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I have been reading a lot about the all electric plane, of which I have one.  Since I do no IFR flying, I never have added a backup battery.  But, if I did, I think I would take a different approach:

When we were building the IV, we used a 9.6 volt Makita electric drill battery to cycle the hydraulic pump during the landing gear installation and fitting.  Interestingly, this small battery was capable of retracting and extending the landing gear a phenomenal 7-9 times on one charge!   And this was with the wheels and brakes assembled.

Now we have even larger capacity batteries available for our drills.

Therefor,  I am thinking of parting out an old Makita drill, scavaging the battery holding sleeve, making a glass overlay mounting assembly for it, and using it as a backup battery in case my primary battery system poops.  Based on my experience with the amperage and work capacity of the battery during my building phase, I think that I would figure out a simple isolation circuit for the Makita battery to run a few essentials such as the radio, gear, gps (mine are hand held dual gps anyway), and whatever else is needed to get me to the closest airport.

My present all electric system is breakered and fused all over the place, and it would be very easy to disable the affected malperforming circuits and enable the backup Makita circuit.

PS:  if you experiment with a Makita to raise your gear, don't hold the wires on the battery terminals with your fingers!

David Jones

LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.