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Why put the starter relay on the firewall?

claycookiemonster

Well Known Member
When I built the rear battery tray for my -8, I wondered what the four big nutplates were on the aft side. Now I see that they are for mounting the battery and starter relays, except I see many posts about mounting the starter relay at the firewall. Even the VP-X directions suggest putting the relay there.

That environment seems a sketchy place for an important electric component. The firewall is a crowded place already. I've still got to wire it from the battery, so no weight savings. It's the same length wire no matter where the relay is, so the voltage drop from the battery is the same. It seems to me that the entire length of that cable is hot all the time, and only the short length from relay to starter becomes hot when the starter is engaged.

I have no doubt there is some good reason for this location, but I'm not getting it.
 
Ease of access for inspection/replacement. I had one fail at about 400 hours.
 
A start relay on the rear battery mount would require two big cables all the way from the back.
 
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Ok, foolish question. Learning in progress. There is only one big cable from battery to start relay, but two big cables from relay to starter?
 
1. One big cable from battery to master relay.
2. One big cable from master relay to starter relay.
3. One not quite so big cable from master relay to main bus.
4. One big cable from starter relay to starter.

Cable # 3 can run all the way from the back, or just from the firewall mounted starter relay to the main bus.
 
BTW Clay, note the use of different terms, relay vs contactor. The silvery drum shaped thing is a contactor. In general, an energized contactor connects two terminals, while an energized relay typically switches a common input from one output terminal to another output terminal.
 
A start relay on the rear battery mount would require two big cables all the way from the back.

Sometimes a simplified picture is worth a thousand words.

Starter Cable.jpg

So why add the extra big cable and its associated possible problems?
 
Go ahead and mount both contactors in the rear if you want.
Use 10 or 12 AWG wire for the main bus feeder. It will only carry battery charging current.
The alternator will power the main bus, not the battery. If the alternator fails, you will shed loads to less than 30 amps.
A lithium battery should have a bigger wire because it can accept higher charging current.
 

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A firewall mounted starter contactor is redundant and unecessary. Adds weight and more failure points. There is already one built into the starter. There is a significant improvement in cranking performance by eliminating the firewall contactor as a few milliohms of resistance at an inrush current of 350A has a significant voltage drop across the contactor.
 
A firewall mounted starter contactor is redundant and unecessary. Adds weight and more failure points. There is already one built into the starter. There is a significant improvement in cranking performance by eliminating the firewall contactor as a few milliohms of resistance at an inrush current of 350A has a significant voltage drop across the contactor.

Following that logic and building upon it -- shorten and relocate the "B" lead from the alternator and run it over to the input of the starter solenoid (on the starter)...

The mind -- it reels ...

:)
 
A start relay on the rear battery mount would require two big cables all the way from the back.

Not really. You would need to run an additional #8 or #6 AWG wire from the main contacter up to the cockpit and alternator. This would add a bit of weight, but nowhere near using two #2 AWG cables. The upside of having the starter relay in the rear is that:

#1 you don't have a constantly "live" #2 cable running through the cockpit.

#2 The starter relay should live longer due to not being exposed to the heat of the "under cowl" area.

Greg,
Access to the rear battery area isn't bad on an 8 or 8A.

I went a 3rd way. If interested in learning more, PM me with your email address.

Charlie
 
Following that logic and building upon it -- shorten and relocate the "B" lead from the alternator and run it over to the input of the starter solenoid (on the starter)...

The mind -- it reels ...

:)

I have thought about doing this but if you have any sort of fuse or breaker on the B-lead for the alternator that wont work. But it could work if you only had an OV protection scheme on the field wire. The fuse/breaker is only good for a short which is highly unlikely to happen, if the cable is supported properly.
 
A firewall mounted starter contactor is redundant and unecessary. Adds weight and more failure points. There is already one built into the starter. There is a significant improvement in cranking performance by eliminating the firewall contactor as a few milliohms of resistance at an inrush current of 350A has a significant voltage drop across the contactor.

I had thought about saying exactly that but figured the peanut gallery would shoot it down, as they have in the past. I wired my -6 like that. The only drawback of doing it this way is you have a hot fat wire firewall-forward in case of a crash if the hapless pilot forgot to shut off the Master before impact.

Following that logic and building upon it -- shorten and relocate the "B" lead from the alternator and run it over to the input of the starter solenoid (on the starter)...

Yep, I did that too on my -6. It's a ~12" long 8-ga wire with a homemade fuseholder in the middle from an ANL 80A fuse supported by a fiberglass strip and several layers of heat shrink tubing. It's held up fine over the past 15 years and 900+ hours.

I have thought about doing this but if you have any sort of fuse or breaker on the B-lead for the alternator that wont work. But it could work if you only had an OV protection scheme on the field wire. The fuse/breaker is only good for a short which is highly unlikely to happen, if the cable is supported properly.

You should have a fuse on the B-lead, see my reply above. It's to protect the wire from the battery in case of rectifier diodes shorting in the alternator, not from the alternator producing "too much amperage". And OV devices always work on the Field lead, so nothing changes there. Below is my DIY B-lead fuse. The other end goes to the starter-mounted solenoid terminal (the fat one on top).
20220312_161144.jpg
 
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Sometimes a simplified picture is worth a thousand words.

Thank you Galin, but let's note the main contactor belongs next to the battery.

Go ahead and mount both contactors in the rear if you want. Use 10 or 12 AWG wire for the main bus feeder. It will only carry battery charging current.

You would need to run an additional #8 or #6 AWG wire from the main contacter up to the cockpit and alternator. This would add a bit of weight, but nowhere near using two #2 AWG cables.

All true. Next time I will define "big" ;)

The upside of having the starter relay in the rear is that:
#1 you don't have a constantly "live" #2 cable running through the cockpit.

Instead you have a live 6, 8, 10, or 12. Either way, circuit protection is the battery contactor. Same thing.

Hey, don't get me wrong. Feeding the main bus direct from the battery contactor is more or less standard for installs with the battery in the front, and yes, it can be wired that way with a rear battery. However, it still means a long main bus feed rather than a short one, plus, if case grounded, the start contactor will need a fused coil control lead run from the panel.
 
snipped
Instead you have a live 6, 8, 10, or 12. Either way, circuit protection is the battery contactor. Same thing.

Hey, don't get me wrong. Feeding the main bus direct from the battery contactor is more or less standard for installs with the battery in the front, and yes, it can be wired that way with a rear battery. However, it still means a long main bus feed rather than a short one, plus, if case grounded, the start contactor will need a fused coil control lead run from the panel.

Using the mid sized #6 or #8 wire allows you to install an ANL fuse for this circuit back by the battery tray, so that wire is protected.
 
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Using the mid sized $6 or 8 wire allows you to install an ANL fuse for this circuit back by the battery tray, so that wire is protected.

Sure, just like an ANL on a main bus feed through the firewall. Still need the contactor.
 
Sure, just like an ANL on a main bus feed through the firewall. Still need the contactor.

Agreed. The big question is, will the ANL fuse blow faster than you notice the short and kill the main contactor. If it doesn't, than installing it is a waste of time/money/weight.
 
Agreed. The big question is, will the ANL fuse blow faster than you notice the short and kill the main contactor. If it doesn't, than installing it is a waste of time/money/weight.

Hey, I'm all for the ANL!
 
If the big wire stays with the starter, a smaller gauge fusible link wire could be used from the alternator, eliminating the fuse.

True that, just have to be careful to keep that fusible link away from anything combustible, like a fiberglass inlet duct. My old Silverado is actually wired this way.

Another drawback I see with a fusible link is there will probably be more resistance than a fuse would have, leading to a lower voltage output at the starter fat wire terminal, where the alt B-lead is connected. I installed an alternator sense wire going to the starter terminal to counteract that resistance.
 
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