Guides

DC‑DC Charging for Campervans & Motorhomes in NZ: What It Does, Why You Need It, and How to Size It

Fri, 17 Apr 2026 22:39:26 GMT

Charge while you drive—DC‑DC + MPPT explained, with quick sizing and install guidelines

A DC‑DC charger is the safest, most reliable way to charge your house batteries from the alternator while you drive. It protects modern alternators, gives your AGM or lithium (LiFePO4) the right charge profile, and works alongside your solar MPPT, not instead of it. This guide covers the why, how and what‑size questions.

What a DC‑DC charger does

Regulates charge properly: Delivers multi‑stage charging matched to AGM or lithium, so you reach full charge without over/under‑charging.
Plays nice with smart alternators: Modern vehicles vary alternator voltage; a DC‑DC maintains a steady charge current anyway.
Protects starting battery: Prioritises the starter and isolates the house bank when the engine’s off.
Safer than a simple VSR/isolator: VSRs connect batteries; with lithium’s high current draw, that can overwork alternators and under‑charge or overheat cables.

Why do you still need DC‑DC even with solar

Different sources, same goal: Solar charges when the sun’s out; DC‑DC charges whenever you drive. Together, they reduce reliance on generators and shore power and reach full charge faster.
Cloud, shade and winter: Driving top‑ups cover days when solar is weak.
Healthy charge cycles: Lithium and AGM both benefit from proper absorption/finish charging, DC‑DC helps you get there on travel days.

DC‑DC + MPPT: how they work together

Parallel charge sources: Both feed the house battery (through proper fusing). The higher‑voltage source will do more work; they naturally “share” as the battery voltage rises.
Keep monitoring accurately: Route all negatives through your battery shunt so your State of Charge is correct (MPPT, DC‑DC, inverter and loads).

What size DC‑DC charger do I need?

Typical sizes: 30 A, 40 A, 50–60 A at 12 V.
Quick picks (12 V house bank):
Up to ~150–200 Ah LiFePO4/AGM, light–moderate loads: 25–30 A
200–300 Ah LiFePO4/AGM, touring loads (fridge, lights, pumps, small inverter): 40–50 A
Balance three limits:

Battery charge rating (C‑rate): Don’t exceed what your battery maker allows (lithium often 0.5C or more; AGM usually much lower).
Alternator spare current: Leave a safety margin for vehicle systems; many alternators can spare ~30–60 A continuously, but verify for your model.
Wiring and heat: Higher currents need larger cables and better cooling.

Wiring, fusing and cable sizes (simple rules)

Keep runs short and fat: Voltage drop kills charge rate. Use quality copper (avoid CCA).
Typical guides (12 V, short runs):
30 A: 10 mm² (AWG 8) with MIDI/MEGA fuses sized to protect the cable
40–50 A: 10–16 mm² (AWG 8–6)
60 A: 16 mm² (AWG 6)
Fuse at both ends when both ends can source power:
Starter side: protect the cable near the vehicle battery/alternator feed
House side: protect near the house battery/busbar
Good earths matter: Use a dedicated negative return of the same size back to the source or to a solid chassis/busbar as designed—no sketchy grounds.

Where to mount the DC‑DC

Close to the house battery to minimise voltage drop on the output side.
Cool, ventilated, dry location; not in hot engine bays or sealed lockers.
Accessible for service; follow the maker’s clearance/cooling guidance.

Setup basics

Select the correct chemistry/profile (LiFePO4 or AGM) and charger current.
Use ignition/sense wiring if required by your vehicle’s smart alternator.
Enable low‑temp protection for lithium (via BMS or charger settings if available).
Confirm charge voltage at the battery terminals under load; adjust settings if your wiring has an unavoidable drop.

Common mistakes to avoid

Undersized cables: Leads to low charge current and hot wires.
No fuse (or only one): Protect both ends where applicable.
Ground shortcuts: Poor returns create a voltage drop and unstable charging.
Mixing VSR with lithium: Can overwork alternators and stress wiring.
Bypassing the shunt: Your battery monitor will read incorrectly if any negative voltage doesn’t pass through it.

FAQs

Do I need DC‑DC if I already have solar?
Yes. Solar can’t charge at night or on bad weather days. DC‑DC ensures reliable charging whenever you drive.
Will a DC‑DC harm my alternator?
No, sized correctly, it caps current draw and protects the alternator better than a simple isolator.
Can I charge lithium from the alternator safely?
Yes, with a DC‑DC set to a lithium profile and appropriate cabling/fusing.
What if I only do short trips?
Even short drives can add useful amp‑hours. A higher‑amp DC‑DC (within limits) helps recover faster between stops.
Where should my negatives connect?
Through the battery shunt on the “system” side so your monitor reads all charge/discharge accurately.

DC Wire Sizes for RVs: mm² vs AWG, Voltage Drop, and Quick-Pick Charts

Thu, 16 Apr 2026 22:20:29 GMT

Simple rules for sizing RV DC cables: mm² vs AWG, voltage drop, and when series beats parallel

mm² vs AWG

International: cable size is cross-sectional area in mm².
US: AWG is used—lower AWG number means thicker cable.

How to pick a cable size (3 steps)

Determine load current (A) and round-trip cable length (out and back).
Pick a voltage-drop target:
1–2% for inverter/battery main feeds
2–3% for most 12 V branch runs
Use a voltage-drop calculator or chart to size the conductor for the run, then verify the wire’s ampacity (continuous current rating) and always fuse to protect the wire. When in doubt, go up one size.

Why higher voltage helps (smaller cable, higher efficiency)

For the same power, doubling the voltage halves the current.
Lower current reduces voltage drop and heating, allowing a smaller, cheaper cable for the same run.

Where to use higher voltage

Solar array to MPPT: Wire panels in series (within controller limits) to raise PV voltage and cut current.
24 V house banks: Halves current on main DC feeds vs 12 V if your gear supports 24 V.

Watch-outs

Stay below the controller’s max PV Voc at the coldest expected temperature.
Ensure appliances and chargers are compatible with 24 V; plan DC-DC converters for 12 V loads and alternator charging if you run a 24 V bank.

Solar wiring: series vs parallel

Series (often best on RV roofs with minimal shade)
Pros: Higher voltage/lower current → smaller cables, less drop, fewer penetrations, good MPPT operation.
Cons: Shade on one panel reduces string current (bypass diodes help but don’t eliminate it); must respect max cold Voc.
Parallel
Pros: Shade on one panel affects that panel mostly; some redundancy.
Cons: Higher current → larger cables, more fusing/combiner hardware, more voltage drop.
Rule of thumb: If shading is limited and your MPPT allows it, use 2–3 panels in series. If shading is common, consider parallel (with proper string fusing) or mixed strings.

12 V vs 24 V house banks

12 V: Most common; widest appliance support.
24 V: Better for larger systems (big inverters/loads) thanks to lower current on DC feeds. Plan DC-DC for 12 V loads and for alternator charging.

Safety must-dos

Use tinned, automotive/marine-rated copper where appropriate; avoid aluminium for RV DC runs.
Fuse or breaker every positive feed as close to the source as practical to protect the cable.
Keep high-current runs short; use correct lug size, proper crimp tooling, heat-shrink, torque, and strain relief.
Account for temperature, bundling, conduit, and duty cycle when sizing.
Hire a licensed electrician for any 240 V AC work.

This chart is a general guide only. Verify cable sizes and fuse ratings with manufacturer datasheets and applicable standards. Adjust for heat, bundling and duty cycle. Ampacity does not account for voltage drop, long runs often need larger wire than ampacity alone suggests.

Accurate RV Battery Monitoring for Caravans, Motorhomes and Campervans

Thu, 16 Apr 2026 01:25:36 GMT

Victron SmartShunt for RVs — What It Is, How It Works, and Setup Tips

If you want reliable State of Charge (SoC) and clear insight into RV power use, a shunt-based monitor is the gold standard. Below is a concise guide to what a shunt does, why the Victron SmartShunt is popular, how to size it, where to install it, and how to set it up with VictronConnect. Professional supply, installation, and configuration available across Auckland.

What a battery shunt does (and why it beats a voltmeter)

A shunt is a precise resistor installed on the battery negative that measures all current in and out of the battery.
The monitor “coulomb counts” (tracks amp‑hours) for accurate SoC, time‑to‑go, and charge/discharge history, far more useful than voltage alone, especially with lithium.

Why choose the Victron SmartShunt

Built‑in Bluetooth: View SoC, amps, volts, and history in the VictronConnect app—no separate display needed.
Flexible inputs: Monitor a second voltage (e.g., starter battery) or use midpoint monitoring on a series bank.
Integrates well with solar MPPTs, DC‑DC alternator chargers, shore chargers, and inverters.
Compact for tidy installs close to the battery.

Model options and sizing

500 A (common for most RVs), 1000 A (larger inverter banks), 2000 A (heavy/specialist systems).
Choose based on your peak DC current. For inverters up to ~1500–2000 W in typical RVs, 500 A is usually sufficient; larger systems may need 1000 A.
Match with correct cabling, lugs, and DC protection for your maximum currents.

Where the SmartShunt fits

Install on the main house battery negative.
Battery side: Only the battery negative connects here.
System/load side: Every house load and charger negative must be on this side (solar MPPT, DC‑DC from alternator, inverter, DC loads, shore charger). Any bypass will cause incorrect readings.
Mount close to the battery in a dry, accessible, ventilated area. Avoid engine bays and wet/spray‑exposed locations.

Setup in VictronConnect (key steps)

Battery capacity: Enter usable Ah (e.g., 200 Ah LiFePO4). For lead‑acid, consider realistic usable capacity.
Chemistry parameters: Set charged voltage, tail current, Peukert exponent, and charge efficiency appropriate to your battery (near‑unity efficiency for lithium; Peukert and efficiency matter more for AGM/lead‑acid).
Synchronise SoC: Fully charge the battery (current tapered, charger in float), then use Synchronise to set 100%.
Optional: Configure the second input for starter battery voltage or midpoint monitoring.

Tips for accurate readings

Periodically reach a true full charge so the monitor can resync and maintain SoC accuracy.
If readings seem off, look for any negative leads bypassing the shunt—a common install mistake.
Ensure DC‑DC charger and MPPT negatives both pass through the shunt.

SmartShunt vs BMV‑712

SmartShunt: Best for app‑only monitoring and a minimal, hidden install.
BMV‑712: Adds a physical display plus Bluetooth for at‑a‑glance data without opening the app.

Need help?
We supply, install, and configure Victron systems across Auckland and can optimise your setup for accuracy and safety.

Victron MPPT Charge Controllers for RVs and Campervans

Thu, 16 Apr 2026 00:15:20 GMT

MPPT vs PWM, Features, 100/30 Explained

If you’re upgrading an RV, caravan, motorhome or campervan solar system, the charge controller you choose has a big impact on performance and battery life. This guide explains why MPPT controllers outperform PWM in most real‑world setups, highlights useful Victron SmartSolar features, and demystifies model numbers like “100/30.” For installation help, we offer mobile on‑site service across Auckland.

What does a solar charge controller do?

It regulates solar panel output to safely charge your battery bank (AGM or LiFePO4).
It applies the right charging stages (bulk/absorption/float) to protect battery health.
It provides protection and monitoring so you can see how your system performs.

MPPT vs PWM — why MPPT usually wins

How PWM works: A PWM controller essentially connects the panel to the battery and “pulses” to control voltage. The panel is forced close to battery voltage, so you lose the extra voltage potential your panels could provide.
How MPPT works: An MPPT controller constantly tracks the panel’s Maximum Power Point (Vmp/Imp) and uses DC‑DC conversion to step the higher panel voltage down to the battery’s charging voltage while increasing current. Result: more harvested energy from the same panels, especially in cool weather, partial shading, or when roof space forces non‑ideal wiring.
Typical gains: MPPT can deliver noticeably higher daily energy than PWM in RV use (often double‑digit percentage improvements), which translates into faster recharging and smaller risk of running flat on cloudy days.
When PWM can be fine: Very small, budget systems with a single 100 W panel and modest loads. For most NZ touring setups with 200–600 W and lithium batteries, MPPT is the better long‑term choice.

Victron SmartSolar MPPT — features and benefits

Bluetooth app monitoring: Pair with the VictronConnect app to see solar harvest, history, live charge current, and to adjust charge profiles. No separate display required.
Lithium‑friendly charging: Presets and custom profiles for LiFePO4 or AGM with correct bulk/absorption/float targets, and protections to avoid over‑ or under‑charging.
Efficient tracking: Fast MPPT algorithms help maintain higher output during variable conditions (passing clouds, partial shading).
Flexible panel choices: Safely run higher‑voltage panel strings (within controller limits), useful when roof space or cable runs make single‑panel, low‑voltage wiring inefficient.
Integrates well with the rest of the system: Works alongside DC‑DC charging from the alternator, inverter/chargers, and battery monitors. Good system design keeps all charging sources balanced so your batteries charge quickly and safely.

What “100/30” means on a Victron MPPT

The first number (100) is the maximum PV open‑circuit voltage (Voc) that the controller can accept. In this case, 100 V max. You must design your panel strings so their total Voc, adjusted for cold temperatures, never exceeds 100 V.
The second number (30) is the maximum charging current the controller can deliver to the battery: 30 A.
Practical PV sizing: Because MPPT steps down the voltage, the allowable PV wattage is higher than 12 V × 30 A. As a rule of thumb, Victron’s recommended array size for a 100/30 is roughly 440 W on a 12 V battery bank or 880 W on a 24 V bank. Always confirm with the latest datasheet and account for real‑world factors (temperature, wiring losses, shading).

Series vs parallel with a 100/30 (quick tips)

Series increases voltage and lowers current on the PV side, which reduces cable losses and suits MPPT. Ensure the total cold Voc stays under 100 V.
Parallel keeps voltage low and increases current, which can demand thicker cables and higher‑amp protection but may reduce shading impact from one panel.
Many RV roofs end up with two panels in series (if safe under cold Voc), feeding a 100/30, or larger arrays stepping up to a 100/50.

Sizing examples for typical NZ RVs

Weekend caravan or campervan: 200–300 W of solar, 100–200 Ah LiFePO4, often a 75/15, 100/20 or 100/30 controller depending on expansion plans.
Touring motorhome: 400–600 W of solar, 200–300 Ah LiFePO4, a 100/30 or 100/50 controller, integrated with DC‑DC (alternator) and shore charging.
Heavy AC use (e.g., induction, espresso): Larger arrays plus higher‑current controllers (100/50+) and appropriately sized inverters, batteries, and cabling.

Installation pointers

Use correct cable sizing, DC‑rated fusing/isolators, tidy cable routing, and watertight roof glands. Good workmanship matters for reliability and safety over bumpy NZ roads.
Combine solar with DC‑DC charging from the alternator for balanced charging while driving.
Configure the controller for your battery chemistry and temperature conditions; verify absorption/float times for lithium vs AGM.
If you’d prefer a pro install, we supply, fit, configure and troubleshoot MPPT controllers on‑site across Auckland.

Common questions

Can I “over‑panel” a 100/30? Within reason, yes. It’s common to install PV wattage a bit above the controller’s ideal rating because the controller will current‑limit at 30 A. You must still stay under 100 V Voc (cold) and use suitable wiring/protection.
Do I need DC‑DC if I have MPPT? Yes, if you want proper alternator charging while you drive. MPPT manages solar; a DC‑DC charger safely manages alternator output to your house batteries.
Is MPPT worth it with lithium? Yes. Lithium’s ability to accept higher charge currents pairs well with MPPT’s improved harvest and faster bulk charging.
Will MPPT fix shading? It can mitigate some loss, but shading still reduces output. Thoughtful panel placement and realistic expectations matter.

We provide mobile RV solar installation, upgrades, and MPPT setup across Auckland, no workshop visits needed.