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I suggest you definitely do some wire upgrades if you go the HO alternator route. It's actually really easy. The hard part was taking that lower bolt off the factory alternator!
Agreed. I am working out the BIG 3 upgrade with 0ga. cable from StingerElectronics.com to go along with my alternator plans.
 

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<snip>.....
Batteries will not be over charged by a properly functioning electrical system. Alternators are demand driven devices, and unless the battery is putting a load on the alt (aka battery needs a significant charge) the alternator will only supply the power that's needed. All batteries, even fully charged ones, still have resistance that will load a alternator. It's a very small amount but it does exist.
Thank you!!

<snip>.....Which brings me to my next question:

What are you trying to achieve by running dual batteries? What is your goal, what do you want? Without knowing exactly this I can't really give you fitting advice.

Why spend hundreds on a dual battery kit and 100 to 200 on a second battery, only to gain the power reserve equal to one larger battery? One single 2150 style battery will equal the power reserve of the factory battery and many secondary batteries people use, combined (and be less then 15% less total reserve then almost all dual battery setups people run).

To do a dual battery system right means you have two identical batteries (such as pc1500) propper sized wiring (2ga or bigger), a 500 amp solenoid (70$), proper crimped/soldered connectors, and everything is properly wired. You will have 200$ into a DIY kit if you do everything right (plus batteries). Cheaper solenoids will fail when you need them. Many people use starter solenoids and they can't handle the task of dual battery setups and will fail.

Most people want dual battery setups for 3 reasons.

1) to gain capacity so they can run winches and whatnot longer.
This makes sense but why run a mix of a stock battery and a much better deep cycle aftermarket battery for a total combined reserve that barely (if it does) beats a single large 2150 style battery? If you want reserve capacity running dual big batteries that offer 30% or more over a single 2150 makes sense, but most never do this. In laymens terms many dual battery setups people run will not power a winch longer then a single 2150 battery will.

2) To gain the ability to have a battery to start the vehicle if the primary one dies

This makes sense as well. However depending on what solenoid/how its wired you might have issues with this working the way you indend. Not to mention you can install a device that will cut power to accessories if the voltage drops to low, giving you the benefit of a dual battery in a single battery setup.

3) because they think they need one
Dual batteries are thought of as a must have with guys with serious sound systems in the car audio world. Guys never look at other things like the alternator and wiring, its always "Throw another battery in it for the power you need". Well where do you think that power comes from? Batteries don't generate power of their own for the most part, they store it. Not enough power from the alternator and it doesn't matter how many batteries you have, you will drain them.

If you are worried about your FJ draining the battery and having the FJ not starting, yes a dual battery setup would give you the ability to start the vehicle. So would installing devices on added on electronics that would kill power before the battery died, which would prevent the battery from being dead in the first place. It is also better to wire the new (should be deep cycle) battery in as the primary battery, and hook all high drain devices to it. Why? Well if you ever are in a situation where you left lights on and the main battery is dead, and thats the factory or other non deep cycle starting battery, that battery will have significant life taken away. Starting batteries are not designed to take a massive discharge and walk away. Which is why using a new deep cycle battery as a second battery in the event the starting battery is drained is putting you in a situation where if that does happen, your main battery will likely be toast after one or two times.

Running a single 2150 style battery is a 200 to 300$ investment, gives you the reserve capacity equal to many second batteries people run combined with the stock factory battery. Its simple to install (no real modifications needed) handles winching just fine (for minutes under high load), and with proper wiring of accessories wont likely ever go dead on you. You wont have battery lifespan issues like you will with dual battery setups either.
Having read this thread for the umpteenth time now,,,, I must say there is a lot of excellent information in just these two posts!! I took the liberty of highlighting red just for my edification.

@Greggry Is there a low voltage disconnect device you prefer and recommend? Something like this Blue Sea one maybe? https://www.bluesea.com/products/7635/m-LVD_Low_Voltage_Disconnect
 

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You know...I also run the same set up and have had battery problems in the past. (Pythons can be started with standard btw). Anyway, I also run a lot of accessory stuff just like some of the other questions people posted (lights, music, amps, winch, heaters, etc). I went with a dual battery setup....still drained power if I didn't drive it for a few days (using optima r ed top as second). Batteries tested good, just drained. Decided on an alternator upgrade with the big 3 wire upgrade and bam...fixed the problem so far. I didn't take the baseline before changing, but I'm guessing the alternator wasn't charging the batteries enough. Just a guess on my experience..not educated! lol
Thanks for the reply. Is there any online tutorial on how to do this mod? For example, what other components alongside the alternator should be replaced (wires, etc.)?
 

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Thank you Greggry for the great write up very nicely done. Quick question, I have some local boys in my FJC group that are doing a mod to the alternator S lead with this mod HKB Electronics. Any thoughts on this, me personally I don't think it's worth the effort or money and it's a bit risky if you ask me. Cheating the charging system into something it wasn't design to do.
 

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^^^^^ i would not expect a reply, the OP has not been active since April 2014, I think I recall he sold his FJ and left the Forum.
 

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This is the first thread I've read since joining. Darn glad I did too. I was seriously considering a dual battery set up in anticipation of future upgrades and accessories. I was going to do it because I 'thought I needed it'. I'm already running a blue top Optima.
Looks like maybe a wire upgrade might be all I really need to do for a couple of light bars and a winch I might use three times a year.
Learn something new everyday. Saved money too, thanks.
 

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One thing that does make sense to do though is replacing your alternator with the Tacoma 130 Amp HD alt with the time comes to replace your stock alt
 

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One thing that does make sense to do though is replacing your alternator with the Tacoma 130 Amp HD alt with the time comes to replace your stock alt
Is that pretty much a direct replacement or does it require more than normal work to fit it? That's definitely on the future to do list.
 

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does this apply to 2011-2014 fjs or do they have a higher output alternator??
It applies yes , 2011-2014 stock alt is 100amp and can be replaced with the Tacoma 130 HD alt

Or another option guys is to have the stock alt rebuilt to 130 amp
 

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It applies yes , 2011-2014 stock alt is 100amp and can be replaced with the Tacoma 130 HD alt

Or another option guys is to have the stock alt rebuilt to 130 amp
Thanks. that is good to know when the time comes to change over, since I have dual battery setup this would give me the extra bump.
 

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Thank you for the info, My Battery light came on shortly after replacing the ORIGINAL battery on my 08 FJ. The symptoms of slow electrics such as windows shows up as well as a lower needle on the charging meter. I was under the impression the rectifier was "built into" the alternator, I see a rectifier on e-bay for about $24....What's experience out there? Completely bad ALT or Rectifier only? RSVP, Thanks GG
 

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This was a very informative thread. Thanks for taking the time to write it. I am ready to upgrade my alternator, and wiring as I will be adding lights, ect.
 
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