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wiring design for the simpleton - the battery end first

L

larryj

Well Known Member
I'm trying to finalize my wiring schematic without becoming a EE. I ordered the Aeroelectric CD but it never came and 2 attempts to contact Bov Nuckolls didn't work - so I've given up on that. I spent some time with the Matronics Aeroelectric forum and my personal spin on it is that it looks great for espousing platitudes and theories on eircraft electrical suystems but has little to no practical value. All I want to do is wire my aircraft. I have just enough "knowledge" to be slightly danderous and make some general decisions.

Given all considerations (like CG, overall weight, all-electric EFIS panel, etc.) I have settled on a 2 battery / one alternator architecture. The batteries are of different sizes and mounted aft with 2 "0" gage wires running forward. I know what I want the panel to be:
toggle switch - Battery 1 (on/off)
toggle switch - Battery 2 (on/off)
toggle switch - Mag 1 (on/off)
toggle switch - Mag 2 (on/off)
toggle switch - fuel pump (on/off)
momentary push switch - starter

toggle switch - E Buss (on/off)
toggle switch - Main Buss (on/off)

My immediate question is at the battery end of this architecture. I believe in simple terms all I need or want is for 2 contactors near the batteries; one for each battery and the 2 "Bat" switches running each contactor. These feed power to the 2 big wires running forward from either or both batteries. Simple. In addition, I run 1 smaller wire (say 16 gage or so; to be calculated) from the bigger battery up to the E-Buss switch so it doesn't need the contactors. Is this all I need ?? The only shortcoming to this that I see is that if the alternator fails I have only the option of running the E-Buss off one of the batteries with no choice - but I think that pretty remote so so-what ?? I have read about Battery Isolators and get confused and seem to think one doesn't really need them.

Can someone smart enough to put this in simple straight forward terms steer me on this ?? Right now I'm just working to finish the aft portion of the aircraft with around 5 wires running forward.
 
Larry,

What is driving your decsion to install two batteries?

There is some good reading on our docs page here: http://www.verticalpower.com/documents.html that might be of help to you.

Specifically, the docs at the top of the page, and you can also reference the "Dual Battery" wiring doc further down. It shows how to wire in two batteries.
 
Good question Marc. A little more background is due. I chose 2 batteries because of flexibility. Most folks use an Odyssey PC680. I'm choosing an Odyssey PC625 and PC310. Combined these two give me more cranking power than a single PC680 at roughly the same weight- but with the addition of the redundancy of 2 batteries instead of one. I have an AEIOX-540 with 10:1 compression so may need both batteries in parallel on a cold morning. Please take no offense; but I don't think I need or want an additional box - all I want is some switches on my panel running things I know about like simple contactors. I think I see on your dual-battery installation the batteries are wired in series which would defeat my purpose??
 
Larry
I really do not think you will need the extra battery. It is cold where I live as well :) If you keep the battery on a maintenance charger and preheat your engine you will have no problems. Keep it light and simple.
 
Keep it simpler

I think I see what you are doing. Here are some comments:

1) Does your EFIS have a backup battery option? If so, I think having two batteries is overkill. If VFR, everything can go dead and you are okay. If IFR, you want enough to get you out of the clouds, and I think an E-Buss and internal backup battery is enough. Your choice of course. Depends a lot on what else you have running on your battery and how much it draws. My backup to the backup is a solid state turn and bank (low draw), a single pointer altimeter, and an airspeed indicator.

2) If you have two batteries and two contactors, no need for two fat wires run forward. Just run each of the contactors to their individual batteries. The other side of the contactors could be wired together with one fat wire running up front from one of the contactors. If one switch is on you feed (and charge) one battery. If both switches are on you feed from both (and charge both). Make sure your alternator can handle charging both and that should work.

3) The single additional wire that bypasses both battery contactors and powers your e-buss sounds fine. You have to ask yourself "What are the odds of both contactors failing?". More than likely, the failure modes is that your alternator craps out. In that case, just a single heavier wire for the ebuss is best, because battery contactors draw power too.

Just my opinion, and there are many on the forum.

Don
 
OK, thanks for the clarification.

The most common reason we see for running two batteries is to be able to isolate one in case of a bus failure and each independently provides power to something critical. We typically see this with dual Lightspeed ignitions or Subaru engines. In such a case you'll want an extra contactor to isolate one battery from the other.

In your case, it seems you just want a big battery. I recommend wiring them together in parallel and the system sees them both together as one big battery. No extra wiring, switches, contactors, etc. Keeps it simple.

Bob Nuckolls wrote a good article on this a while back: http://www.aeroelectric.com/articles/Batteries/Multiple_Battery_Myths_A.pdf

Hope that helps.
 
Thanks a ton for the help so far. Let me address some specifics:
Tom, I'm seriously considering this option. I think the way I have it the second battery gives me no additional backup and just complicates things. I have a friend with an -8 / IO-360 and the PC625 works fine; but my big 540 beast may need the PC680. If I go with 1 battery I'll probably try a single 625 first and only go with the 680 if this proves inadequate.
Don, I'm putting in a Garmin G600 and it has no internal backup battery; and this originally drove my 2 battery decision and still does. The 2 big wires are + and -; I'm not using an airframe ground because of potential problems (pretty good pun!). My goal with the "e-buss" is to totally simplify my pilot workload; alternator fails during IFR - switch off field switch, switch off main-buss (dumping all non-essential load), choose which battery to run off of, fly out of IMC, land and get the problem fixed.
Marc, I can see that the one higher priority over basic nav would be electronic ignition. However, I'm still too conservative to go that route (I'm looking at Lasar or equivelant) so my primary motive for redundancy is just nav out of IFR in the case of a problem. Bob Nuckoll's article does seem to lead to a single battery conclusion for me.
Dave, thanks .... I'm downloading it now.

More clarification: even though I'm putting in a G600; I do not enjoy or intend a lot of IFR flight; it just makes flying long cross-countries easier and more enjoyable for me and I like the "look and feel" of the installation. So my design goal is NOT to build a super-IFR panel. I'm just trying to find a reasonable amount of safety while keeping it logical to operate for me (= linear in thought). That's why I like switches and wires :)

So am I to conclude that I leave out the second battery and second contactor and proceed with a 1 alternator / 1 battery design ????
 
In case you're not aware of this, make sure the switches are rated for DC. A DC switch is made differently than an AC switch since if you have an arc in an AC switch the alternating voltage going through zero will extinguish it; not so in a DC switch. So even though a switch is rated for 20A at 120VAC, you may get a failure at 2A, 12VDC.
 
larryj said:
I have a friend with an -8 / IO-360 and the PC625 works fine; but my big 540 beast may need the PC680. If I go with 1 battery I'll probably try a single 625 first and only go with the 680 if this proves inadequate.
Click to expand...

Larry, I was cranking an IO390 with a rear mounted PC680 and +/- 2ga cables. Worked fine, although not a lot of cranking reserve. Just switched to a PC925 for CG improvement as well as more AH. Plus I have GRT's backup battery, so there's no reason to ever run out of electrons. You can create your own backup with any small battery and a Schottky diode to the panel buss. Many choices; might search Wicks or Spruce for the little 5AH Ultrastart Red. Nice battery.
 
Dan,
I have a friend that has had no problems using a PC625 on a IO-360 (in an RV-8 and over 700 hrs TT). Here are the numbers I have used as key:
the PC310 is rated with 100 CCA and weighs 5.9 lbs
the PC625 is rated with 265 CCA and weighs 13.2 lbs
the PC680 is rated with 220 CCA and weighs 15.4 lbs
the PC925 is rated with 380 CCA and weighs 26.0 lbs
Given these numbers I would first choose two PC625's for a combined 530 CCA and 26.4 lbs. This is what I built my airframe to hold and can ultimately use this combo if needed. But, in the interest of weight I thought I'd try a PC625 / PC310 combo; and now I'm looking at just a single PC625.
Of course if the extra weight of the PC925 helps your CG and it turns out to be the most desirable way to get you where you want to be; sounds good. I moved my batteries back a little further in keeping with my lust for light-weight and maintaining good CG.
 
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