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  #21  
Old 11-15-2018, 05:16 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by Walt View Post
Sounds like you need to rethink that "plan", or have plenty of spare diodes on hand
What would you recommend? Do you see problems / risks, other than letting charge level currents through diode?

I am open to help and suggestions. I had also considered a 30A diode for better margin.

I also have a backup. If the diode blows, I see the b/u bus voltage drop and energize the batt 2 contactor to keep it charged. Then drop the contactor if I lose the main bus or alternator.

Larry
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Last edited by lr172 : 11-15-2018 at 05:28 PM.
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  #22  
Old 11-15-2018, 05:31 PM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by lr172 View Post
I am setting up a 2 batt system (PC680's) in the 10, with the second batt running a b/u bus. I am putting a 20A diode from the main bus to the b/u batt, but also have a second contactor to put batt 2 on the main bus. The plan is that any significant discharged state on batt #2 will require me to remember to have the #2 contactor closed before the alternator starts charging or my diode will blow. The second contactor also lets me use both batt's for starting, if necessary. Larry
I like to see a diagram to understand what you are doing AND how the diode can blow. Here are my thoughts.

1) Reconsider having a dual large battery starter idea, it may not be necessary, adding weight and complexity, with undesirable failure mode. The #2 battery can be much smaller, with the intent to provide a second independant power source for essential in-flight electrical systems.

2) At no time should you be worried about blowing a diode.


1) Two full size batteries to have extra crank power will add weight and complexity. A good battery, starter and engine in good repair, makes starting on one battery routine, minimizing the value of dual batteries. Typically the #2 battery for the E-bus (essential bus) is a small battery, eg 7.5 Ahr (for weight and size) and good to drive the emergency or essential bus for +45 minutes. The whole idea is to isolate EFIS and or Elect Ignition for min flight power until you can get on the ground.

You might consider battery terminals accessible from cockpit or outside fuselage so you can attach a battery charger, without removing a lot of access panels. If you end up with dead battery you can charge it. Another tried and true design (to add battery to start) is an external AUX jack or receptacle (AN2551, MS3506-1) to charge or jump the airplane with dead battery. You can carry one of those Li-ion jump packs. I'm not a fan of jumping a totally dead battery, puts a heavy strain on the alternator. When jumping of make sure polarity is correct, check and double check. Don't trust FBO ramp'er. https://www.aircraftspruce.com/categ...ceptacles.html

2) A Schottky diode as you know is a one way valve. In the Fwd mode you get 0.5 volt drop. In the reverse mode you get 100% voltage drop... Schottky come in ratings to handle +100 of amps. Typically (in my mind) the Schottky diode allows the main bus (alternator) to charge the second battery (E-bus), but it keeps the second battery from being drained by main bus (one way flow). The Schottky diode connecting the #2 battery main bus will see little or no current when starting. As you said you will provide second battery contractor (relay) to parallel #2 battery for starting (IMHO not needed). Relay, switch, heavy gauge wires to do this adds weight, complexity and cost. One way you might overload diode fwd current rating is due to battery charging + bus load. I doubt a battery charging will ever take more than 15 amps. You should not have more than 15 amps load on the E-bus (EFIS, EIS, EI, Com). 30 Amp should be enough, but you should run engine until current drops down before turning off your parallel relay.

Another way to go is use the EFIS and Electronic ignition (CPI2) back up batteries integral to their design... A lot of electrical items now have their own back up scheme...
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Last edited by gmcjetpilot : 11-15-2018 at 06:27 PM.
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  #23  
Old 11-15-2018, 06:08 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
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Default Shottkey diode

I bought one recommended here on VAF. It had huge amperage rating but the attractive part was relatively low voltage drop. If anyone wants details, I will dig up the info.
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  #24  
Old 11-15-2018, 11:05 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Quote:
Originally Posted by gmcjetpilot View Post
I like to see a diagram to understand what you are doing AND how the diode can blow. Here are my thoughts.

1) Reconsider having a dual large battery starter idea, it may not be necessary, adding weight and complexity, with undesirable failure mode. The #2 battery can be much smaller, with the intent to provide a second independant power source for essential in-flight electrical systems.

2) At no time should you be worried about blowing a diode.


1) Two full size batteries to have extra crank power will add weight and complexity. A good battery, starter and engine in good repair, makes starting on one battery routine, minimizing the value of dual batteries. Typically the #2 battery for the E-bus (essential bus) is a small battery, eg 7.5 Ahr (for weight and size) and good to drive the emergency or essential bus for +45 minutes. The whole idea is to isolate EFIS and or Elect Ignition for min flight power until you can get on the ground.

You might consider battery terminals accessible from cockpit or outside fuselage so you can attach a battery charger, without removing a lot of access panels. If you end up with dead battery you can charge it. Another tried and true design (to add battery to start) is an external AUX jack or receptacle (AN2551, MS3506-1) to charge or jump the airplane with dead battery. You can carry one of those Li-ion jump packs. I'm not a fan of jumping a totally dead battery, puts a heavy strain on the alternator. When jumping of make sure polarity is correct, check and double check. Don't trust FBO ramp'er. https://www.aircraftspruce.com/categ...ceptacles.html

2) A Schottky diode as you know is a one way valve. In the Fwd mode you get 0.5 volt drop. In the reverse mode you get 100% voltage drop... Schottky come in ratings to handle +100 of amps. Typically (in my mind) the Schottky diode allows the main bus (alternator) to charge the second battery (E-bus), but it keeps the second battery from being drained by main bus (one way flow). The Schottky diode connecting the #2 battery main bus will see little or no current when starting. As you said you will provide second battery contractor (relay) to parallel #2 battery for starting (IMHO not needed). Relay, switch, heavy gauge wires to do this adds weight, complexity and cost. One way you might overload diode fwd current rating is due to battery charging + bus load. I doubt a battery charging will ever take more than 15 amps. You should not have more than 15 amps load on the E-bus (EFIS, EIS, EI, Com). 30 Amp should be enough, but you should run engine until current drops down before turning off your parallel relay.

Another way to go is use the EFIS and Electronic ignition (CPI2) back up batteries integral to their design... A lot of electrical items now have their own back up scheme...
The 10 probably needs the weight of two 680's. My EFIS and two EI's will draw around 5 amps and I would like the ability to run the GPS and possibly the transponder (I fly a lot of IFR). So my desired emergency load is 8+ amps. A 680 discharging at that rate will only give me 1.5 hours in perfect condition and will get closer to an hour with some age.

The second battery has three connections. 1) The bridge to the main bus with the diode for trickle charge. 2) A contactor to bridge it to the main bus when needed/desired. 3) A feed through a breaker switch (20A) for the backup bus. Each leg of this bus bridges with the main bus feeds just before each breaker that is backed up. The main bus branch feeds to these CB's have diodes to prevent back draining.

My concern with the diode is that if I don't energize the batt 2 contactor and batt 2 is low, it can easily draw more than 20 amps once the alternator starts pumping out juice. I have seen my 680 on the 6 take 40+ amps for a short period after an extended starting effort. I believe that with the contactor closed, it should take the bulk of the load.

I have also considered two diodes in parallel to increase throughput and possible redundancy. However I don't know the downside if any. I also believe that if I blow a diode, I will see the volts drop on the b/u buss and can close the contactor to keep the battery charged.

I have two charging jacks cut into the baggage bulkhead for charging.

Larry
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Last edited by lr172 : 11-15-2018 at 11:10 PM.
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  #25  
Old 11-16-2018, 06:26 PM
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gmcjetpilot gmcjetpilot is offline
 
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Quote:
Originally Posted by lr172 View Post
The 10 probably needs the weight of two 680's. [snip]...

My concern with the diode is that if I don't energize the batt 2 contactor and batt 2 is low, it can easily draw more than 20 amps once the alternator starts pumping out juice. [snip]

Larry
Sounds like you have a plan... Good luck. Have fun. As far as 40 amp draw to charge the #2, that is a lot, even if transient. I believe you, but I'm also of the opinion for a PC680 (18 Ahr) battery, the initial draw will drop off fairly quickly.

I have a spare PC680 that is 50% charged to test. My charger is micro processor controlled so it won't allow current spike. I think attaching to a large fully charged automotive lead acid battery with car running might be best. If the transient is too fast I might miss it. However if it is that fast I wounder if the diode would notice. I think those diodes are good for transient (suppressing spike)? Interesting & fun to think about. The previously mentioned charger would take care of this and moderate charging current.

Go to a bigger diode? That might increase forward voltage drop, which you don't want. Anyway I understand what you are doing now.
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Last edited by gmcjetpilot : 11-16-2018 at 06:33 PM.
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  #26  
Old 11-16-2018, 09:36 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by gmcjetpilot View Post
Sounds like you have a plan... Good luck. Have fun. As far as 40 amp draw to charge the #2, that is a lot, even if transient. I believe you, but I'm also of the opinion for a PC680 (18 Ahr) battery, the initial draw will drop off fairly quickly.

I have a spare PC680 that is 50% charged to test. My charger is micro processor controlled so it won't allow current spike. I think attaching to a large fully charged automotive lead acid battery with car running might be best. If the transient is too fast I might miss it. However if it is that fast I wounder if the diode would notice. I think those diodes are good for transient (suppressing spike)? Interesting & fun to think about. The previously mentioned charger would take care of this and moderate charging current.

Go to a bigger diode? That might increase forward voltage drop, which you don't want. Anyway I understand what you are doing now.
I have seen 40+ amps on the screen for maybe 30-60 seconds. In fairness, my steady state draw is around 15 amps, so the battery was only taking around 30 amps.

Larry
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  #27  
Old 11-16-2018, 10:59 PM
moosepileit moosepileit is offline
 
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Default Best Schottky source, I think

https://www.periheliondesign.com/pow...ttkydiodes.htm
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  #28  
Old 11-17-2018, 07:39 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by lr172 View Post
The second battery has three connections. 1) The bridge to the main bus with the diode for trickle charge. 2) A contactor to bridge it to the main bus when needed/desired. 3) A feed through a breaker switch (20A) for the backup bus. Each leg of this bus bridges with the main bus feeds just before each breaker that is backed up. The main bus branch feeds to these CB's have diodes to prevent back draining.

My concern with the diode is that if I don't energize the batt 2 contactor and batt 2 is low, it can easily draw more than 20 amps once the alternator starts pumping out juice. I have seen my 680 on the 6 take 40+ amps for a short period after an extended starting effort. I believe that with the contactor closed, it should take the bulk of the load.

I have also considered two diodes in parallel to increase throughput and possible redundancy. However I don't know the downside if any. I also believe that if I blow a diode, I will see the volts drop on the b/u buss and can close the contactor to keep the battery charged.

I have two charging jacks cut into the baggage bulkhead for charging.

Larry
This back up battery charging challenge has been around for a long time, a search of the archives will provide much reading material.

Implementing a system with a known failure point if "procedures" are not followed is a bad idea, you're just asking for problems (or anyone else who may operate the aircraft).

Options are switch to a dual battery system, examples from the 'Z' diagrams will help you there, or limit the charge current on the "backup" battery.
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  #29  
Old 11-17-2018, 09:12 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Walt View Post
This back up battery charging challenge has been around for a long time, a search of the archives will provide much reading material.

Implementing a system with a known failure point if "procedures" are not followed is a bad idea, you're just asking for problems (or anyone else who may operate the aircraft).

Options are switch to a dual battery system, examples from the 'Z' diagrams will help you there, or limit the charge current on the "backup" battery.
Thanks for your input. I'll do more searching.

Larry
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  #30  
Old 03-03-2020, 06:53 AM
WingsOnWheels WingsOnWheels is offline
 
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Reviving this thread a bit since I am planning to switch to dual EI in the future and need to come up with a simple system. Battery charging is on issue. I did some searching after reading through this thread and found this charger:

Tekonsha Two Stage 12 V DC Battery Charger. It is 5.5x3.75x1.5". A little larger than what I would like, but more compact than some other options. Listed as 0.25 pounds.



The plan would be to connect this from the main buss. I assume it has internal diodes, so it doesn't back feed from the aux battery.

The ignition system would be fed from the battery buss and Aux battery via diodes. The system would then draw power from whichever has the higher voltage. In a resistor trickle charge system, this would work since the Aux battery voltage would always be lower than the main voltage (with the alternator working). However, I couldn't find a spec on the output voltage for this charger. For all I know it could have a step-up regulator. If it just uses system voltage it will likely be lower. I don't want a manual switch of any kind to change power source.
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