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My 4Runner came with a Panasonic battery which measured 10"x6.5"x8" (LxWxH). This battery was working fine, but I'd been researching alternatives that would provide a higher reserve capacity for camping so that the battery would not be run down after a few days of running accessories.
Battery Configuration
There are two main ways to increase the electrical storage capacity of a vehicle, one is to move to a single larger battery and the second is to run multiple batteries. Each setup has its benefits and downsides. A single battery allows for simplified wiring, easier installation, is usually more space efficient, and is less expensive. Dual batteries take up more space and require additional electronics and installation hardware, but they usually store more power and allow you to reserve one battery for starting while using the other for accessories. If the goal is to be 100% self sufficient then having a dedicated starting battery is the best plan.
Battery Selection
In the past I've had good success with Optima batteries but many people have reported quality issues with the more recent models, supposedly this is being resolved by Optima but I didn't want to take the risk. Other good alternatives for sealed batteries are Odysseys and the DieHard Platinum from Sears which are apparently internally the same. I ultimately decided to get a DieHard Platinum based on the positive reviews, the lower cost, and the 48/100 month warranty. There are several different options available that would work for expedition use, here's a quick overview.
P-1
http://www.sears.com/shc/s/p_10153_12605_02850034000P
G34 880cca 135rc 68ah 53lbs (7-4/5" x 6-4/5" x 10-9/10")
Supposedly the same as a Odyssey PC1500. Terminals are backwards for use in a 4Runner.
P-2
http://www.sears.com/shc/s/p_10153_12605_02850065000P
G65 930cca 135rc 75ah 60lbs (7-2/5" x 7-1/5" x 11-4/5")
This is a fairly massive battery. Terminals are backwards for use in a 4Runner.
P-4
http://www.sears.com/shc/s/p_10153_12605_02850090000P
G34/78DT 880cca 135rc 68ah 53lbs (7-4/5" x 7-1/10" x 10-9/10")
Has side terminals. Terminals are backwards for use in a 4Runner.
P-5
http://www.sears.com/shc/s/p_10153_12605_02850035000P
G35 740cca 100rc 59ah 50lbs (8-9/10" x 6-9/10" x 9-1/2")
Terminals are correct for a 4runner.
P-6
http://www.sears.com/shc/s/p_10153_12605_02850025000P
G25 740cca 100rc 59ah 50lbs (8-9/10" x 6-9/10" x 9-1/2")
Terminals are backwards for use in a 4Runner.
Only the P-5 has the terminals on the correct side for a 4Runner, but I was not impressed with the capacity of this battery. The P-2 has very good numbers, but it's too heavy and too big for my use. The P-1 and P-4 seemed like viable options. I was lucky and found someone local who had received a P-4 as a gift but didn't want to deal with making the terminals work in his Tacoma, so I bought his brand new battery for a really good price.
Installation
The 4Runner is setup to have the positive terminal on the left and the negative on the right. There's a good amount of room available in the stock mounting location, and if the washer bottle is moved to the other side of the engine you can fit two batteries in this space.
The P-4 has the terminals reversed. The plastic grid on top is a spacer required to use the stock tie-down, this is comes with the battery from Sears.
The size of the stock drip tray limits the footprint of the battery. It can be replaced or removed, but the P-4 fits fine.
As you can see the stock battery cables are designed for the opposite terminal configuration. The good news is that using the spacer the height of the battery is perfect for the stock clamp.
Initially I was going to extend the positive terminal using 4g wire and terminals. This would work, but would have required brazing the terminals on to the wire.
Instead I built a buss bar out of scrap steal and sanded the ends clean.
This buss bar extends the positive terminal so that it reaches the wire. The positive terminal bolts on to the cable which makes it very easy to bolt the buss bar in place. I considered installing some military terminals (such as the ground terminal shown here), but decided to keep them stock for now.
I added a layer of heatshrink and sleeving for protection, and then put the stock terminal cap back into place.
Here's the final setup. The negative terminal is a tight fit but reaches, I added some wire protection to keep it from rubbing on the battery clamp. The positive terminal is insulated using a heat shrink and electrical tape. I hate using electrical tape, but it's working fine in this case.
One reason I kept the stock battery terminals is that I strongly believe that all positive connections should be insulated and I had no way to insulate the military terminals. There are options available which I might switch to in the future.
https://www.vtewarehouse.com/content/termins/battterm/html/466Series/466SeriesPage.php
http://www.electerm.com/battery.html
The DieHard is a little narrower than the stock battery so I modified the hold down bracket using some Delrin and foam to make it a tighter fit.
Resources
http://www.ttora.com/forum/showthread.php?t=67738
http://expeditionportal.com/forum/showthread.php?p=89760&highlight=Dual+batteries#post89760
http://www.tundrasolutions.com/forums/members/rambrush/albums/dual+battery+install-623/
http://expeditionportal.com/forum/showthread.php?t=14285&highlight=sears+platinum
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