So, you want to add a “house battery” to your van or an “auxiliary battery” to your touring rig? Adding a lithium iron phosphate (LiFePo4 or LFP) secondary battery is not as simple as simply placing it in parallel with the vehicle’s starter battery! Believe it or not, a passenger vehicle’s alternator is NOT appropriate for charging lithium batteries (dedicated secondary alternators are an exception). First, its 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-based batteries will draw far higher charge current than a standard alternator should supply. My particular LiFePo4 battery management system (BMS) will take as much as 120 amps of charge current while others can take 200 amps or more. That can lead to alternator damage and possibly start a fire under the hood! The goal is to find a solution that adjusts charge voltage, limits charge current, and isolates the lithium battery from the starter battery.
There are several choices of DC-DC chargers from companies such as REDARC, Bluetti, EcoFlow, Renogy, and others. I’m going to share from my experience with Victron Energy. My Orion XS Smart 12|12-50A 700-watt DC-DC charger (and its 360W predecessor) achieves all three goals: First, it converts alternator voltage to a variable charging voltage of my choosing. In this photo, you can see that the charger is pulling 13.3V from the starter battery, which includes ~0.5V of drop along 14 feet of 4-gauge copper wire, and adjusting the output to 14.0V. The voltage varies in order to manage current flow while charging. Next, the charger limits input current to 50 amps to prevent damage the alternator. Most alternators are capable of operating continuously at 50% of their rated capacity. My alternator is rated for 150A; so, it’s capable of supporting my “XS” charger’s maximum output of 50A. The XS’s input can be reduced via smartphone app to accommodate less powerful alternators. Since the charger uses power in its voltage conversion process, the output current will almost always be less than its input, 46.5A in this example photo. Finally, the XS acts as a battery isolator so that the two systems cannot back-feed, drain, or otherwise interfere with each other.
The lines in this image depict the three charging stages. During the “Bulk” stage of charging, voltage is gradually increased as required to maintain maximum charge current until the battery’s state of charge reaches ~80%. During the “Absorption” stage, voltage is held constant while current decreases as a consequence of the battery voltage slowly rising to match the charge voltage. “Float” begins when the battery is fully charged. The battery is held at a constant voltage that is slightly below its Absorption voltage in order to prevent overcharging the battery. If the battery discharges to below the “Re-Bulk” voltage for beyond a preset period of time, then the charger will begin a new charge cycle, which could be brief if the charger was off for only a short duration. Why would the charger be off? Unlike “shore power” chargers, alternator chargers get turned off for both brief and extended periods. Examples include brief stops for fuel, meals, or errands, a typical work day at the office, or when parked overnight.
Smart chargers can be monitored remotely via Bluetooth connection. I usually don’t monitor the charger since my BMS has Bluetooth monitoring, too. Overkill Solar’s BMS app allows me to monitor the battery cells and to estimate when charging may be complete. It also shows me the battery’s state of charge, its input or output power, and the temperature at three different sensors, which is good for evaluating the effectiveness of my battery’s heating pad in cold climates. Still, sometimes I like to see the charger’s phase of charging, shown in the images above. Is the charger in its Bulk, Absorption, or Float stage of charging? Or perhaps it is in “Storage” mode. I look at the Victron app when I want an explanation for why the charge current is not what I expect. Knowing the charger’s stage of charge can help me to diagnose potential problems or to simply have a better understanding about how my system works.
What’s NOT being reported by the BMS (shown as “KE4WMF-Shunt” in this image) is the fact that the electronics that are fed by the LiFePo4 battery are pulling power from the charging path. For example, each of my ham radios consumes about 900mA during receive and my dash cam takes 500mA. The fridge uses about 3.5A when its compressor is running and the D-STAR Raspberry Pi3 draws 500mA. That equipment takes from what would be charge current; so, it’s not uncommon for the BMS to see less charge current than what’s being reported by the charger. In this image, the charger is drawing 50A from the alternator and sending 46.6A to the battery. However, the shunt is showing that only 44.27A is charging the battery, a difference of 2.33A. That difference can be accounted for by one ham radio, the Raspberry Pi3, and the dash cam. The values can vary wildly if one or more of my ham radios is transmitting.
To summarize, an automotive DC-DC charger adjusts the alternator’s output to a voltage that’s appropriate for lithium batteries, limits its input current to prevent damage to the alternator, and isolates the lithium battery from the starter battery. It sounds complicated, and perhaps it is, but it’s fairly easy to just let the charger do its job without constant monitoring. I can go weeks without looking at the app because I have a lot of faith in the system. But it’s nice to have a peek from time to time, just to see that everything is working as designed. See this video for a complete look at my previous setup with a 360-watt charger. The same principles apply to my 700W charger.
Dual-Charging!
Scott
