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I would suggest getting the plane pretty much built but not having the battery permanently installed. Then get it onto some scales with all the weights in place on the aiplane even if they are not installed yet. In other words, if you plan on wheel pants but they are not yet mounted, set them on top of the tires so the weight is in the right place, if the instruments are not installed yet, lay them on the floor directly below the panel so that much weight is in the right spot on the plane, etc. Put the battery where you think you would like to have it, just prop it in place somehow.
Figure out where the empty CG presently is. If it is not where you want it, move the battery fore and aft in the airframe to move the empty CG to the right place. If the empty CG is too far forward, try the battery someplace further aft and check the weight and CG. Keep moving it around until you find the battery location that gives you the desired aircraft empty weight CG. That location is where you want to mount the battery. In other words, you need to lug the battery around anyhow so use it as the last installed item to go into the plane. You can locate the battery to get your empty CG right where you want it.
My two cents.
*Disclaimer* I don't have a Corvair motor! (yet) However I understand they are pretty heavy for our application. You didn't specify the type of battery you are planning on using, that will make quite a bit of difference as to where you need to put it. If it's an EarthX battery I'd keep it on the firewall, otherwise the length of wire may weigh more than the battery if placed out back. An Odyssey PC680? How about under the passenger seat, that seems to be a good location. And at about 13lbs it's fairly light.
I guess I should have been more specific, I planning on an earth X 680 John Auston says that his weighs about 4 lbs., the corvair weighs in at around 228 with all the acs. I had to wait for a good while for my motor mount e.t.c. and have been closing up seat pans and baggage areas so I'm getting somewhat limited on where I can go. W.W. says that you can use #4 wire but if I have to move very far back I'll have jump up to #2 and then you are all right it will weigh more than the battery. I'll not have access to scales till I move to the airport.
The EarthX weighs so little that it is almost not worth using it as ballast. I was thinking a lead acid battery (even a small one like the Odyssey PC680 is enough weight to significantly move the CG around, but a four pound EarthX not so). I agree with the folks that say keep that EarthX as close to the starter as you can get it and keep the wire run short (and light). Or, if you need to put ballast in the plane anyhow, go with a lead acid battery and save the considerable cost of the fancy lithium battery. If you need to add weight to put the CG in the right place you might as well have the weight be something useful like a battery (a cheap battery, even). If you go with the four pound expensive battery and then need to put twelve pounds in the tail to get the plane balanced, you might as well just use a battery and the cables hooked to the battery as the weight in the back.
Are you too far into the build to move every moveable object (ELT, rack mounted avionics, autopilot actuators, whatever) that you can to the tailcone? Every thing you put in the back is that much less ballast you will need to put back there. The ballast should be as far back as you can possibly get it, the further aft it is the less you will need to move the CG to where you want it. It is good form to paint it red and label it boldly with something like "PERMANENT BALLAST, DO NOT REMOVE" to avoid a future problem caused by a helpful but ignorant friend or mechanic.
thanks Bob, I really think I'll be using the earthx batt. A lot of folks are using corvairs and don't appear to be have too much trouble with w/b, although the weight will tell. I do concur that if I have to ballast the tail that would be a good way.
Earth X inside firewall co-pilot side.
thanks Tim there is positively no room on the motor side
Thanks Jeff this all helps I've been back and forth on the earthx 680 (4lbs) and the 680 (14lbs) I love the small weight of earthx but the price OUCH.