I'm thinking of doing the same with my FJ40. I really like the completeness of Man-A-Fre's kit (200 Amp alt, battery swith, Optima Red Top, battery tray & wiring), but that $895 price tag is just a little much.
To break it down, here is how it is listed on their site:
200 amp Mean Green alternator for $370
Optima Red Top 750cca for $225 :-/ --> a dual-terminal Red Top @ 4-Wheel Parts lists for $140...what's with the additional $85?!
Good Samaritan Dual Battery Current Control System for $200
Dual batter tray for $95 - $109 (have a couple diff't ones listed)
So, like you, I am kind of hunting around for a way to do this, but cheaper than the above...heh. As I haven't gotten really serious about this part of my project, I haven't actually taken the time to see if I can piece together the whole kit through various other sources.
If anyone has any pointers on where to go for any of this stuff, or if someone has engineered another solution, I'm sure that there are a couple of ears willing to listen!!
Well I set up a dual battery system with a rebuilt 95 amp chev alt, an old tool tray with a real nice piece of angle iron bracket, a couple battery hold down that were of course discontinued and blow out priced, some welding cable with auto wreckers post connectors, and a brand spankin new solenoid for isolating the batteries. With exception of the batteries the whole set up cost me approx. $100. Now that is duals on a budget(or in my case as a cheap ass) Just wait til I do my suspension.. whoa!
I have a 97 FZJ80, so a bit different than the FJ40. I went with www.wranglernw.com and purchased their 200AMP solenoid - http://www.wranglernw.com/commerce/ItemDetail.asp?ItemNo=29-983 , their battery manager, military grade terminals (4) http://www.wranglernw.com/commerce/ItemDetail.asp?ItemNo=34-003N and had them make custom cables for the setup. I found that it was easiest to get the tray and second battery in and to then take string and measure all of the distances - then just call up wranglernw and have them made. If you decide to hook up your winch to the aux battery (a good idea soas not to drain the main) you can put another relay between the battery and winch to shut the power off when not in use and to eliminate tampering. I think all in I spent around $700 on the project and it works like a charm.
I made my own system some 3 years ago, and it's still working great. See the diagram below for a schematic. Basically took the output from the alternator and ran it to 2 separate charge diodes. (The size of these will depend on your alternator, but I put in 2 60A diodes for a 100Amp alternator) The diodes should be mounted on heat sinks to dissapate the heat.
Each diode was then in turn connected to a battery. This method ensures that each battery will only get the required charge, and also ensure they will not cross drain each other.
I then fitted a battery isolator switch between the +ve terminals, so that in the event of the starter battery being flat, I can use the spare to start the engine.
As said earlier, this solution has been running for 3 years, with my second battery for my engel freezer, lights, compressor, etc. The wire, diodes, heat sinks are all available from the local electronics store, and cost me about $15!!