DC-to-DC Charger Cable Sizer
Sizes both circuits for a dual-battery DC-DC charger: the input run from the starter battery/alternator and the output run to the auxiliary house battery. For single alternator primary runs without a DC-DC charger, use the Alternator Cable Sizer.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Engineering Notes on DC-DC Chargers
Calculation Assumptions
- Accounts for step-up/step-down voltage differential and 90% internal converter efficiency.
- Separate calculations for vehicle alternator input run and auxiliary battery output run.
Engineering Limitations
- Assumes smart alternator voltage profile; mechanical voltage drops across ignition relays or isolators must be added separately.
A 50A charger mounted 16 feet from the vehicle starter battery draws ~58A input. To maintain <3% voltage drop and enable the charger to detect engine-run voltages, 2 AWG (33.6 mm²) input cable is required.
DC-to-DC chargers are boost-buck switch-mode power converters. Because the converter operates at roughly 90%–92% efficiency, the input current drawn from the starter battery is always greater than the output charging current delivered to the house battery:
Under-sizing the long input cable from the front engine bay causes significant voltage sag. If input voltage drops below the charger's smart-alternator engine-shutdown detection threshold (often ~12.2V–12.8V), the charger will continuously turn on and off in an erratic short cycle.
Frequently Asked Questions
What size wire do I need for a 30A or 50A DC-DC charger?
For a 30A DC-DC charger with a 15-20 ft run from the starter battery to campervan living area, 4 AWG or 6 AWG copper wire is standard to maintain input voltage drop below 3%. For a 50A charger (e.g., Renogy DCC50S or Victron Orion-Tr Smart), 2 AWG or 4 AWG is typically required depending on total round-trip distance and engine bay ambient temperatures.
Why is the input current higher than the rated output current on a DC-DC charger?
DC-DC chargers operate at 88-93% electrical efficiency and boost voltage to lithium absorption levels (14.4V-14.6V). When boosting a depleted 12.5V starter alternator feed to 14.4V at 50A output, the input stage must draw approximately 63A to 68A of DC input current. Wire and fuse sizing on the input side must be based on this maximum input current.
Do I need a fuse on both ends of a DC-DC charger input wire?
Yes. A DC-DC charger input wire connects between two live power sources: the starter battery/alternator at the front of the vehicle and the charger electronics. An overcurrent fuse must be installed within 7 inches (ABYC) or 18 inches (automotive standard) of the starter battery positive post to protect against chassis short circuits along the length of the vehicle.
Can I share a common chassis ground for a DC-DC charger?
Isolated DC-DC chargers do not share ground, whereas non-isolated models share a common negative. While chassis ground return is possible on non-isolated units, running a dedicated, full-size negative return cable directly between the starter battery and auxiliary ground busbar eliminates high ground-loop resistance and ensures stable smart-alternator voltage sensing.