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jluckey(at)pacbell.net
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 Posted: Mon Oct 09, 2023 9:33 am Post subject: #2 or #4 AWG? |
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Group,
I am installing a battery behind the baggage compartment in an RV-14. It is the second battery in a two-battery system but will be used for cranking in case of emergency. Length of run is ~15 ft. (there is also a battery on the firewall that will normally be used for cranking). Both batts are PC-680s (which has a new part #: ODS-AGM16L).
Trying to decide between #2 or #4 AWG Welding Cable. Leaning towards #2.
Any thoughts or considerations?
TIA,
-Jeff
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alec(at)alecmyers.com
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| Posted: Mon Oct 09, 2023 11:27 am Post subject: #2 or #4 AWG? |
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In the context of a previous discussion on insulation material, what is welding wire insulated with? What is its fire-resistance status, and fume potential?
On Oct 9, 2023, at 13:33, Jeff Luckey <jluckey(at)pacbell.net> wrote:
Group,
I am installing a battery behind the baggage compartment in an RV-14. It is the second battery in a two-battery system but will be used for cranking in case of emergency. Length of run is ~15 ft. (there is also a battery on the firewall that will normally be used for cranking). Both batts are PC-680s (which has a new part #: ODS-AGM16L).
Trying to decide between #2 or #4 AWG Welding Cable. Leaning towards #2.
Any thoughts or considerations?
TIA,
-Jeff
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Ceengland
Joined: 11 Oct 2020
Posts: 391
Location: MS
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| Posted: Mon Oct 09, 2023 11:45 am Post subject: #2 or #4 AWG? |
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The specs are available for most welding wire's insulation. Temp is likely to be at least as good as Tefzel; welders expect it to be dragged across red-hot steel fairly frequently. I suspect that nasty fumes are going to be there regardless of material, if you mess up and get the wire hot enough to burn the insulation, but if it gets that hot, you've got more immediate issues with the fire that's already burning. Worst thing about welding cable for me personally is that the insulation is so thick; it just consumes a lot more space. But I still prefer it over fat A/N wire; all its other characteristics are better, for my purposes.
On Mon, Oct 9, 2023 at 2:29 PM Alec Myers <alec(at)alecmyers.com (alec(at)alecmyers.com)> wrote:
| Quote: | --> AeroElectric-List message posted by: Alec Myers <alec(at)alecmyers.com (alec(at)alecmyers.com)>
In the context of a previous discussion on insulation material, what is welding wire insulated with? What is its fire-resistance status, and fume potential?
On Oct 9, 2023, at 13:33, Jeff Luckey <jluckey(at)pacbell.net (jluckey(at)pacbell.net)> wrote:
Group,
I am installing a battery behind the baggage compartment in an RV-14. It is the second battery in a two-battery system but will be used for cranking in case of emergency. Length of run is ~15 ft. (there is also a battery on the firewall that will normally be used for cranking). Both batts are PC-680s (which has a new part #: ODS-AGM16L).
Trying to decide between #2 or #4 AWG Welding Cable. Leaning towards #2.
Any thoughts or considerations?
TIA,
-Jeff
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nuckolls.bob(at)aeroelect
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| Posted: Mon Oct 09, 2023 12:31 pm Post subject: #2 or #4 AWG? |
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At 12:33 PM 10/9/2023, you wrote:
| Quote: | Group,
I am installing a battery behind the baggage compartment in an RV-14. It is the second battery in a two-battery system but will be used for cranking in case of emergency. Length of run is ~15 ft. (there is also a battery on the firewall that will normally be used for cranking). Both batts are PC-680s (which has a new part #: ODS-AGM16L).
Trying to decide between #2 or #4 AWG Welding Cable. Leaning towards #2.
Any thoughts or considerations? |
2awg 0.16 mOhm/Ft
4awg 0.24 mOhm/Ft
Assume 15' wire
2awg 0.16 x 15 = 2.4 mOhm wire path
4awg 0.24 x 15 = 3.6 mOhm wire path
Assume 200A load:
2awg 200A x 2.4 mOhm = 0.48V drop
4awg 200A x 3.6 mOhm = 0.72V drop
4awg will drop ~0.24 Volts MORE than the 2awg
Of course, there are other resistances that add to
total path drop but the DIFFERENCE between 2
and 4awg is pretty small.
What kind of failure does your FMEA anticipate
that would be mitigated by the second battery?
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
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jluckey(at)pacbell.net
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| Posted: Mon Oct 09, 2023 2:10 pm Post subject: #2 or #4 AWG? |
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Bob,
Thanks for the numbers.
The 15 feet is one way, therefore it is actually 30 feet round trip, which doubles your numbers to .48 volts more drop using #4.
You used 200A. Is that typical starter current? I typically use 250A. Is 200A a more accurate assumption? In this aircraft we will be cranking a brand new IO-390.
I updated your calcs and did it for both 200 & 300 amps:
[img]cid:55a11853-6c86-90d7-e945-22e77f777669(at)yahoo.com[/img]">
The scenario in question is as follows:
1. You have attempted to start a flooded or cold engine and exhausted the primary starting battery on the firewall.
2. Now, you can engage the "back-up" battery for cranking. (with a paralleling solenoid that is designed into the system)
Is the .72 volts significant is this case?
Battery voltage during typical cranking is ~8.5 volts --- that's more of a question 'cuz I don't know for sure.
But, if that is correct then .72/8.5 = 6% at 300A. Is that significant in this scenario?
Seems like we are pretty far down the rabbit hole... 
-Jeff
On Monday, October 9, 2023 at 01:37:32 PM PDT, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:
At 12:33 PM 10/9/2023, you wrote:
| Quote: | Group,
I am installing a battery behind the baggage compartment in an RV-14. It is the second battery in a two-battery system but will be used for cranking in case of emergency. Length of run is ~15 ft. (there is also a battery on the firewall that will normally be used for cranking). Both batts are PC-680s (which has a new part #: ODS-AGM16L).
Trying to decide between #2 or #4 AWG Welding Cable. Leaning towards #2.
Any thoughts or considerations? |
2awg 0.16 mOhm/Ft
4awg 0.24 mOhm/Ft
Assume 15' wire
2awg 0.16 x 15 = 2.4 mOhm wire path
4awg 0.24 x 15 = 3.6 mOhm wire path
Assume 200A load:
2awg 200A x 2.4 mOhm = 0.48V drop
4awg 200A x 3.6 mOhm = 0.72V drop
4awg will drop ~0.24 Volts MORE than the 2awg
Of course, there are other resistances that add to
total path drop but the DIFFERENCE between 2
and 4awg is pretty small.
What kind of failure does your FMEA anticipate
that would be mitigated by the second battery?
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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nuckolls.bob(at)aeroelect
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| Posted: Mon Oct 09, 2023 5:40 pm Post subject: #2 or #4 AWG? |
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At 05:09 PM 10/9/2023, you wrote:
| Quote: | Bob,
Thanks for the numbers.
The 15 feet is one way, therefore it is actually 30 feet round trip, which doubles your numbers to .48 volts more drop using #4. |
You're not using the airframe as a ground return?
| Quote: | | You used 200A. Is that typical starter current? I typically use 250A. Is 200A a more accurate assumption? In this aircraft we will be cranking a brand new IO-390. |
It's a wag driven by numerous variables
of temperature, engine condition, etc.
The purpose of the study was to illustrate
the DIFFERENCE between the two conductors in
a constant condition.
If you constantly used both batteries to crank,
then they will share the load and current in
the rear battery path will be less than 1/2
of total due to the longer conductor.
| Quote: | I updated your calcs and did it for both 200 & 300 amps:
[img]cid:55a11853-6c86-90d7-e945-22e77f777669(at)yahoo.com[/img]
The scenario in question is as follows:
1. You have attempted to start a flooded or cold engine and exhausted the primary starting battery on the firewall.
2. Now, you can engage the "back-up" battery for cranking. (with a paralleling solenoid that is designed into the system)
Is the .72 volts significant is this case?
Battery voltage during typical cranking is ~8.5 volts --- that's more of a question 'cuz I don't know for sure.
But, if that is correct then .72/8.5 = 6% at 300A. Is that significant in this scenario?
Seems like we are pretty far down the rabbit hole... |
Agreed. If you HAVE two batteries then I
would recommend using BOTH for cranking
all the time (Z-14 philosophy from 20+
years ago).
Battery life is predicated on a complex model
that accounts for depth of discharge, rates
of discharge and discharge cycles. Running
the batteries in parallel to crank will get
you best return on investment for the
constellation of batteries.
But if you've got a particularly difficult
cranking situation, it's ALWAYS better to
use ground power assist . . . even it it's
just a battery cart or even jumper cables to
a modern vehicle. That car battery + alternator
is more than 2x the energy reservoir of your
ship's battery(ies). A successful hard start
is greatly enhanced by keeping the cranking
voltage high . . . the idea of planning
for a 'flat battery' contingency kinda smacks
of poor design/protocol.
When we ran K1K1, our renters were advised
to ALWAYS use available ground power if their
cranking prospects were compromised. We
would deduct the ground power fee from their
rent!
The extra weight plus time/labor/expense of maintaining
TWO batteries to cover what should be a rare event
is . . . well . . . A set of jumper cables
with ground power plug is a lot less expensive
and lighter than that extra battery installation.
Bob . . .
////
(o o)
===========o00o=(_)=o00o=========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
=================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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wsimpso1
Joined: 04 Nov 2018
Posts: 33
Location: Saline MI
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| Posted: Sun Jul 07, 2024 5:53 am Post subject: Re: #2 or #4 AWG? |
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Hmm, I find this interesting from a couple fronts. Yes, 4AWG gives us a little more voltage drop but with two batteries carrying the load (according to both the OP and for my Z-14 arrangement) that seemed fine. Z-14 diagrams show both 4AWG and 2AWG in the starter circuit. Why both sizes?
Perhaps answering my own question, perhaps the examples shown were postulating short runs of 4 AWG and a long run from the start contactor to starter?
My plan with a non-conductive airframe, two batteries separated by four feet and both well aft is 4AWG between the batteries, a Bus-Tie contactor for starting, and 2AWG for the combined starter circuit. But with a two battery start and a combined loop (power and ground) for starting of 20 feet, it sounds like weight for nothing.
Thoughts?
Billski
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