DC-DC Charger Sizing Calculator
A DC-DC charger is one of the biggest upgrades you can make for a reliable dual-battery setup. It helps charge your auxiliary battery properly while driving, even in modern vehicles with smart alternators, and it can protect battery health by delivering the correct charge profile.
Our DC-DC Charger Sizing Calculator estimates a practical charger size (in amps) based on your battery bank, typical depth of discharge, and how many hours you usually drive between camps. Use the result as a planning guide, then match it to a charger that suits your touring style.
DC-DC Charger Sizing Calculator
Estimates are a planning guide. Real charge rates depend on battery chemistry, temperature, wiring size/length, alternator behaviour, and charger limits.
DC-DC chargers that match your results
Based on your inputs, here are DC-DC chargers that suit your setup. Not sure? Give us a call and we’ll help confirm the right fit.
How to use this calculator
- Select your battery type. Lithium typically allows deeper usable capacity than AGM/lead. If you’re using a custom setup, choose Custom and set the usable % you’re comfortable using.
- Enter your battery bank size (Ah). This is your auxiliary battery capacity (or combined bank capacity if you run multiple batteries).
- Set your average depth of discharge (DoD). Think of this as how much of your usable battery you typically use between charges (for example, 30–50% is common for touring).
- Enter your usual driving / charge time. Estimate the hours you typically drive between camps or stops where you want the battery topped back up (for example, 2–4 hours for touring days).
- Set charge efficiency. This accounts for real-world losses (heat, wiring, conversion losses). A practical default is 85%.
- Choose your trip style. Weekend / Touring / Off-grid helps match you to the right charger tier.
- Select alternator type (guide only). Modern vehicles often use smart alternators that vary voltage — DC-DC chargers help maintain stable charging.
What you’ll see:
- Usable battery capacity (Ah) - your bank capacity adjusted for safe usable percentage.
- Energy to replace (Ah) - the typical amount you’ll want to put back during your drive time.
- Minimum charger output (A) - the idealised current required to replace that energy in your available hours (after losses).
- Suggested DC-DC size (A) - a practical charger size rounded up to common real-world options.
Quick example
Let’s say you have a 200Ah lithium auxiliary battery, and you usually use around 40% before driving again:
- Usable capacity ≈ 200Ah × 80% = 160Ah
- Energy to replace ≈ 160Ah × 40% = 64Ah
- If you drive 4 hours and allow 85% efficiency, you’ll often land around the 20–25A minimum range
- A practical recommendation may be a 25–40A DC-DC charger depending on your inputs and desired recovery speed
Tips for choosing the right charger
- Bigger isn’t always better: oversized chargers can push more current than some batteries like to accept — always check battery manufacturer recommendations.
- Wiring matters: long cable runs and undersized cable can reduce real charging performance. Good cabling can be the difference between a 40A charger behaving like a 25A charger.
- Smart alternator vehicles: a DC-DC charger is commonly recommended to maintain reliable charge voltage.
- Solar input models: DC-DC chargers with MPPT can simplify your setup if you also run solar.
- Heat and mounting location: under-bonnet installs may derate in high temperatures — consider airflow and mounting position.
If you’d like help matching the right DC-DC charger, wiring, and fusing for your battery and vehicle, contact Battery Force — we’ll help you build a setup that performs in real WA conditions.