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Alright, I've decided to install a 100ah agm aux battery in the rear. Nothing too serious, just enough to power a fridge, basic lighting and charge batteries. The aux will power a small panel (usb, 12vdc, etc) I'll also be charging via solar (eventually).

Main questions are....
  1. Is 4awg cable from the starter to the aux battery ok or would 6awg be alright?
  2. Redarc bcdc 1225 (25a) VS Renogy 20a. There is a big price difference, but the Redarc has an mppt built in.
  3. I'll have a 100ah circuit breaker at the starter battery, should I also have one next to the aux battery? Maybe, between the aux and fuse panel?
  4. Should I install an on/off switch before the charger?
 

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How I charge my spare battery. Run a ~14 gauge wire from the 7-pin trailer plug through the tailgate and connected to the battery. Go drive around for a couple days. That is how I charge spare batteries before I had an actual battery charger. Even with a fridge attached. Works VERY well. Doesn't cost very much. That charge wire on the 7-pin is designed to charge batteries on trailers you are towing. Surprise, you don;t have to be towing the battery to use it to charge a battery.

The only advantage you have with a 4-gauge wire is you don't have to get the jumper cables out to jump start yourself, providing you have a properly sized charge relay. But if you have one of those fancy high dollar chargers to charge a 12V battery from a vehicle designed to already be charging a 12V battery, you will probably still have to break out the jumper cables to jump start yourself.

I have no idea why the battery to battery charging systems are suddenly the hot thing right now. I would understand if you were running a different battery chemistery (some lithium varient of a battery) that has a slightly different charge voltage. But you are charging a lead acid battery with a charging system already set up to charge a lead acid battery. And there is even a special (nothing really that special about it) dedicated charge wire.

Oh my, you will have a voltage drop across such small gauge wire. As properly engineered, that is correct. You are not putting a 12V drop across the wire, that would be a dead short and blow the fusible link. You might have a 2, or you are really abusing the battery, 3 volt difference between the batteries. At that point the wiring simply acts as a ballast resistor. Regulates the charge rate. As the batteries get closer to a match, the amp rate goes down, the voltage drop goes down, and eventually both batteries have a balanced charge with no significent amp draw and both batteries are fully charged. For 50+ years that is how they charged batteries in travel trailers. But someone has made a fancy gadget that is pretty expensive and is making a small fortune selling it to "overlanders" as something that they must have it it will never work right. Oh, gee, the battery might charge in 2½ hours of driving instead of 3 hours without. It must be better. If you feel you have to empty your wallet, feel free.

As for needing a MPPT solar charge controller over a PWM, how much charge do you need and how large is the panel. Very often in this size of a system it is more affordable to go up a panel size and use the less expensive PWM charge controller. A 100W panel with MPPT could make a therotical ~97W output in standard test conditions. You can probably get that 97W from a 120W panel with a PWM charge controller. Panel costs $20 more, charge controller $60 less. The MPPT is great when you need that 97W and only have room for a 100W panel. My 70W panel feeding my PWM charge controller has been running my fridge for about 9 years now.
 
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I have been wanting to do this myself, but I keep running in to technical issues. I have no doubt over complicated it.

Early2010's suggestion is interesting as it solves a lot of problems. I tried to follow the wiring diagram of how this works and I HATE nissan's schematics. I found it from two connectors under the truck (assuming its not already wired to tow) to the main body harness connector then couldn't find it any further. I presume it just goes back to the IPDM, is fused, and is on when the key is on? If so then your essentially putting your house battery in parallel with your starting battery when the rig is on. So the plus's are that the wiring is already there, its only on when the key is on, and your current limited by the thin gauge of the wire - and everything I have read says you shouldn't charge your deep cycle battery at more than 15% the rating of that battery, so if you have a 100Ah battery you don't want to exceed 15A, and since in theory the alternator is 130A, this would likely solve that problem.

Having said that, its a catch 22. First is your putting your battery in parallel with your start battery. Most aux batteries in RV's etc are wired through an isolator so that the house battery can never drain the start battery - even if you leave something on or whatever. Which leads to one of my technical question is how would the Nissan Smart charging system interact with the Voltage Sensitive Relay of the isolator? Would I need to disconnect the smart charging system to make it work, and even if I don't need to should I? The second issue with charting from the trailer Aux is that your now pushing charge through your IPDM. I suppose technically the IPDM is supposed to be for putting power to your rig, but given the issues the IPDM can cause, I really would rather avoid that idea altogether. Last thing I need out in the woods is a cooked IPDM. The last issue I see is the trailer wiring is going to be very low current - the gauge can't be that great - which is fine if you want to drive around and charge your battery, but if your out and you really just want to run your rig long enough to charge the battery its going to take a really long time.

Most RV type sites recommend just wiring your Aux battery to the starter battery with a isolator. That is likely the simplest, but there is no current limiting in that case other than your wire gauge, but most people seem to think that isn't an issue.
 

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I have been wanting to do this myself, but I keep running in to technical issues. I have no doubt over complicated it.

Early2010's suggestion is interesting as it solves a lot of problems. I tried to follow the wiring diagram of how this works and I HATE nissan's schematics. I found it from two connectors under the truck (assuming its not already wired to tow) to the main body harness connector then couldn't find it any further. I presume it just goes back to the IPDM, is fused, and is on when the key is on? If so then your essentially putting your house battery in parallel with your starting battery when the rig is on. So the plus's are that the wiring is already there, its only on when the key is on, and your current limited by the thin gauge of the wire - and everything I have read says you shouldn't charge your deep cycle battery at more than 15% the rating of that battery, so if you have a 100Ah battery you don't want to exceed 15A, and since in theory the alternator is 130A, this would likely solve that problem.

Having said that, its a catch 22. First is your putting your battery in parallel with your start battery. Most aux batteries in RV's etc are wired through an isolator so that the house battery can never drain the start battery - even if you leave something on or whatever. Which leads to one of my technical question is how would the Nissan Smart charging system interact with the Voltage Sensitive Relay of the isolator? Would I need to disconnect the smart charging system to make it work, and even if I don't need to should I? The second issue with charting from the trailer Aux is that your now pushing charge through your IPDM. I suppose technically the IPDM is supposed to be for putting power to your rig, but given the issues the IPDM can cause, I really would rather avoid that idea altogether. Last thing I need out in the woods is a cooked IPDM. The last issue I see is the trailer wiring is going to be very low current - the gauge can't be that great - which is fine if you want to drive around and charge your battery, but if your out and you really just want to run your rig long enough to charge the battery its going to take a really long time.

Most RV type sites recommend just wiring your Aux battery to the starter battery with a isolator. That is likely the simplest, but there is no current limiting in that case other than your wire gauge, but most people seem to think that isn't an issue.
I've had a message discussion with early2010 about this but didn't put it here because I didn't want to derail OP's thread .... here's the schematic in question with the paths of importance highlighted. The ignition switch fires up the Ignition Relay which then turns on the Trailer Tow Relay 2 which connects battery to pin 5 of the 7pin connector. If you charged the Aux battery from the 7pin connector you would NOT be pushing current through the IPDM (the current would go through that 30A fuse branch). Also, from the schematic, as long as your ignition key is either set to OFF or ACC the 7pin connector is not connected to the starting battery. No chance of discharging the starting battery so long as the ignition key is in ACC or OFF.
Whenn the key is in the ON or START position then the 7pin connector pin 5 IS connected to the Aux battery which puts the two batteries in parallel.
OP if this is not helping you let me know and we can stop talking about this on your thread.

132592
 

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Discussion Starter #5
So here's what I've figured out and how I'll likely run my setup in (just the basics)...
  • 4awg from starter to 100a circuit breaker.
  • 4awg from circuit breaker to dcdc charger in rear of vehicle.
  • Dcdc charger to aux battery
  • 8awg from aux battery to fuse panel
  • Fuse panel to Powerwerx panel (usb, 12v, etc).
  • Pure sinewave inverter connected to aux battery.
I'll be using the Redarc bcdc 1225D (you could also use the Renogy dcc30s) to charge the battery from the starter.
Both the Redarc and the Renogy are compatible with smart and non smart (or in our case semi smart) alternators.
Redarc recommends the 25ah charger for aux batteries between 75-100ah.
Both are multi stage and support multiple battery chemistries including liFePO4.

These 2 dcdc chargers also have an mppt built in for solar.

I've chosen this route so that I can skip over Nissan's wiring and isolate my own. It also makes it easier to repair/upgrade as needed.
 

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I've had a message discussion with early2010 about this but didn't put it here because I didn't want to derail OP's thread .... here's the schematic in question with the paths of importance highlighted. The ignition switch fires up the Ignition Relay which then turns on the Trailer Tow Relay 2 which connects battery to pin 5 of the 7pin connector. If you charged the Aux battery from the 7pin connector you would NOT be pushing current through the IPDM (the current would go through that 30A fuse branch). Also, from the schematic, as long as your ignition key is either set to OFF or ACC the 7pin connector is not connected to the starting battery. No chance of discharge with key off. When the key is in the ON or START position then the 7pin connector pin 5 IS connected to the Aux battery.
OP if this is not helping you let me know and we can stop talking about this on your thread.

View attachment 132592
THanks!

If I can ask which part of the manual did you find that schematic. I have been looking for a while - I can't find it in my PG section?
 

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Some of these things are difficult to find! This schematic was in the Exterior Lighting System seciton of my 2013 FSM.
 
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