A battery is only as useful as your ability to refill it. Most US van and RV builds use two or three charging sources so they're never stuck in any situation: solar for days parked in the sun, the alternator for travel days, and shore power for campgrounds and city stops.
This guide covers each source in technical depth — how they work, how to size them, how to wire them correctly, and how to coordinate all three when they're active simultaneously.
For what you're charging, see our house battery guide. For the complete system, see the electrical system guide.
Plan your charging sources
Pick solar, DC-DC and shore power and we'll size each one to your battery — free.
Why You Need More Than One Source
No single charging source is reliable for all situations:
Solar alone: Excellent in the Southwest in summer. Inadequate in the Pacific Northwest in winter. Useless when parked in a city garage or under tree cover for multiple days.
DC-DC alone: Charges every time you drive — reliable, but requires movement. Park in Moab for a week and your battery slowly drains.
Shore power alone: Unlimited 120V charging at RV parks, but you're dependent on hookup availability and paying campsite fees.
The right combination for US van life:
Minimum viable: 200–300W solar + 30A DC-DC charger. Covers most sun-belt van life year-round. Add shore power if you'll use RV parks regularly or need reliable winter charging.
Source 1: DC-DC Charger (Alternator Charging)
How It Works
A DC-DC charger (also called a battery-to-battery charger or B2B charger) draws power from the vehicle's starter battery — which is kept topped up by the alternator while the engine runs — and converts it to the correct charging voltage for your house LiFePO4 battery.
The sequence: engine running → alternator charges starter battery → DC-DC charger draws from starter battery → delivers a proper multi-stage charge to house battery.
The DC-DC charger monitors the starter battery voltage and only activates when the engine is running (starter battery voltage rises above a threshold, typically 13.1–13.3V). When the engine stops, the DC-DC charger disconnects immediately — your starter battery is protected even if the house battery is completely flat.
Why Not Just a VSR (Voltage Sensing Relay)?
VSRs (also called battery isolators or split charge relays) connect the starter and house batteries directly when the starter battery voltage crosses a threshold. On older vehicles with conventional alternators, they work. On modern vehicles, they don't work reliably.
The smart alternator problem:
Most vans built after 2015 — Ford Transit, Mercedes Sprinter, Ram ProMaster, Dodge Grand Caravan, Nissan NV — have smart (variable-voltage) alternators. These vary their output voltage to improve fuel economy: the alternator charges hard when regenerative braking opportunity exists (deceleration), then drops output when cruising to reduce engine load.
Smart alternator voltage can drop to 12.5–12.8V while cruising — below the VSR's closing threshold. The relay opens, disconnecting the house battery, even though the engine is still running. You're driving without charging your house battery.
A DC-DC charger doesn't depend on input voltage to stay active — it detects engine running via an ignition trigger wire and maintains output regardless of alternator voltage fluctuations. It handles smart alternators correctly; a VSR doesn't.
Check your van: Look up your specific model year and alternator type before specifying your solution. Some older ProMasters and certain vans still have conventional alternators where a VSR is acceptable. For everything 2015+ with a turbocharged engine, assume smart alternator and use a DC-DC charger.
Sizing the DC-DC Charger
DC-DC charger output current (amps) determines how fast your house battery charges from the alternator.
Rule of thumb: Size to approximately C/7 of your battery capacity. A 200Ah battery → 200 ÷ 7 ≈ 30A charger.
More precisely, consider how much energy you need to replace per day of driving:
| Daily Energy Use | Driving Time | DC-DC Size Needed | Energy Restored per Hour |
|---|---|---|---|
| 600Wh | 1–1.5 hrs | 20A | ~256Wh |
| 1,000Wh | 1.5–2 hrs | 30A | ~384Wh |
| 1,500Wh | 2–3 hrs | 40A | ~512Wh |
| 2,000Wh+ | 2–4 hrs | 50A+ | ~640Wh |
A 30A DC-DC charger at 12.8V delivers approximately 384W of charging power. If you drive 90 minutes per day, that's ~576Wh added — a meaningful contribution to a 1,200Wh daily budget.
Alternator capacity check:
Your alternator has a rated output (typically 150–220A for a Transit or Sprinter). The engine management system, HVAC compressor, headlights, heated seats, and other loads take priority. A DC-DC charger adds to the alternator's load. For a 30A charger, the actual draw from the alternator's perspective is slightly less (the charger isn't 100% efficient at input), but plan on 35–40A of effective alternator draw.
For a 50A DC-DC charger, verify your alternator can spare the current during typical driving load conditions. Most van alternators handle it comfortably; smaller alternators in heavily loaded vans may not.
US Product Picks
Victron Orion-Tr Smart 12/12-30A (~$287):
The most popular DC-DC charger for US van builds. Bluetooth configuration via Victron Connect, VE.Smart networking (coordinates with Victron MPPT controllers), and a proper LiFePO4 charging profile. The 30A output handles most builds. The Orion XS 12/12-50A (~$330) is the upgrade for larger systems.
Renogy 40A DC-DC Charger (~$200):
The Renogy DCC50S is a 50A DC-DC charger with an integrated MPPT solar input — solar panels can connect directly to the unit, which charges the house battery from both solar and the alternator simultaneously through one device. Good value if you want to simplify the install, though it's less configurable than the Victron.
Sterling B2B 30A (~$225):
Sterling Power is a UK company popular in the overlanding community. Solid build quality and proven reliability. Less ecosystem integration than Victron.
Wiring the DC-DC Charger
Four connections: Input positive (from starter battery), input negative, output positive (to house battery bus bar), output negative.
The cable from the starter battery to the DC-DC input runs through the engine bay and firewall. Use high-temperature wire rated for 105°C for the engine bay section — standard 75°C PVC insulation isn't appropriate for the heat near the engine.
AWG sizing for DC-DC charger cables:
| Charger Rating | Cable Run Length | Minimum AWG |
|---|---|---|
| 30A | Up to 10 ft | 10 AWG |
| 30A | 10–15 ft | 8 AWG |
| 50A | Up to 8 ft | 8 AWG |
| 50A | 8–15 ft | 6 AWG |
Fusing: Fuse the input cable (starter battery side) within 18 inches of the starter battery positive. Fuse the output cable within 18 inches of the house battery bus bar. Use MIDI fuses or ANL fuses for these circuits (not blade fuses — blade fuses aren't rated for sustained high DC current at the temperatures a DC-DC charger input cable may reach in an engine bay).
Ignition sense wire:
Rather than relying solely on voltage sensing to determine when the engine is running, connect a dedicated ignition sense wire from the DC-DC charger to a switched 12V circuit (one that is only live when the ignition is on). This is more reliable on smart alternator vehicles and ensures the charger starts and stops with the ignition rather than chasing voltage.
The ignition sense wire is small gauge (typically 18 AWG is fine) and carries minimal current — it's a signal wire, not a power wire.
See how to wire a DC-DC charger in a van for the complete step-by-step guide.
Source 2: Solar
Solar is covered in depth in our van solar setup guide. Key points from a charging system perspective:
How Solar Integrates with Other Sources
The MPPT controller's output connects to the same positive bus bar as all other charge sources. They don't conflict — each independently regulates its output voltage; they share the charging current the battery will accept.
When all sources are active (driving with solar panels producing and connected to shore power), the battery receives the combined current from all three sources simultaneously. A battery in bulk phase simply accepts all of it and charges faster.
US Solar Reality: Peak Sun Hours by Region
| Region | Annual Average Peak Sun Hours | Winter Minimum | Summer Peak |
|---|---|---|---|
| Southwest (AZ, NM, NV, S. CA) | 5.5–6.5 | 4.5–5.5 | 6.5–7.5 |
| Mountain West (CO, UT, WY) | 4.5–5.5 | 3.0–4.0 | 6.0–7.0 |
| Southeast (FL, TX, GA) | 4.5–5.5 | 3.5–4.5 | 5.5–6.5 |
| Mid-Atlantic / South | 4.0–4.5 | 2.5–3.5 | 5.0–5.5 |
| Midwest | 3.5–4.5 | 2.0–3.0 | 5.0–5.5 |
| Pacific Northwest (OR, WA) | 3.0–4.0 | 1.5–2.5 | 5.0–6.0 |
| Northeast (NY, MA, ME) | 3.5–4.0 | 1.5–2.5 | 4.5–5.5 |
| Alaska | 2.0–3.5 | 0.5–1.5 | 5.0–6.5 |
A 400W solar array in the Southwest in summer produces: 400 × 6.0 × 0.85 efficiency = 2,040Wh/day — more than most builds need. The same array in the Pacific Northwest in January: 400 × 2.0 × 0.85 = 680Wh/day — barely enough for a fridge.
This regional variation is why DC-DC charging is critical for builders who travel the full country, and why van lifers who winter in the Southwest can often rely on solar alone.
Source 3: Shore Power (120V Hook-Up)
What Shore Power Means in the US
US RV parks and campgrounds provide power at a dedicated "pedestal" — a weatherproof outlet box at the campsite. The two connector standards:
NEMA TT-30 (30-amp shore power): The standard for most RV parks. A three-pronged 30A/120V outlet. Total available power: 30A × 120V = 3,600W. This is enough for a large battery charger, an air conditioner, and basic loads simultaneously on separate circuits.
NEMA 14-50 (50-amp shore power): Found at premium sites and full-hook-up campgrounds. A four-pronged 50A/240V outlet providing two legs of 120V at 50A each — effectively 12,000W total. More than any van build needs, but useful if you want to charge at maximum speed.
Most van conversions use NEMA TT-30. A NEMA 14-50 to TT-30 adapter (~$25) lets you use 50A pedestals with a 30A setup, limited to 30A.
The Converter/Charger
The device that converts 120V AC shore power to 12V DC to charge your house battery is called a converter or converter/charger (distinct from an inverter, which goes the other direction).
For LiFePO4 batteries, the converter/charger must:
- Have a configurable absorption voltage (14.2–14.4V for LiFePO4)
- Have no mandatory equalization phase (equalization at 15V+ destroys LiFePO4 cells)
- Ideally have a dedicated LiFePO4 charging mode
Sizing your converter/charger:
| Charger Output | Charging Power | Time to Charge 200Ah LiFePO4 (from 20%) |
|---|---|---|
| 20A | 256W | ~9 hours |
| 30A | 384W | ~6 hours |
| 40A | 512W | ~4.5 hours |
| 50A | 640W | ~3.5 hours |
For an overnight stay at a campground (8 hours), a 30A charger comfortably fills a 200Ah LiFePO4 from 20% to 100%.
US product picks:
| Model | Output | LiFePO4 Mode | US Price (2026) |
|---|---|---|---|
| Victron Blue Smart IP22 12/30 | 30A | Yes | ~$215 |
| Victron Blue Smart IP22 12/20 | 20A | Yes | ~$170 |
| NOCO Genius 40A | 40A | Yes | ~$200 |
| PowerMax PM4-55A | 55A | No — set custom voltage | ~$180 |
The Victron Blue Smart range has the best integration with Victron monitoring systems and true LiFePO4 profiles. The NOCO Genius 40A is a good standalone option with a proper LiFePO4 mode.
Warning on older converter/chargers: Many older RV converters (WFCO, Progressive Dynamics) designed for AGM apply equalization voltages (15V+) that damage LiFePO4. If upgrading an existing RV to LiFePO4, replace the converter/charger as well.
Shore Power Wiring: NEC Article 551
The 120V side of a shore power installation — the inlet, wiring to the breaker panel, GFCI protection, and grounding — must follow NEC Article 551 (Standard for Recreational Vehicles). This is the National Electrical Code section specifically for RVs and converted vehicles.
Key requirements:
- Shore power inlet must be rated for the amperage (30A TT-30 or 50A 14-50)
- Inlet must be GFCI-protected (or the outlets it feeds must be GFCI)
- A main breaker panel (or at minimum a properly rated breaker) must be installed between the inlet and the branch circuits
- All 120V wiring must be in proper NEC-compliant conduit or cable types (typically 12 AWG minimum for 20A circuits)
- The grounding conductor must be bonded correctly
GFCI (Ground Fault Circuit Interrupter) protection is the US equivalent of the UK's RCD. GFCI devices disconnect within 25 milliseconds of detecting a 5mA leakage current — protecting against electric shock. Install GFCI outlets or a GFCI breaker on all 120V circuits accessible in the van living area.
Grounding:
Unlike in the UK where the inverter neutral-earth bond is a significant concern, US practice (following NEC) grounds the 120V neutral to the vehicle chassis at the main panel or at the inverter/charger. When on shore power, the ground comes from the pedestal's equipment ground wire. When on inverter, the inverter's ground output bond must be confirmed. A proper transfer switch (or inverter/charger with built-in transfer relay, like the Victron MultiPlus) manages this automatically.
Hire or consult a licensed electrician for the 120V side. The DC output of a converter/charger is DIY-friendly. The 120V input wiring, panel, GFCI protection, and grounding involve shock and fire risk that warrants professional verification.
Using Shore Power in Europe (For Cross-Continental Travelers)
If you'll ship your van to Europe or drive from Canada into Mexico, the grid voltages and connector standards change. European campgrounds use 230V with CEE17 connectors (rated at 6A, 10A, or 16A per site). Your US shore power inlet won't plug into a European site without an adapter, and your 120V converter/charger can't run on 230V.
This is a complex scenario — most US van lifers traveling Europe buy a dedicated European-voltage converter/charger and use a travel adapter for the inlet. Consult an RV electrician specializing in international travel.
Combining All Three Sources
Simultaneous Operation
When all three sources are active simultaneously — engine running with DC-DC active, solar panels producing, and plugged into shore power — all three share the charging current the battery will accept. Each regulates its output independently; none "fight" the others.
In bulk phase (battery below ~80% SoC), all three deliver maximum current simultaneously. If your MPPT produces 25A, DC-DC delivers 30A, and the shore charger delivers 30A, the battery receives 85A combined — charging very rapidly.
As the battery approaches full, each source's current tapers in proportion to the battery voltage rising toward the absorption target. All three naturally share the absorption phase together.
Victron VE.Smart Networking
Victron's "Smart" products can communicate over Bluetooth in a VE.Smart network. This enables:
Synchronized absorption: When one device (say the MPPT) detects the battery is at absorption voltage, it broadcasts this to all other Victron devices in the network. All devices transition to absorption simultaneously, preventing any single source from overcharging while others are still in bulk.
Battery voltage accuracy: The MPPT controller naturally senses voltage at its output terminals, which includes cable resistance losses. With VE.Smart enabled, the battery monitor (SmartShunt) measures the actual battery terminal voltage and shares it over the network. All chargers then regulate to the true battery voltage, not the voltage at their terminals.
Temperature-compensated charging: If a battery temperature sensor is connected to one device, it broadcasts the temperature. All devices adjust their charging voltage to compensate — critical for winter camping where cold cells need slightly lower absorption voltage.
To enable VE.Smart: in the Victron Connect app, join all devices to the same VE.Smart network (under network settings in each device's configuration). One device acts as master (typically the MPPT); the others follow.
When to Use an Inverter/Charger
If you'll regularly use both an inverter and shore power, an inverter/charger (like the Victron MultiPlus) combines both functions in one unit with an automatic transfer switch. When shore power is available, the MultiPlus:
- Passes shore power directly through to your 120V loads (with no efficiency loss)
- Uses the excess shore power capacity to charge the battery
- Disconnects the inverter function
When shore power is removed, it instantly (within 20ms) switches to inverter mode — your loads don't notice the transition.
This is cleaner than separate inverter + converter/charger because:
- The transfer switch is built in — no manual switching
- The neutral-earth bond is managed automatically
- Shore power and inverter can't accidentally feed each other
See van inverters & 120V power guide for inverter/charger details.
Sizing Your Complete Charging System
Working through a complete example:
Van build: 200Ah LiFePO4 battery, 1,200Wh daily consumption, full-time van life, travels the western US.
Required daily charging: 1,200Wh (consumption) + ~50Wh (losses) = 1,250Wh/day to maintain charge.
Solar contribution (assuming 4.5 peak sun hours, mountain west average):
- 300W array × 4.5h × 0.85 efficiency = 1,148Wh/day from solar
- This nearly covers daily consumption on average
DC-DC contribution (30A charger, 1.5 hours driving/day):
- 30A × 12.8V × 1.5h = 576Wh/day from driving
- Combined with solar: 1,148 + 576 = 1,724Wh — comfortable surplus most days
Result for this build: 300W solar + 30A DC-DC charger handles average conditions with buffer. Add shore power for cloudy streaks or when camped in shade.
Troubleshooting
DC-DC charger won't start:
- Engine running? The Orion monitors input voltage — starter battery must be above ~13.1V to trigger start
- Ignition sense wire: confirm it reads 12V when ignition is on
- Check input fuse (starter battery side) — common failure point
- Check Victron Connect app — the Orion reports its state (standby/bulk/absorption/error)
Solar MPPT producing less than expected:
- Partial shade — even a narrow shadow across one cell can reduce output significantly
- Check panel Voc at the MPPT input — if reading battery voltage rather than panel voltage, there's a wiring issue
- Verify MPPT is configured for correct battery chemistry
- Controller may be current-limiting due to high battery temperature or voltage near absorption level — this is normal behavior when battery is nearly full
Shore power charger not starting:
- Check the inlet for correct voltage (measure between live and neutral — should read ~120V)
- Check GFCI outlet — may have tripped; press reset button
- Check the charger's indicator or fault codes — Victron Blue Smart has LED error codes documented in the manual
- Verify shore power cable is firmly seated at both ends
Battery depleting despite all chargers active: Your consumption exceeds charging input. Calculate actual daily Wh consumption from battery monitor history and compare to measured charging output. If solar is producing less than expected, check for shade. If DC-DC isn't activating, check the troubleshooting steps above.
Related Guides
- House battery guide — what you're charging, and how to size it
- Van solar setup guide — your primary charging source
- Van wiring & safety guide — safe wiring of charging circuits in AWG
- Van inverters & 120V power guide — inverter/charger integration
- Complete van electrical system guide — the full overview
FAQ
Can I charge my van battery while driving?
Yes — that's what the DC-DC charger does. It pulls power from the alternator (via the starter battery) and charges your house battery at the correct multi-stage profile. A 30A unit adds approximately 384Wh per hour of driving.
Do I need a DC-DC charger, or can I connect the batteries directly?
For modern vans (2015+) with smart alternators, you need a DC-DC charger. Connecting batteries directly with a VSR (split charge relay) doesn't work reliably — smart alternators drop output voltage when cruising, which opens the relay and stops charging even with the engine running. See the smart alternator explanation above.
What size shore power inlet do I need?
NEMA TT-30 (30A) is correct for most van conversions — it provides up to 3,600W of 120V power, more than enough for a 30A converter/charger and other 120V loads simultaneously. Install it with a matching 30A TT-30 power cord.
Can all three charging sources run at the same time?
Yes. All three connect to the same battery bus bar and can operate simultaneously. Each independently regulates its output; they share the charging current the battery accepts. Combined output is the sum of all three — faster charging when all sources are active.
What's the difference between a converter and an inverter?
A converter (or converter/charger) turns 120V AC shore power into 12V DC to charge your battery. An inverter does the opposite — it turns 12V DC battery power into 120V AC for household outlets. An inverter/charger (like the Victron MultiPlus) does both, with an automatic transfer switch between the two modes.
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