Originally shared at StealthGTI.com on December 9, 2021
A car’s alternator is NOT appropriate for charging LiFePo4 batteries. First, it’s output voltage is not quite correct; nor, is it intended to charge two batteries (starter and aux) with differing chemistries at the same time. Next, and perhaps more important, lithium batteries will charge at far higher currents than a standard alternator should supply. My particular LiFePo4 BMS will gladly take as much as 120 amps of charge current; others can take 200 amps or more. That can lead to overburdening the alternator and even starting a fire under the hood! My goal was to find a solution that would adjust the charge voltage, limit charging current, and isolate the lithium battery from my starter battery.
My Orion-Tr Smart 12|12-30 360-watt DC-DC charger by Victron Energy achieves all three of my goals: First, it converts alternator voltage to a charging voltage of my choosing, 14.2V in the case of charging LiFePo4 batteries. Next, it limits charge current to 30 amps, which ensures that excessively high charging current can not damage the alternator. Finally, it acts as a battery isolator so that the two systems cannot back-feed each other. The smart charger can be monitored remotely via Bluetooth connection; but I find that monitoring the charger is unnecessary since my BMS has Bluetooth monitoring, too. I’ve explained the BMS functions in my previous article. Still, I’ll share this image to illustrate that the Orion-Tr “30-amp charger” is probably more accurately described as a “360-watt charger.” In this photo, I’m charging the LiFePo4 battery at 27 amps. Why not the full 30 amps? If you look at the “Power” block, you’ll see that it’s showing 366W. In this case, the BMS is working at 13.5V: 366W divided by 13.5V equals 27.1A. When the charger is outputting 14.2V instead of 12V, the current has to necessarily decrease to 25.3 amps to avoid pulling too much power through the charger. Remember: Power creates heat; so, the charger needs to manage its POWER output to avoid overheating.
What’s NOT being shown is the fact that the components fed by the LiFePo4 battery are consuming power from the charging path, too. For example, my dual-band ham radio consumes about 900mA and my Blackvue dash cam takes 500mA. The fridge consumes about 3.5A when the compressor is running and the D-STAR Raspberry Pi3 draws 500mA. All of that pulls from what would be charging current. So, it’s not uncommon for me to see about 19-21A of charging current most of the time. On top of that, the unit gets hot fairly quickly. The charger outputs 27 amps first thing in the morning, even with the radios and Blackvue running, then pulls back to 25 amps within 20 minutes. That is consist with the “360/14.2” math I shared above. By then, I can hear my add-on cooling fan running, which indicates that the charger’s chassis has exceeded 105°F (40°C). Again, “power creates heat.” The owner’s manual states that the output power will derate by approximately 3% for every 1°C over 104°F. So, it’s operating at 90% power when the chassis is 110°F, which I have observed. Ninety percent of 25 amps is 22.5 amps, which is consistent with my measurements during extended testing.
The Orion-Tr can function either in “Charger Mode” or “Power Supply Mode.” I used it as a power supply during my first battery cycle due to some readings that I didn’t understand during its first charge. Now, I’m running it as a charger. I’ve been seeing the usual 21-27 amps during the “Bulk” phase of charge, then gradually tapers to zero during “Absorption.” Once in “Float,” the battery is allowed to discharge until it reaches a “Re-Bulk” point, then it will begin a relatively brief charge cycle to keep the battery topped-off.
Space is very tight in my application! I thought I was going to mount the unit “face-up” until I saw that had to be mounted with the fins oriented vertically. I found space to do that. Unfortunately, my first location did not provide adequate clearance for ventilation and easy cable management. I addressed the ventilation challenge by mounting the charger directly over one of my “rear seat delete“ cooling fans. It didn’t take long for me to move the charger just six inches away to outside of my rear seat delete where the unit can breathe easier. Giving it better heat dissipation yielded a charging improvement of ~3 amps!
My cable management is not as flattering as I’d prefer. But I think it turned out well enough. I also used ferrules on my wire leads, shown in this photo. This is a new practice for me that has me wondering why I had not known about them before. The terminals are fairly close together. So, it would be easy for a single strand of wire to veer away from the bundle and short to something that it shouldn’t. By crimping the wire in a ferrule first, installers can ensure that all strands are where they belong and then crimped into their appropriate screw terminals. I now keep a bag full of 4-gauge and 8-gauge wire ferrules.
That’s my explanation for using a “12-volt charger” in a 12V automotive system. It adjusts the charging voltage to a level that’s appropriate for lithium batteries, limits charging current to prevent damage to the alternator, and isolates the lithium battery from my AGM starter battery. Since I still prefer a “stealth” appearance, I was a little concerned that it might be “too visible” to passing thieves. However, it’s almost impossible to see if I’m carrying my Dometic CFX3-35 refrigerator/freezer. See this video for a live look at the setup.