I wanted to move my battery to the trunk, but didn't want to sacrifice space I use all the time. The cargo container behind the passenger seat is a good place for the battery. It's a little closer to the engine bay, I rarely use it, it can be latched and locked, and best of all, its almost a perfect fit for an OEM sized battery. I've seen this done with smaller batteries, like a Harker Odyssey, which is pretty easy given their tiny size. I decided to use an Optima Yellow Top, #D51, which is the same size as the stock CRX battery. You can get these at auto parts stores or order them online. I picked mine up at the local Sears auto center for about $140. I don't know about the other places, but Sears won't give you a core back for the old battery.
What you need:
Optima D51 Yellow Top 2 or 4ga wire, about 2m for the longest piece, and 2m to cut up for the rest Battery terminals if the cables don't have them already ring terminals heat shrink (don't use tape! do it right!) distribution block (or alternative, see below) hammer phillips head screwdriver 10, 12, 14mm sockets utility knife crimper multimeter (optional) sandpaper/wire brush dremel/air grinder
Removing the Old Battery
First off, you need to take out the old battery and tray, and remove some of the stock wiring. Begin by taking the leads off the battery. The negative goes right to the frame underneath the stock tray. The positive splits and goes to the underhood fuse box and the starter. Pull them all out, and If you can, try to save the boot that covers the starter lead. Now unbolt the battery itself, and take it out. It's going to be very dirty, I was surprised, mine looked clean but was sitting in a puddle of grime. Now take out that battery tray and get rid of it. At this point I cleaned that side of the bay, it was filthy. Also you can now remove the main ground for the battery.
Fitting the Battery
Unscrew your cargo container and pull it out. Lay the battery in it and cut out the bottom of the container all the way against the passenger side edge for it. You also need to cut out the bottom corners so it can sit flat, otherwise the bottom edges will sit in the round corners and the compartment won't close flush. Finally, cut the lip in the front of the compartment so your cables will fit. Now comes the painful part: you need to hammer a little bit of frame. I suppose you don't need to do this, but if you don't the compartment will close and latch, but it won't sit completely flat. You don't need to go crazy, just flatten down the front part of that depression so the battery won't sit with the front corner tilted up (you'll see what I mean when you fit it). Screw the container back in, but don't install the battery yet.
For this part, run all the wires first without putting any terminals on them, just find the right length. It's a lot easier to use the heat gun/torch/whatever you use for heat shrink if it's not in the car. Don't use tape on the terminals, do it right and heat shrink them all, it won't come off and it's a lot cleaner. For this, I had a 2m piece of 2awg red wire a friend gave me, and then I got some more 4awg at an auto parts store. Both had battery post terminals on one end already. The positive post will be closer to the driver's side of the car, so start your red cable there, and go across the front to the passenger side, then back down the side and out the back/bottom of the compartment. Run the cable under it and out the front. Now pull the carpet up along the door all the way up to the front of the passenger footwell. There is a little grommet behind the glove box you should easily be able to pop out, take it out and save it. Honda conveniently left a bunch of wire clips along the bottom of the door, so clip the wire in there (2ga fits perfectly) and then send it out into the bay with about 30cm slack. This wire doesn't need a terminal, so you can strip the sheathing off the end and then tuck your carpet back in. Now you are going to need to make the two power wires, one to the starter and the other to the fuse box. For this I used 4ga wire. These won't be too long, you just need a couple of short pieces to go from where the power comes in from the back to the two components. They each need a bare/stripped end and a ring terminal. the terminals for this size wire won't fit in the fuse box without some modifications though. You need to grind/cut the sides of the ring slightly, and cut away some of the plastic fins that hold the stock wire in the fuse box. When it's done, installed and box closed, it will fit and look stock (except for the huge wire) so don't go crazy trimming it down. The starter wire just needs a ring terminal, and you can try to put that boot over it too if you like (mine was torn when I took it off). Ok time to distribute power. If you want, you can get a distribution block at a stereo place for $20-25. OR, you can get a cheaper one at Home Depot that won't look as nice, but will work just as well. The Home Depot one is basically a bolt with a slot up the middle, and a nut that clamps down on the slot. You put the wires in, and tighten the nut, then seal it up with some big heat shrink and creative stretching. If you use this method, MAKE SURE IT"S SEALED. Since it's metal and not necessarily secured to anything, you need to make sure it's not going to contact the frame or body anywhere.
Ok, finally, the last wire. You can ground to almost any bolt in the frame,a lot of people like the spare tire bolt hole. I chose not to do that because I have a spare in there and I have to remove it for autocross and stuff, so I didn't want to have it in the way. If you pull out the rear interior, you can see that there are quite a few places you can use. I chose one of the mount screws for the "wall" that separates the trunk from the compartment. You will notice that it is in fact a plastic anchor, not a screw into the body. I only used it because it's close and when I cut my last wire I didn't realize it was plastic. It's been working fine, here's how I did it: Pull the screw and the anchor out of the car. Now take your sandpaper and get all the paint off it and make it smooth. To make sure it won't oxidize, put some Vaseline on there, then set the ring around the hole, and finally push the anchor through it. You will probably need the hammer to get it all the way down. Some Vaseline should come out the sides, that means you're forcing it out and making a metal-metal contact that isn't exposed to air (especially the very moist air that is usually found in the CRX trunk). You can use the meter here to test the ground if you like. Now you can put everything back together. Engine grounds to replace.
Battery Ground Location
Drop the battery in and start the car. You might want to check the battery with your multimeter, I got mine in and THEN realized it was just a little too low to start the car. The other thing that you can do is make some new grounds with the scrap wire. If your car is like mine was, the stock grounds are old and frayed 10ga wire, they could be replaced.
Optima D51 Yellow Top installed
Engine Grounds To Replace
Write up by Tom Downes.