Above is a diagram of my 12 volt wiring. I have protected the positive take-offs from the batteries with T-fuses. These should ideally be within 18 inches of the battery. You can mount the T-fuses within special fuse holders made for them if you have room. Again, I recommend John Drake's web site for a good price on these fuses. Due to space limitations, I bolted the fuses directly to the cable lugs without using a holder and made up special protective housings using plastic plumbing parts (a 1 ¼ inch union works well). I also installed cutoff switches to both positive battery lines. This enables me to effectively isolate the batteries from the rest of the system if I need to either work on part of the system or on the batteries. I used Blue Sea Systems Mini Battery Switches, which are rated at 300 Amps. The Blue Sea products that I've purchased are of very high quality. All the components on the top portion of this drawing reside within the existing electrical compartment. In my Roadtrek 190V, this is the small compartment under the cushion directly below the TV cabinet. See below for discussion on the BatteryLink ACR devices.
Overview of electrical compartment
Electrical compartment 12 volt DC detail - I moved the existing Intellitec Relay further back to give me more room. The BatteryLink ACR can be seen at the center top of the photo with the red cutoff switch to the right of it. I didn't have to move the two existing circuit breakers. I have enough room to install a second BatteryLink ACR later (proposed) if I decide to go that route.
Using the Existing 12 Volt Converter in Conjunction with the Inverter Charger
When I first installed my inverter, I removed the existing 12 volt Parallax converter, since the people at Xantrex had assured me that the charger on my inverter could supply all the 12 volt needs of my RV in addition to charging the batteries. This turned out to be true. The charger had no problem supplying all my 12 volt needs. One problem did surface however. When the batteries are being charged, the charger tends to keep the voltage up between 14 and 14.5 volts (closer to 14 volts once the charge current starts dropping). As the batteries get filled, the charge current will gradually taper and once the charge current drops down to nearly 0, the charger will switch to 'ready' mode and provide about a 13.3 volt float voltage to keep the batteries topped up. The trouble arises when you have a few lights on in the RV. These draw several amps of load from the charger. Since the charge current does not drop down to 0, the charger will not switch over to ready mode, and the voltage supplied to the batteries is kept at around 14 volts. One simple thing you can do to remedy this situation is to turn the 12 volt Intellitec relay to off for a while using the wall switch. This takes the load off the charger, it senses that the current has dropped to 0 and will switch over to the desired 'ready' status. You can then turn the relay back on. The charger will nicely supply any needed 12 volt power to the RV while still staying in a 'ready' state providing a float charge to the batteries. The problem with this is that you loose your 12 volt power for a short period of time (typically a few minutes until the charger switches over to 'ready').
Since I had the Parallax converter anyway, I decided to reinstall it. Now, when I'm plugged into shore power, I just keep the 12 volt Intellitec relay turned off using the wall switch. The converter supplies the 12 volt power to the distribution panel and is electrically isolated from the batteries. Now the charger on my inverter is no longer fooled by any other loads and it will drop down to 'ready' status as designed.
For your information, the people at Xantrex said there would be no problem charging the batteries using both the Parallax converter and the inverter charger at the same time. So you don't really have to turn the Intellitec relay off if you don't want to. The only problem is that the Parallax converter puts out 13.8 volts, which is a little high for a long term float voltage. If you're plugged in to shore power for a long time, it would be better for the batteries if you isolated the converter voltage from the batteries using the Intellitec relay.
Considerations about Charging the Batteries from the Van Alternator
There have been a few discussions on various forums about charging your batteries using the automotive alternator when you drive (see Open Roads Forum, Tech Issues or for a summary see the post by Smitty on this page). After going through all this trouble and expense to make sure the AGM batteries are charged properly, I don't want them abused by the engine alternator while driving many hours. You could just not connect them to the alternator (via the battery isolator), but then you would lose the ability to recharge them while driving; something I very much want to have. When you first turn your engine on, the alternator will put out up to 15 volts. This voltage will soon decrease as the regulator in the alternator senses the starter battery voltage and gradually cuts back. After about 5 to 10 minutes of driving, I find that the voltage will drop to about 14.6 volts. More driving time and the voltage will drop further.
The AGM batteries should never be charged above about 14.5 volts (depending on the temperature). Some people say that the short time that the engine alternator is above this voltage will not be significant, and that the majority of the time, the alternator will be putting out a safe voltage. In addition, the battery isolator will drop the alternator voltage by about 0.3 volt (actual measurement), further providing protection. This is probably true, but I still feel a little uncomfortable about it.
There are several possible solutions to this problem. A battery charger exists that can take 12 volts DC and use it to power a sophisticated 3 stage battery charger programmable for AGM batteries - see Sterling 50 Amp 3 stage charger ($345 here ). Less expensive alternatives include the PowerStream 15 Amp charger ($150 - not programmable for AGM) and possibly the MajorPower 42 Amp charger (? price). Alternatively, some have proposed that you just put an inexpensive 120 volt inverter on your engine starter battery and use the power from this inverter to run your 3 stage charger for your house AGM batteries. All these solutions, while viable, seem to be too expensive or difficult to implement for the problem at hand; a problem which may not even be significant.
I took a conservative approach and simply installed a BatteryLink ACR from Blue Seas Systems ($67). This is an intelligent 60 amp electronic relay that will open if the voltage coming from the alternator is too high. You can set the high voltage cutoff to whatever you like. I set mine to 14.3 volts so I never have to worry about the batteries being exposed to too high a voltage. This seems to work nicely. When I first start the engine, the BatteryLink will remain open and indicate an over voltage condition with a red light. After about 30 minutes of driving, it will close and indicate a connection with a green light.
Note: This BatteryLink ACR is a versatile devise that also has a low voltage cutoff. If you like, you can install an additional BatteryLink on the output line going to the 12 volt distribution panel, as I've shown on my 12 volt schematic. You can then set it to disconnect if the battery voltage drops below your preset value, probably 12.2 volts. This will prevent your batteries from discharging below 50% charge and help prolong their life.