minutes, remove the top cowling and disconnect the ‘ground terminal’ on the main bat.
4. or leave them hooked up as you have for an hour or two an you’ll still have plenty to start your plane with both connected…then fly it for 15 minutes or so and your fully charged.
I have used a combination of 2 thru 4 depending on my intentions.
Hope this helps,
Don rennie

Just so I don’t kick myself around the block for buying the 12 volt model 8 months ago, how expensive is the 24 volt? (For your sake, hope it was economical. For my sanity, hope I wouldn’t have sprung for it if it were available when I bought!)

Thanks

Andy

All of the power tugs reported on this site talk about powertow.com which only sells 2 electric tows which are both 12 volts only. So where did you get yours?
Brain

I believe if you pull the Engine Instruments breaker, that will cut out the hobbs. Not recommended for in-flight use!

-Curt

I can’t check this right now, but my understanding is that the hobbs is wired to the alternator output, and only counts when the alternator is producing voltage, which it won’t do if the engine is not turning.

Where can you get a four volt battery? I currently have two 12 Volt auto batteries in series for ground power and would like to get up to 28.

Be very careful, gents. Although it’s true that a charging system typically provides 28 volts, the current is regulated. This is true whether it’s the engine alternator or an external charger. However, simply having 28 Volts worth of Lead Acid batteries on the outside of the airplane hooked up to your SR2X is not likely to be a good thing, because with the very low internal resistance of lead acid batteries, you may have a VERY high current flow that is damaging to (at least) your on-board battery. I’ll have to go back and check the schematics, but I don’t believe that there’s anything in the airplane to limit the current, and unless you find there is, I’d stay away from this method.

Mike.

1. you can geta 4 volt batteryh at most 'battery warehouses
2. Cirrus reccommended the 12/12/4 combo for 28.
don r

Post deleted by Bill_Dobson

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.

I can’t check this right now, but my understanding is that the hobbs is wired to the alternator output, and only counts when the alternator is producing voltage, which it won’t do if the engine is not turning.

Clyde,

I was told the same thing.

Mike.

In the SR22, the Hobbs records time when BAT1 and either ALT1 or ALT2 are on. Ref page 7-56 of the POH in the training manual. That should allow working on the any of avionics with just the BAT1 on, or using just BAT2 for programming GPS1, without the Hobbs cranking up the hours.

Don,

Is the 4V battery you use a Lead-Acid battery?

Mike.

I’d have to ask a Cirrus mechanic, but the circuit schematic in my POH seems to imply that there is a relay connected to the gound power connector which takes the battery out of the “loop” when proper external power is applied; anyone know if this is the case?

Bill,

This much I do know for sure… in an SR20 there is in fact such a relay. This means that if the battery is stone dead (or close enough), even external power won’t help - the relay you refer to needs sufficient power to turn on before the external power can do any good.

Mike.

http://members.aol.com/aerotow/....aero-tow makes the 24volt.

Mike,

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?

Do you know the cost of the aerotow E-200?
Brian