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  #1  
Old 09-12-2016, 09:10 AM
bigwheel bigwheel is offline
 
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Default Single Alt Dual Battery System Design

Hi everybody, new guy here. Been lurking on VAF for a few years after finding lots of the information I was seeking could be found here. Up till this point I've been able to find everything I've been looking for in existing threads. Unfortunately this time I have a specific question and I couldn't find a existing thread I felt it was appropriate to tag on to.

Hoping you guys will take a look at my power supply schematic and tell me its flaws. I have Bob Nuckolls book, which is full of great information, and I have read the pertinent parts several times. Because my plane will be fairly basic I want a simple yet robust electric system. So please, tell me what you think.



I should mention: This is a VFR only plane with a GRT Sport EX, airspeed and altimeter steam gauges.

Last edited by bigwheel : 09-12-2016 at 09:31 AM.
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  #2  
Old 09-12-2016, 09:33 AM
Mike S's Avatar
Mike S Mike S is offline
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Default Welcome to VAF!

Quote:
Originally Posted by bigwheel View Post
Hi everybody, new guy here.
Jon, welcome aboard the good ship VAF
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Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #3  
Old 09-12-2016, 10:25 AM
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Carl Froehlich Carl Froehlich is online now
 
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John,

Some thoughts:
- Your use of two master solenoids and two batteries eliminates one "single failure takes out the world" risk. I don't know what your "voltage sensing relay" is supposed to do but if you are thinking of an automatic way to isolate a fault, this might not be it.
- Now that you have two master solenoids, you have twice the dead weight of the current draw from these solenoids. Some of these draw 3 amp to remain closed. So at up to 6 amps of current draw, you have created a load larger than most panel items. There is no way to avoid using two master solenoids (and still retain the needed robustness of design) as you need the engine start current capacity. What you don't need is these solenoids sucking up current to just provide power to you panel when the alternator dies. You need smaller, 30 amp or so relays to feed your panel (typical current draw to hold shut is 100 ma). That why when the alternator dies you open both master solenoids to drop that load. If you also power the typical loads that you would dump for a loss of alternator (landing lights, strobes, boost pump, pitot heat, etc.) from the output of the master solenoids, when you open the solenoids when the alternator dies you drop the load to what you really want - so you enhance your electrical reserve. If you need one of these loads (like boost pump for landing) you just shut one of the master solenoids when you want the load.
- Once you decide to have these independent feeds to your panel, you can now enhance your redundancy by splitting your panel so that half runs on each battery (e.g. Comm#1 and Nav #1 on #1 battery, Comm #2 and Nav #2 running on #2 battery. This way if you have a battery fault you can feed the panel via the other battery.

If you send me your email I'll send you what I did.

Carl
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  #4  
Old 09-12-2016, 10:57 AM
bigwheel bigwheel is offline
 
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Quote:
Originally Posted by Mike S View Post
Jon, welcome aboard the good ship VAF
Thanks Mike!

Quote:
Originally Posted by Carl Froehlich View Post
John,

Some thoughts:
- Your use of two master solenoids and two batteries eliminates one "single failure takes out the world" risk. I don't know what your "voltage sensing relay" is supposed to do but if you are thinking of an automatic way to isolate a fault, this might not be it.
- Now that you have two master solenoids, you have twice the dead weight of the current draw from these solenoids. Some of these draw 3 amp to remain closed. So at up to 6 amps of current draw, you have created a load larger than most panel items. There is no way to avoid using two master solenoids (and still retain the needed robustness of design) as you need the engine start current capacity. What you don't need is these solenoids sucking up current to just provide power to you panel when the alternator dies. You need smaller, 30 amp or so relays to feed your panel (typical current draw to hold shut is 100 ma). That why when the alternator dies you open both master solenoids to drop that load. If you also power the typical loads that you would dump for a loss of alternator (landing lights, strobes, boost pump, pitot heat, etc.) from the output of the master solenoids, when you open the solenoids when the alternator dies you drop the load to what you really want - so you enhance your electrical reserve. If you need one of these loads (like boost pump for landing) you just shut one of the master solenoids when you want the load.
- Once you decide to have these independent feeds to your panel, you can now enhance your redundancy by splitting your panel so that half runs on each battery (e.g. Comm#1 and Nav #1 on #1 battery, Comm #2 and Nav #2 running on #2 battery. This way if you have a battery fault you can feed the panel via the other battery.

If you send me your email I'll send you what I did.

Carl
The VSR is used to ensure batt1 is fully charged before allowing the alternator to begin charging batt2. The the engine is shutoff the batteries are again isolated. Here is an example of a VSR

I hadn't considered the draw of the contractors. I looked up some and found that there are continuous duty contractors available that draw 0.7A. That's still nothing to shrug off.

Points well taken. Thanks Carl.
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  #5  
Old 09-12-2016, 11:13 AM
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az_gila az_gila is online now
 
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I read the diagram differently.

If only one battery is connected to the Power Bus at a time then the current drawn by the contactors is identical to the usual one alt./one batt. system.

If your EMS can monitor two voltages simultaneously then it would be a good electric source backup with an easy indication of its status.
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  #6  
Old 09-12-2016, 11:28 AM
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As shown, battery 1 seems to be used for starting the engine. If batt 2 is also used as an alternate start battery, the wiring to and from the power buss has to carry the starter current load------as does the power buss-----which I suspect is not how you intend to do things. If you want to use battery 2 as an alternate power source for the starter, you might consider adding the needed wiring.

If batt 2 is only there for backup to the electrical load of the panel, and not to be a backup to the starter, then IMHO you do not need a second contactor. Just a switch will do the job just fine.

This is how I wired my plane, the backup battery is run through a DPST switch which is rated to carry the need load on each set of contacts, but I paralleled them for extra robustness, and in case one set of contacts failed.

Do you have a low voltage alarm to let you know to turn on the backup battery if the main battery goes TU??? Or, do you plan to run both batteries at the same time, and let the one with the highest power do the work?? If the latter, consider putting in blocking diodes so the good battery will not back feed to the weak battery.

Also, when using a VSR, and having the main battery go way low in voltage so that you have to engage the backup battery----------will the backup battery be taken off the charge due to the VSR sending all the current to the bad battery?? If so, you are running on borrowed time with the backup battery as it is now a total loss system.
__________________
Mike Starkey
VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."

Last edited by Mike S : 09-12-2016 at 11:32 AM.
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  #7  
Old 09-12-2016, 11:59 AM
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Carl Froehlich Carl Froehlich is online now
 
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[quote=bigwheel;1110895]Thanks Mike!



The VSR is used to ensure batt1 is fully charged before allowing the alternator to begin charging batt2. The the engine is shutoff the batteries are again isolated. Here is an example of a VSR

/QUOTE]

I offer the VSR is not required or desired. This adds an unnecessary failure point. Use two identical batteries (e.g. PC-625) and have them in parallel for normal operations (including charge and engine start). Split them if the alternator fails.

Batteries will charge based on internal voltage (terminal voltage minus the voltage drop from the battery's internal resistance). As such, the battery that has the lowest internal voltage will always take the most charge current - until the battery internal voltages are the same.

Carl
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  #8  
Old 09-12-2016, 12:05 PM
bigwheel bigwheel is offline
 
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Quote:
Originally Posted by az_gila View Post
I read the diagram differently.

If only one battery is connected to the Power Bus at a time then the current drawn by the contactors is identical to the usual one alt./one batt. system.

If your EMS can monitor two voltages simultaneously then it would be a good electric source backup with an easy indication of its status.
I'm not sure if the GRT can monitor two different voltage sources simultaneously.

Quote:
Originally Posted by Mike S View Post
As shown, battery 1 seems to be used for starting the engine. If batt 2 is also used as an alternate start battery, the wiring to and from the power buss has to carry the starter current load------as does the power buss-----which I suspect is not how you intend to do things. If you want to use battery 2 as an alternate power source for the starter, you might consider adding the needed wiring.

If batt 2 is only there for backup to the electrical load of the panel, and not to be a backup to the starter, then IMHO you do not need a second contactor. Just a switch will do the job just fine.

This is how I wired my plane, the backup battery is run through a DPST switch which is rated to carry the need load on each set of contacts, but I paralleled them for extra robustness, and in case one set of contacts failed.

Do you have a low voltage alarm to let you know to turn on the backup battery if the main battery goes TU??? Or, do you plan to run both batteries at the same time, and let the one with the highest power do the work?? If the latter, consider putting in blocking diodes so the good battery will not back feed to the weak battery.

Also, when using a VSR, and having the main battery go way low in voltage so that you have to engage the backup battery----------will the backup battery be taken off the charge due to the VSR sending all the current to the bad battery?? If so, you are running on borrowed time with the backup battery as it is now a total loss system.
I do intend to use batt2 as a backup start battery. If leads from batt1 and batt2 connect to the same lug on the buss then the buss won't have to carry any load. But I am working on a schematic that may work better, don't know yet though.

The GRT has a built in low voltage alarm. I do not plan to run the batteries at the same time; when batt1 gets low I'll switch to batt2.

Your last point about the VSR is well taken. I need to investigate this further. Only reason I could see the main battery go way low with a working alternator is if it fails. If it fails and the alternator keeps the voltage above 13.3VDC then the VSR will keep the batteries connected. What that will do to batt2 I'm not sure but I imagine it wouldn't be good.
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  #9  
Old 09-12-2016, 12:31 PM
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Mike S Mike S is offline
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Quote:
Originally Posted by bigwheel View Post

I do intend to use batt2 as a backup start battery. If leads from batt1 and batt2 connect to the same lug on the buss then the buss won't have to carry any load. But I am working on a schematic that may work better, don't know yet though.
This requires big fat wires going to the buss from each battery----as drawn.

Quote:
Originally Posted by bigwheel View Post
I do not plan to run the batteries at the same time; when batt1 gets low I'll switch to batt2.
How do you plan to switch in batt 2?? Do you intend to switch off batt 1 when you engage batt 2??

A DPST switch could do that, but I would suggest you make sure it is a "Make before Break" switch, otherwise the EFIS and other goodies most likely will need to reset after switching power.

If using separate switches for swapping battery feeds, be sure to have a way to disable any back feed from the good battery to the bad one. Either a diode or a warning light------wired correctly a pair of DPDT switches can activate a warning light when the backup battery is engaged, and turn it off when the main batt is then disconnected.
__________________
Mike Starkey
VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #10  
Old 09-12-2016, 12:39 PM
bigwheel bigwheel is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
[/url]
I offer the VSR is not required or desired. This adds an unnecessary failure point. Use two identical batteries (e.g. PC-625) and have them in parallel for normal operations (including charge and engine start). Split them if the alternator fails.

Batteries will charge based on internal voltage (terminal voltage minus the voltage drop from the battery's internal resistance). As such, the battery that has the lowest internal voltage will always take the most charge current - until the battery internal voltages are the same.

Carl
I will give some more thought to paralleling the batteries. When I evaluated this in the beginning I decided I'd like to keep my batteries isolated from each other but I didn't write the the why's. It is possible, even likely, that a VSR isn't the right component to use. I had planed on using an old school diode isolator because they maintain isolation between the batteries at all times but they burn up several amps to operate. This means the batteries aren't seeing their intended charging voltage.
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