12V Ute Guide

What size DC-DC charger do I need?

Short answer: size the charger to suit your battery first, then check it can put back what you use in the time you actually drive. For lithium, a common guide is about 30% of capacity; for AGM or lead-acid, about 20%.[1] Your battery's spec sheet always wins.

Method: a battery-friendly charge rate of about 30% of capacity for lithium and 20% for lead-acid (Enerdrive's rule of thumb), compared with the current needed to replace your daily use in your drive time. Rounded up to a common size (12, 20, 25, 40 or 50 A). Your battery's own spec overrides this.

Step 1: what your battery wants

Enerdrive suggests a charge current of around 20% of capacity for lead-acid and 30% for lithium, for both performance and lifespan. On that basis, a 200 Ah lithium battery suits about 60 A and a 200 Ah lead-acid battery about 40 A.[1]

Manufacturers often publish two numbers: a maximum charge current and a lower recommended current for long life. Enerdrive's B-TEC G2 manual, for example, lists a recommended charge current of 33 A or less for its 100 Ah battery and 60 A or less for its 200 Ah battery, well under the short-term maximums.[4] Size to the recommended figure.

Step 2: what you use and how long you drive

If you use 50 Ah a day and drive three hours, you need roughly 50 ÷ 3 ≈ 17 A going into the battery. If you only drive one hour, you'd need 50 A, which most small batteries shouldn't take. In that case the answer is more solar or a bigger battery, not a bigger charger.

Step 3: allow for loads while driving

A charger feeds running loads first; only what's left charges the battery. Enerdrive's example: 10 A of loads on a 20 A charger leaves 10 A for charging.[1] The calculator adds your driving loads on top.

Step 4: check the vehicle side

A bigger charger draws more current from the alternator and needs heavier cable and fusing. Enerdrive recommends its 40 A setting for systems with an appropriately sized alternator, cables and fusing.[2] Use the cable size calculator to check your run, and see 25A vs 40A if you're torn between the two most common sizes.

Smart alternator? Solar?

If your vehicle has a variable-voltage ("smart") alternator, you need a DC-DC charger that's wired to support it. See smart alternators and dual batteries. If you want solar too, see solar input explained.

FAQ

What size DC-DC charger do I need for a 100Ah lithium battery?

Enerdrive's general guide is a charge current of about 30% of a lithium battery's capacity, so roughly 30 A for 100 Ah. In practice that usually means a 25 A charger, or a 40 A charger turned down if the battery's spec allows. Check your battery's recommended charge current first.

Can a DC-DC charger be too big for my battery?

Yes. If a charger's output exceeds the battery's recommended charge current, choose one with adjustable output and set it to the battery's spec.

Do loads like a fridge reduce the charge going into the battery?

Yes. A charger supplies running loads first and the rest goes into the battery, so a 4 A fridge on a 25 A charger leaves about 21 A for charging.

Sources

  1. Enerdrive Support: What size battery charger do I need?
  2. Enerdrive Support: How do I program my DC2DC charger for lithium?
  3. REDARC Help Centre: How to use a BCDC charger to charge a secondary battery in any vehicle
  4. Enerdrive B-TEC G2 lithium battery manual (PDF, hosted by Batteries Direct)

This is general information, not a substitute for the manuals for your specific charger, battery and vehicle. Fixed wiring and anything near the starter battery should be installed by a qualified auto electrician.

Last updated: 15 October 2026