June 9, 2011
hotrod, first off, getting a deep cycle battery is not going to benefit you unless you intend on running a dual battery setup with one battery dedicated to starting the truck (using a standard battery) and then putting all of your auxilary loads on the deep cycle battery and then isolating the two batteries from each other using a battery isolator (not a relay setup). All the deep cycle battery is going to do is to make it so you can run your toys with the truck off a little bit longer, but it will still leave you at the point that you can`t start the truck. Seems a bit silly in my opinion.
As for your alternator issue, are you running under drive pulleys by chance? If so, there is your problem. You are actually spinning the alternator too slow for it to make any useable power (especially with the fan running). Contrary to what most people think, the output of the alternator is a function of how fast it is spinning with it reaching a maximum at some point (which is where an alternators output is stated at). So, at idle, assuming you are running all stock pulley sizes, a 130 amp alternator will only output by maybe 60 amps. So, now when you figure it takes 10 amps just to run the truck, another 25 amps to run the A/C, another 10 amps for a decent stereo, you are already up to 45 amps of load assuming you are running nothing else. Now, let the fan kick in and you are adding around 30-40 amps to the alternator. So, now you are up in the 75-85 amp range. The alternator is only able to output 60ish amps, so, the 25 amp difference comes out of the battery. You add the under drive pulleys and you can drop that mere 60 amps at idle to under 50 amps pretty easy. This is only compounding your problem.
If you need me to go into more detail about this or have other questions, let me know. Electricity is what I do for a living and will help you get the setup you need to make sure that at the end of the day, you can still start your truck and make it home without needing a jump.
Chris "Thermo" Coleman, Nukie the radioactive 97 Ford X, and K`RE Ann the 03 Jag X