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Discussion Starter · #1 ·
This is what I am proposing to do for my dual battery mod. I welcome any feedback or suggestions, particularly with which 2nd battery to get. See my list of questions at the end.

Objectives
  • Listen to stereo while camping/tailgaiting without worrying about a dead battery.
  • Ability to "self-jump" if main battery goes dead.
  • Automatic process.
  • Clean looking install.
  • Run lots of accessories at the same time (high current draw) with or without car running.

Solution
  • Install two batteries in the engine compartment. Main battery will be used primarily for starting, while the aux battery would be a deep cycle battery used to power accessories.
  • Connect batteries together using a Power-Gate Dual Rectifier Isolator. This allows each battery to be charged by the alternator while the car is running, but isolates the current from each battery so that the accessories (winch, stereo, amps, aux lights) don't drain the main starting battery.
  • Install a switch in the car to combine the two batteries. The Power-Gate has an option that will combine the two batteries together. This can be used to "self-jump" by using the auxiliary battery as a jumping battery. This can also be used to get more power if needed (while winching, etc.).
  • Wire up the stereo head unit, amps, winch, aux lights, etc. to the aux battery while keeping all stock items (except stereo head unit) on main starting battery.
  • Install a stereo switch + relay that will allow the stereo to be run w/o the keys in the ignition.
  • [optional]Low battery warning indicator light. Install some kind of volt-sensing relay that would light an LED when the aux battery voltage drops below a certain level (~11V) and needs to be recharged.

Power-Gate Dual Rectifier Isolator


OTRATTW Aux battery switch (to combine batteries from in the car). Depending on price, get a custom switch that reads "Combine Batteries."


OTRATTW Stereo switch (for powering up stereo w/o keys in ignition)


Cost
Power-Gate Dual Rectifier Isolator (150 Amps) with battery combine feature: $383 + shipping
Optima YELLOWTOP battery: Already have
2nd battery: $100-300
Switches + relays + wires: ~$75

Known issues
  • If I were to play the stereo off of the aux battery, and leave my headlights/dome lights on, both batteries could eventually be drained leaving no way to start the car. I have left the dome lights on all night and the car started fine the next morning, and I am generally very careful about this, so it's a risk I'm willing to take. I could install LED dome lights to further mitigate this risk.

Questions
  • Should I use the yellowtop for my deep cycle aux battery and get a smaller starting battery, or should I use the yellowtop for my starting battery and get another yellowtop, a bluetop, or an odyssey for my aux battery?
  • Does it make more sense to run everything but the starter motor from the aux battery? This way the main battery would only be used for starting and it would greatly reduce the risk of a depleted starter battery.
 

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Discussion Starter · #4 ·
I did look at what others have done, but they did not meet my requirements (automatic, clean install, dual charging, separate current draw).
 

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I think this could be headed in a good direction ... and don't mind t-rex, he just doesn't want to see wasted effort. To me it looks like a good set of requirements/solution, at least at first blush - I agree, in the installs I've read through, I have yet to see one that fully meets your requirements. All dual battery systems are not created equal ... just like everyone's requirements of said dual systems are not the same.

One thing I'd like to see a bit more about is the Power Gate ... if it were me, I'd make sure that there's not a big voltage drop across it (at any point - either when charging the primary battery, or when charging the secondary, or between the two when they're linked) ... if so, that's power that's wasted through an inefficient device. You want it to be essentially 'lossless' ... which is a bit of a misnomer, but there are some solid-state devices out there which are much worse than others, in this regard.

The other thing to keep in mind, is that if you fully isolate the secondary battery it will probably play nice with the factory system ... specifically with the return current sensor that we have on the negative battery lead ... but how will linking the batteries interact with that device? As I understand it, that sensor is used to help regulate the alternator output to match vehicle charging to the load being placed on the system at a given point ... it's how the vehicle keeps track of and balances the battery inputs/outputs so that you don't kill your battery by discharging it too far or by charging it too hard.

For my money? I'd seriously consider just upgrading the stock battery to a group 31 and keeping something like the Micro Start in the gear bag. ... but like I said, your requirements do seem to lend themselves to a true dual setup.
 

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Discussion Starter · #6 · (Edited)
I appreciate that t-rex was trying to save me time/frustration. Sorry if my reply seemed terse at all... it's just usually how I write. :)

The Power-Gate has a typical voltage drop of 20-50 mOhms. In the installation directions, it actually states that there should never be more than a 0.05V drop across the isolator, and if there is, shut the car down immediately and call their support line.

I think the only time I'd ever really combine the two batteries is in a self-jumping situation, so I wouldn't be terribly worried about the stock current sensor. Plus, the negative terminals would be connected together in parallel (since they're both connected to ground), so they could be wired so that the current sensor takes both into consideration (the alternator and current sensor would see the two batteries connected together in parallel while the car was running, but the current sensor would only see the primary battery while the car was off).

You have seriously got me considering ditching this whole plan and getting a micro start instead. I was hesitant to go that route because of size/reliability, but this looks pretty good. I could still have the stereo switch for running that w/o the keys in the ignition, and the low battery indicator light, and then just use the micro start if I got myself into trouble. Much cheaper, but slightly less cool. :) Also, I'd have to remember to actually keep the thing charged... but that could easily happen on the way to the campsite.
 
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