Get a 200 amp 24 volt alt the kind that fits on an emergcy vehicle, get four tractor trailer batteries 985 Amps each 12 V ,heavy cable I think we used welding cable I forget what size we use but I can check we used a trans cold cooling unit off of a 53 foot reefer trailer to run the alternator 30 horse diesel works great.

Just to chime in here but electric resistance heat requires radically more power than a heat pump (which is what an air conditioner is). Citations don't have electric heat - only an electric heat pump for use in hot weather before the bleed air system (ACM) can take over in cooler temps.

6700 Watts would be a little over 20,000 Btu/hr. However, most vapor cycle systems will have a COP of about 2 so the actual cooling capacity would be over 40,000 Btu/hr. Enough for a small house.

Mike,
I did the same calculations for the batteries, and they should run the ac for several hours.
They should also start the engines if needed.

I assumed 24volts would latch in the aux power Contactor, but will not.
If I add another 6v battery that ups the full charge to almost 32 volts. They are new batteries.
Is that too much voltage?

Lou

That seems weird to me.

Maybe the aux power contactor is getting sticky or isn't quite right. It would be unreasonable to expect it to be that precise, to stop 24 but pass 28 volts, so I don't expect it was designed that way.


Probably if it is tied to the main battery as well, which most GPU plugs are. That will lead to a large amount of current flowing from GPU battery to aircraft battery and possibly overcharge it.

When you do a start on a C414, does the aircraft battery remain attached to the start bus or does it drop off and let the GPU do the start entirely?

In my Citation V, the battery drops out during GPU start. In my MU2, it remained attached to the start bus. So planes vary.

If you aren't going to use it for start, a well charged 24 volt set works well enough to keep from depleting your aircraft battery. Even if used for start, it helps take some of the load off the aircraft battery, but not as much as a 28 volt system would do.

I'd rather be cautious and not over charge the aircraft battery with too high a voltage.

There are ways to take a 30 V battery bank and make it 28 volts, but it requires some circuitry that isn't trivial, but not that complex either.

Mike C.

I am thinking my aux power relay may be getting weak. I think 25.5 volts should close it.
The cessna style 3 prong plug is new. I’ll check to make sure it is making good contact.

The aux power relay energizes the battery bus and then the main bus. Manual says keep the battery relay open, battery is isolated. Everything is energized except battery.

This is probably a dumb question, but have you verified that your battery cart plug has voltage on the small socket. The small pin in the connector on the plane needs voltage to close the ground power relay.

No, I did not check the voltage on small pin. I used a factory made cable with alligator clips on one end and the cesssna 3 pin on other end.
I will check this out today. Thanks for the info.

I too am skeptical that 24V is not tripping the relay. There is nothing to control the voltage level, just the resistance of the coil. I'm also skeptical of running 70A continuous through alligator clips.

The thing I'd be most cautious about is if the power is being hooked up backwards. There is a diode in series with the relay terminal (small third terminal) that will prevent closing the relay if negative voltage is applied. I wouldn't trust anyone's cable without verifying the wiring since the price is possibly frying every piece of electronics in the aircraft.

Yeah, agreed on 70a through alligator clips, presumably near fuel in the wing.....no thanks

I did carefully check the polarity and voltage at the 2 large pins. They are correct.
I have yet to check the voltage at the small pin. My guess is that it is missing, and that is my problem.
As for alligator clips, I considered replacing them with battery terminals. But alligator clips work fine for starting a car when a lot more amps flow, but for a shorter time.
Once I figure out the contactor issue,I can decide.
I really appreciate the help so far

Lou

Generally I find that jump-starting works best by allowing the donor car to charge the sick car's battery, and then it actually starts mostly off the sick but recharged battery. I usually leave them hooked up for a couple minutes and rev the donor car to make sure the alternator is making 14V and putting some juice into the sick battery. Drawing starter current through the cables and the alligator clips is a tall order.

Success!,
I checked voltage at small pin. It is 25.5 volts. I inserted plug into airplane and nothing happened. I then put side pressure on plug and could hear the relay engage and disconnect rapidly.
The 3 pin connector is new, and internals are shiny, but I sprayed contact cleaner on contacts and connection worked!,
I ran air conditioning for about 2 minutes and everything seemed fine.
Next time I will put an amp meter on cable to look at amp draw.

My wife just had an trigeminal neulsgia attack, so I have to go. It is so painful, I should start a thread on that to get more advise.

Lou

A 13 HP air conditioner? Really?

My vapor cycle (aka "freon") system on the V draws about 70 amps at 28 VDC, ~2 KW. I highly suspect yours is similar.

Drawing a load at C/3 (1/3 the AH rating) for a lead acid battery should get reasonably close to nameplate capacity. You aren't going to lose half of it at that moderate draw.

Cables can get warm, but that's actually not that much heat. Say it has 50 watts loss, that's still 97% efficient as far as the cable, and 50 watts into the cable will make it quite warm.

Mike C.