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  #11  
Old 09-25-2019, 05:58 PM
lon@carolon.net lon@carolon.net is offline
 
Join Date: Aug 2015
Location: Santa Monica, California
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Thanks for all the suggestions. A friend gave me a 12-volt battery that's identical to the one in my plane. When I'm using the avionics with the engine off, I attach that battery to my plane's battery, positive terminal to positive terminal and negative terminal to negative terminal.

It seems to work. I haven't damaged or depleted my plane's battery. But I don't understand why it works or what's happening.

My plane's ammeter shows that my avionics are drawing 7 amps (-7). But I don't understand where those amps are coming from. Are the avionics drawing power from the plane's internal battery, with the external battery feeding power to the internal battery? Or are the avionics drawing power from both batteries simultaneously?

I began thinking about this because I wondered why the external battery was feeding the plane's battery, rather than the plane's battery feeding the external battery.

I have attempted to answer this question with online research, but haven't been successful. Most of what I've come up with are explanations of how to jump-start a car with an external battery.

I'm not expecting anyone to write an article about electricity (though I'd happily read one). But a link to an online explanation of what's happening when I connect an external battery to my plane's battery would be thankfully welcomed.
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  #12  
Old 09-25-2019, 06:10 PM
BobTurner BobTurner is offline
 
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Location: Livermore, CA
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Assuming the two batteries are in a similar charge state, they will share the load, 50-50. What you should do: Remove the negative lead from the aircraft battery. Jumper the portable battery, positive to positive, and negative to a good ground (if convenient you can use the negative battery lead you disconnected). This way only the portable battery will supply the power for testing.
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  #13  
Old 09-25-2019, 06:55 PM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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Connect the spare battery to the load side of the master contactor and leave the master switch off. That will eliminate 3/4 of an amp that the battery contactor draws.
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  #14  
Old 09-25-2019, 07:10 PM
NinerBikes NinerBikes is offline
 
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Location: Granada Hills
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Quote:
Originally Posted by Mich48041 View Post
Connect the spare battery to the load side of the master contactor and leave the master switch off. That will eliminate 3/4 of an amp that the battery contactor draws.
Is this 3/4 amp draw only when the master switch is turned on, that the master contactor is pulling current, or is there also a phantom load from the master switch when it's turned off?

Trying to help a friend trouble shoot his battery draw down problems.
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  #15  
Old 09-25-2019, 09:19 PM
Mich48041 Mich48041 is offline
 
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The battery contactor uses about 3/4 amp more or less. If you touch a contactor after it has been on for a while, it will be very warm, maybe hot. No, there is no phantom load when the master switch is off. If a phantom load is suspected of running the battery down, here is a test: With master switch off, disconnect one of the battery leads (negative is safer), and put an ammeter in series between the battery post and disconnected lead. If there is no current with the meter set to amp range, set it to read milliamps. No current should be flowing with the master switch turned off unless something is connected directly to the battery.
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  #16  
Old 09-26-2019, 01:13 PM
lon@carolon.net lon@carolon.net is offline
 
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Location: Santa Monica, California
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Quote:
Originally Posted by BobTurner View Post
Assuming the two batteries are in a similar charge state, they will share the load, 50-50. What you should do: Remove the negative lead from the aircraft battery. Jumper the portable battery, positive to positive, and negative to a good ground (if convenient you can use the negative battery lead you disconnected). This way only the portable battery will supply the power for testing.
The battery in my -12 is difficult to get to, so the builder added a quick connect plug that is easy to reach through the oil door. I connect the external battery using a cable that has a compatible quick connect plug, so doing what you suggest would be difficult if not impossible.

However, I have two identical external batteries -- identical to one another and to the battery in my plane. If, as you say, one external battery and the plane's internal battery share the load 50/50, would adding another external battery (attached to the first external, positive-to-positive, and negative-to-negative) mean that the three batteries would share the load 33/33/33? Or would adding a second external battery overload something?
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  #17  
Old 09-26-2019, 02:06 PM
Canadian_JOY Canadian_JOY is offline
 
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Location: Ontario, Canada
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Lon - you're likely over-thinking this and driving yourself into unfounded fears...

It doesn't matter how many batteries you have tied together outside the airplane, as long as they are all of nearly equal quality and state of charge. They need to be connected in parallel (+ of Batt1 to + of Batt 2, - of Batt 1 to - of Batt 2). You could have a hundred batteries connected like this and it would have no difference on the effect on your airplane.

Paralleling batteries just increases the total amount of stored energy available to be delivered to the load. The more energy available, the more slowly the voltage of the storage batteries will drop as their stored energy is consumed.

Think of it this way... Picture each battery to be a 5 gallon jerry can of gas. Connect a single jerry can to the airplane and the motor will run an hour. Connect two cans and the motor will run two hours. Now picture a long line of jerry can turned upside down with their spouts tied together in a big collector. If you've got 20 jerry cans, that's a hundred gallons of capacity. Each will flow at the same rate, so each will deplete at the same rate, 1/20th of the consumption rate of the engine. The engine will run for 20 hours. All jerry cans will empty out at very nearly the same time.

This analogy is a good way of visualizing parallel batteries.

If you're really concerned about doing this, just splash out the cash to purchase a 12V power supply (not a battery charger) and make a way to connect it to the aircraft bus without having to involve the master switch. (Remember to put a fuse in line with the supply so you have over-current protection.) A power supply will take 115Vac and convert it to nice clean 12V DC. OK what you're really looking for is something like 13.75V or so, but most power supplies put out something higher than exactly 12V.
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  #18  
Old 09-26-2019, 02:59 PM
NinerBikes NinerBikes is offline
 
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Get a switching Power Supply. I run my very expensive Elecraft K3S ham radio off of one of these Mega Watt switching power supplies. You will need to get some 10 or 8 gauge wire to handle up to 30 amps.

If you want to use it as a power supply for your battery, set the voltage up for 13.6V and connect to your plane battery, with something like Anderson Power Pole connectors.

If you want it to recharge your battery, set it to 14.4V for recharging and remove from battery when your recharge take rate reaches 0.5% of battery capacity in milliamps 16 amp at 1/2 a percent is 80 milliamp charge rate. A RC Wattmeter in line in the recharging or power supply circuit, is a very helpful device in recharging expensive plane and LiPO RC type batteries.


Always measure the voltage with the power supply turned on and not connected to anything with a DMM.

These guys are local to you, the power supply is a very good, noise free model.

http://www.megawattpowersupplies.com...ick_33_Amp.jpg

Last edited by NinerBikes : 09-26-2019 at 03:02 PM.
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  #19  
Old 09-26-2019, 03:24 PM
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Piper J3 Piper J3 is offline
 
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See post #1, the guy's plane is located where there is no electric power available...
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  #20  
Old 09-26-2019, 04:23 PM
lon@carolon.net lon@carolon.net is offline
 
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Canadian Joy: You're right. I am overthinking this, for two reasons. This is the first plane I've ever owned. (I rented for years.) And I'm like a first-time parent -- afraid to do anything that might hurt the baby. The other reason is that years ago, when I was just a boy, I used to know stuff about electricity. (I built transistor radios when transistors were new gadgets and just beginning to replace vacuum tubes.) Now I'm having trouble remembering what I used to know back then, and that makes me nervous. I have taken off the shelf my copy of Rod Machado's Private Pilot Handbook, and am reviewing the chapter on Electrical Systems. Thank you, a lot, for your gas can analogy. It did help me, and has calmed me down.
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