Charging house batteries from the alternator

I've been focusing on the exterior work, including propane, plumbing and electric. Lots of stuff in progress, but I've finally completed the alternator charging circuit. Almost all of my materials came from the AM Solar 230A Alternator Charger Kit (Drop in Lithium). I did substitute for a panel mount breaker and 5/16 lugs rather than 3/8 lugs. The hall sensor is here


  1. Remove the drivers seat, battery box, and panel cover.

  2. Decide where to put the Victron Cyrix-ct-li unit, 200 Amp breaker, and associated wiring. The only real space was to the left of the battery box but it would be tight.

  3. Install a 4/0 cable from as close to the Ford alternator circuit as I could get to the Cyrix unit. To do this I removed the busbar connecting the alternator to the panel, and drilled a 5/16" hole. I had to grind out a bit of plastic underneath the busbar to make room for the head of my new bolt.

  4. Bolt the Cyrix unit to the left side of the seat base. With a 4/0 lug on the stud, I needed to add some clearance to prevent the lug from potentially grounding out on the seat base, so I put some 1/8 inch foam between the unit and the seat base.

  5. Because of the tight spaces, I used a .25” x .75” copper busbar instead of a cable to connect the Cyrix unit to the 200A breaker. It's a panel mount breaker accessible from the outside of the drivers seat base.

  6. Put a Hall effect sensor on the busbar to allow monitoring of the charge rate to the house batteries. I'm planning on incorporating it into the e-brake trim, but my vehicle was missing this trim when I took delivery (thanks Ford!)

  7. Drill a hole through the floor at the forward left corner inside the seat base for the 4/0 cable run. Run the cable from the seat base along the main left side front/back frame, above the spare tire, over the tailpipe (with some heat shield in place), and up into the 4” access hole in the right rear corner. I used a mix of ¼” plusnuts and 8mm bolts, with stainless steel clamps along the way. The entire cable is sheathed in polyethylene split loom with grommets where it passes through metal.

  8. The access hole underneath the vehicle leads into the D pillar, but there is a a 1" x 3" slot that allows access to the D pillar from the van wall. 

When I did my initial testing with the batteries directly adjacent to the seat, I observed current flow from the alternator of up to 270 A, with 230 A going to the batteries, which is greater than the alternator capacity. I considered moving my connection point to the three customer connection points to reduce the current flow (assuming that the higher resistance of the small Ford busbars would do this), but decided to wait until I had the batteries in their final location with the full wire run. Since then, the maximum current I've observed going to the batteries has been 175 A - and that’s only for the first 10 minutes of engine run time. I have tested with various states of charge of both the house bank and the starter battery trying to stimulate the alternator to put out the higher currents I observed in my first few tests, but no success. At this point I'm going to leave the wiring I have in place as I think it maximizes the current I can get to the house bank. There is a 470 amp fuse just upstream from my connection point, so the busbar I tied into is certainly capable of managing my load. And I have the meter showing charge current which I can monitor. I'm now seeing the following currents to the house bank:

• Engine start – 120 A
• After warm-up and idled down – 60 to 80 A
• At 2000 RPM – 140 A

This is with the house bank at 40% state of charge, and the starter battery down to 11.7 V. So far the Cyrix unit has performed flawlessly – combining the batteries whenever a charging voltage is detected, and disconnecting them when the charging voltage goes away. My alternator will push a maximum of 14.4 volts, so I cannot overcharge the Lithium bank. 

The unit has a starter boost feature – by pushing a button twice the batteries are combined for 30 seconds even if no charging voltage is detected on either side. This allows me to recover from a dead starter battery by using the house bank, but prevents overcharging of the starter battery. 

I will still need to test its behavior when the inverter charging function is enabled.

Overall I'm happy with how it turned out. Good performance from the alternator, minimal space used for the install. I'll be curious to see what Ford thinks of my mod if I have a warranty issue with the electrical system. Let's hope I don't have to find out.

One of the photos shows the cable entering that rear access hole. I also wired up my 30A shore power service and external solar panel jack in the same location, fabricating an aluminum panel to hold it all in place. The solar panel and 30A service are in their own conduit. Another photo shows the wire run underneath the vehicle. That fan housing is the condenser for the CruiseNComfort AC. Install still in progress.