Dear Dave,
I’m thinking of replacing my lead acid 100Ah battery with LiFePO4 batteries for longer off-grid capacity. The biggest draw is from the 12V refrigerator, which consumes about 60W and runs most of the time (or so it seems). I understand that I will need to replace the converter/charger to one that is LiFePO4 compatible.
My question concerns the tow vehicle. What happens if I don’t install a DC/DC converter? Also, it seems I would need to add new heavy-duty wiring and connectors between the alternator and the new batteries, correct? Will the tow vehicle trickle-charge the batteries while they run the refrigerator during travel?
Finally, am I correct in calculating that 200Ah LiFePO4 supply will run 100 watts of load for only about 24 hours? Thank you. —Wayne, 2021 Puma 20RLC
Dear Wayne,
For the past year, I have been conducting seminars for the Ford Towing Bootcamp. One of the items we discuss is the voltage that will come back from the larger alternators or two alternators in some of the tow vehicles. I’ve always sent them to Battle Born® to look at the DC-to-DC charger you are referring to.
You are correct in the fact that the lighter gauge wiring will limit what charge goes to and from the trailer and only provide a trickle charge. So, I sent your question to my contact at Battle Born for clarification and got the following response.
From Battle Born technical support
Most trailers receive charging power through the 7-pin connector while towing. You can continue using the factory’s 7-pin connection with lithium batteries, but it will typically function more like a maintenance or trickle charge. The factory wiring between the alternator, truck, 7-pin connector, and trailer batteries is usually relatively small gauge wire, which limits the amount of current that can flow to the batteries. This helps prevent excessive charging current and also limits the amount of power that could potentially flow back toward the tow vehicle.
With a lithium battery upgrade, many owners find that the 7-pin connection will help offset loads such as the refrigerator while traveling, but it generally will not provide enough charging current to quickly recharge a depleted lithium battery bank.
If you want to optimize charging from the tow vehicle, we recommend installing a DC-to-DC charger. A DC-to-DC charger regulates the charging profile for lithium batteries and allows you to run larger gauge wiring from the truck to the trailer for significantly improved charging performance. In most installations, an Anderson connector is added at the rear of the truck and front of the trailer to provide a secure high-current connection between the two vehicles.
For many RV owners, the factory’s 7-pin connection is sufficient to help maintain the batteries and offset some refrigerator consumption during travel. However, if you’re regularly boondocking, running large battery banks, or want meaningful charging while driving, a properly sized DC-to-DC charger with dedicated heavy-gauge wiring is the preferred solution.
As for his battery capacity question, a 200Ah lithium battery bank provides approximately:
200Ah × 12V = 2,400 watt-hours of usable energy
Assuming a continuous 100-watt load: 2,400Wh ÷ 100W = approximately 24 hours.
So, your estimate of about 24 hours is correct. In real-world use, loads cycle on and off, and factors such as temperature, charging sources, and battery age can affect runtime, but that’s a good planning estimate.
Battle Born® batteries are available at Amazon.
You might also enjoy these posts from Dave
- Switching to 600Ah lithium batteries. Is 200W solar panel sufficient?
- Should I upgrade RV house battery system to 48-volt for boondocking?
- Can different type and age RV batteries be mixed? Can I add a lithium battery?
- My RV’s “larger” batteries are dead. Can I replace them with lithium?
- Will RV’s house batteries ‘play nice together’ if I replace chassis battery with lithium?
- Will my RV’s charger be sufficient for new lithium batteries?
DAVE HAS ANSWERED MORE THAN 1,000 readers’ maintenance and repair questions. Read a directory here. There is so much to learn!
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
HAVE A QUESTION FOR DAVE?
Send your inquiries to him using the form below.
RVDT2930
