Understanding the Question Behind Lithium Battery Upgrades
Upgrading an RV to lithium batteries raises important electrical questions, especially for owners who spend time boondocking. In this Todd’s Tech Tip Tuesday, Todd answers a common concern about whether the 7 pin connector should remain connected between a truck and trailer after converting to lithium batteries. The question comes from a viewer who dry camps without solar or a generator and wants to avoid battery drain or electrical damage.
This scenario is increasingly common as more RV owners switch to lithium for its higher state of charge and consistent voltage output. Understanding how power flows between systems helps prevent costly mistakes.
What Happens When the Truck Is Running
When the truck engine is on, the alternator produces a higher voltage than the resting voltage of the batteries. That higher voltage first supports the truck systems, then supplies power to the RV through the 7 pin connector. Any remaining energy flows downhill to the trailer batteries.
This is normal operation and does not create a risk for lithium batteries. Electricity must be pushed by a higher voltage source. The alternator does exactly that, making charging through the 7 pin acceptable while driving.
Why Lithium Batteries Do Not Pull Power From the Truck
A common misconception is that lithium batteries will pull power from the truck batteries while parked. Todd explains that electricity does not work that way. Batteries do not pull power uphill. If lithium batteries have a higher voltage than lead acid batteries, they cannot draw energy from them.
If directly connected while the truck is off, the higher lithium voltage would actually try to push energy toward the lower voltage battery. That is why isolation matters.
What Happens When the Ignition Is Turned Off
Many modern trucks use a relay that opens when the ignition is turned off, stopping power flow through the 7 pin. However, this varies by manufacturer and model year. Because behavior is not consistent across all vehicles, relying on automatic isolation is not always safe.
Best Practice for Boondocking With Lithium Batteries
Todd recommends a simple and reliable solution. When boondocking, disconnect the 7 pin connector. This ensures there is no electrical connection between the truck and trailer, preventing unintended drain or voltage transfer in either direction.
A DC to DC charger is not required if charging through the 7 pin during travel. The wire length and gauge already limit current enough to protect the alternator. A DC to DC charger is only needed when higher charging rates are desired.
Learn More About RV Electrical Systems
Understanding lithium battery behavior is part of mastering RV electrical systems. At the National RV Training Academy, students learn how batteries, alternators, converters, and chargers work together in real world scenarios. Proper education helps RV owners and technicians make informed decisions that protect equipment and improve reliability.