Is there any reason why everyone uses a solenoid and master switch . . . . .

over a battery selector / disconnect switch?

especially when planning on using a back-up battery.  Some of the selector / disconnect switches come with a lock for added security.

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I have a master contactor/solenoid - isolates the battery and associated cables on the engine side of the firewall to reduce possibility of shorts and fires inside the cabin and keeps heavy battery cables shorter and therefore saves weight.



You have the ability to keep the unprotected batt. positive lead as short as possible between the batt. and the solenoid, after the master solenoid should a short occur it can be controlled with the master switch. The risk can be small if your installation is sound. My own preference other than the safety perspective is, it is much easier to mount the solenoid out of the way with a small switch on the panel. "The AeroElectric Connection" is a great reference reguarding all things aero electric reguarding light aircraft. The book has spacific wireing schematics related to single/duel batt. and covers various engine models. I have found it very helpful.


I did read up on "The AeroElectric Connection" they have sample circuits for dual batteries but it is all wired with solenoids, no mention of a direct selector switch.


If you're using a second battery strictly in case of a dead battery some cold morning, I posted info on a back up battery on the 750 forum. Only 14 oz, 400A peak power. 

Walt Snyder

I saw that, its pretty cool, the same company makes powerful light-weight batteries as well.  I was thinking of going with a dual battery installation, one on the engine side and another on the cabin side of the fire wall. Depending on my C of G, I may need to use a heavier battery on the FWF side. The selector switch is the complete opposite to the new high tech Primary Power System (PPS) which Vertical Power is developing. I guess nothing wrong with KISS.

The reason, in part, is tradition, in my opinion. That is how it has always been done in certified aircraft and so we all do it that way in our homebuilts. In the old days wire quality and insulation quality and durability were not what they are now and being able to disable the electrical system was a very good thing. One can make a very strong argument for wiring an aircraft in the same manner as modern cars -- battery cable directly to starter solenoid, smaller gage wire feeding the rest of the electrical system. One could have a "sort of" master switch by putting a 40 amp switch/circuit breaker into the line feeding the electrical system and putting it on the panel and marking it Master Switch. The only thing you would not disable with that type master switch would be that heavy lead to the starter motor. You could put your battery disconnect switch in that line, if you wanted to, just in case. This would work especially well if your battery is mounted aft of the firewall for CG purposes. Just a thought at this point, but I am seriously considering wiring my plane without a master solenoid.

I do not have a master solenoid either, just a starter solenoid. I kinda agree in part with Bob on this, and I asked my A&P his opinion on this, as he works exclusively on Rotax and Jabiru. He is seeing less and less of the master solenoids, and says it's coming close to about 50/50 now on that type of installation. His feeling was the master solenoid was a source of failure at times, and pilots wanted that link removed. He also made the point that with our low draw panels and avionics, even the new low draw LED landing lights, the high amp wiring and draws are gone, unlike in the past with the older GA aircraft and panels.

I'd have to check, but I think my heaviest gauge wire on my side of the firewall is about a 16 inch lead  of 10 or 12 gauge to my main breaker, which is way overkill for my draw. I posted on another site that I am going to put a disconnect switch by Summit Racing that will be on my inside firewall (the switch is small and light). There will be about a 6 inch lead from my battery positive  to the switch, which I can use to completely isolate everything if need be. I did this because I wanted to be able to totally shut everything off in a remote area to prevent a dead battery the next day by "forgetting" the master or something, and it's a conscience step for me to turn it off. (do you think my friends have had to hustle a jump a few times?) One of the disadvantages of having a Rotax or Jabiru, not being able to hand prop it. 

One note, I did become a firm believer in Tefzel wire after a major short in another aircraft that could have been disaster. That stuff will melt, but will not burn. 


I have a master disconnect switch similar to the one pictured above.  It's on a 701 with a 912uls.  I placed the battery in front of the passenger seat and ran a short length of cable to a box on the floor.  The power goes from the battery to the switch then distributes to the avionics buss, main buss, starter solenoid.  It also has the shunt, fuses and a switch for the charging system and ammeter.  This minimized my starter cable run, and also I did not protect this cable with a fuse because the only time there is power to it is when the engine is cranking  ( the power leads to the busses have circuit breakers).  It eliminated the need to have a switch on the dash to an electric solenoid master switch which I felt was just more complexity then needed.  It's a nice big red reminder to de-energize the system prior to exiting the aircraft (especially important with the Rotax).            

Kitplanes magazine just published a good article on master switches.




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