You’re not getting it. During flight your CG is several inches aft of the empty CG. This due to the occupants and any baggage. The CG will move aft as you remove weight at the 70 inch location. This discussion reminds me of the one where the FAA inspector wanted the empty CG within W&B flight limits.
-Andy
I'm pretty sure I have a very good handle on C of G.
My aircraft has an empty weight C of G arm of 70.18". The fuel arm is located at 70.00". If I put pilot, passenger and baggage into the aircraft the C of G will move aft, certainly, however from full of gas to empty in this configuration the C of G change is minimal at only +0.71" - lets call it three quarters of an inch. In this configuration I have not exceeded the aft C of G limit of 76.8" as I'm still sitting at 76.66".
However, if my EW C of G started at, say 72.00", then the maximum I could take off with is 54 lbs of baggage because in the same fuel configuration the C of G would be at 76.78" upon landing. This is of course all assuming we haven't exceeded the MAUW. Starting with a more rearward EW C of G means the C of G change is greater as the lever arm is greater.
It is true that a number of RV-6's with an O-320 and VFR panel have ended up with an EW C of G at or beyond the 72" datum and this immediately restricts your baggage carrying capacity. I read of one aircraft on this forum where the guy stated his EW C of G was at 74" and I cannot understand how that could be so far aft - it must be an error as it would be a terrible aircraft to fly and very limited if this is correct.
So going back to my original response - if the OP places his battery forward or aft of the firewall (never in the rear behind the cabin) then his EW C of G will hopefully be at or around the 70.00" fuel arm and the use of fuel from the tanks will have very little effect on his C of G change during flight.
Lets take the example that he places the battery forward of the firewall and that in the end his EW C of G ends up at 69.00 inches, forward of the fuel load at 70.00 inches. In the zero pilot configuration with full fuel at take-off and zero fuel at landing the C of G will actually move forward by 0.14 inches. If he adds the same two people and the same 100 lbs of baggage as me, the total maximum C of G change is just +0.64 inches. He could take another 38 lb of baggage more than I could in my aircraft and still not exceed the aft C of G limit on landing with zero fuel.
So again - it is in the best interests of the OP to place his battery at or about the firewall. It is certainly not in his interests to place it any further aft and certainly not aft of the cabin. The EW C of G does not have to end up being aft of the fuel datum. The further forward you can get it to begin with the less effect fuel consumption can have on any change that can occur in flight.
With the exception of the error I made that DanH picked me up on (I was a bit exuberant there) - "fuel, bums and baggage" - clearly I should have just said "bums and baggage" - I don't see anything incorrect in my statements.