I was trying to build an auto-reloading system for geothermal generators today, and I discovered that you cannot stick lava buckets or cells into anywhere but the bottom slot. This is a problem because there isn't a way to insert a bucket and then remove a bucket using buildcraft or redstone power because you need at least TWO sides that can access the bottom slot : one to insert a bucket full of lava with a tube and another side to remove the empty bucket.
Looking at the decompiled code, I found the error in "TileEntityGeoGenerator.java" There's a switch statement that makes every side of the machine go to slot 0, the top slot, except for the bottom slot. This is a bad decision because if a player is trying to access the inventory of this machine from a side of the cube, they will most likely be trying to insert or remove fuel - it would be rare for a player to want an automated battery charger.
So I just added case 5 : return 1; to the switch and it works now. The machine also accepts lava buckets from the left side if you are looking at it from the front, and it works great.
Please take the 30 seconds to copy this fix into the next release.