A DC-DC charger controls how your auxiliary battery is charged, so it suits lithium batteries and vehicles with smart alternators. A battery isolator or combiner is a simpler switch that connects the batteries when the voltage allows. The right choice depends on your alternator, your battery type and how much you rely on the second battery.
DC-DC Charger vs Battery Isolator at a Glance
| DC-DC charger | Battery isolator or combiner | |
|---|---|---|
| What it does | Charges the auxiliary battery using a set charge profile | Connects the batteries when voltage conditions are met |
| Charge control | Multi-stage charging | No charge profile of its own |
| Battery type | Often matched to a chemistry, such as lithium | Check your battery suits the voltage the alternator supplies |
| Smart alternators | Some models list a smart alternator mode | Check the manufacturer's instructions |
How a Battery Isolator Works
An isolator keeps your start battery and auxiliary battery separate, so you can run a fridge or lights without flattening the battery that starts the engine. When the alternator is charging, it joins the two batteries. The Intervolt 100A Electronic Isolator is a solid-state unit rated at 100A continuous, with adjustable voltage and time delay. The Victron Cyrix-ct 12/24V-120A Battery Combiner detects 12V or 24V automatically.
How a DC-DC Charger Works
A DC-DC charger sits between the batteries and manages the charge itself. The Projecta IDC25 3-Stage is listed with smart and conventional alternator modes, a 9-32V input, 25A output and IP67 sealing. The Projecta IDC25L 5-Stage Lithium has the same alternator modes and is listed for lithium iron phosphate batteries.
How to Choose
Choose a DC-DC charger if you have lithium batteries, a smart alternator or want a controlled charge for your auxiliary battery.
Consider an isolator if you have a conventional alternator, a compatible battery type and want a simple setup.
Not sure? Check your vehicle manufacturer's information on its alternator type before buying.
Our dual battery systems guide covers the wider setup.
Wiring and Safety
Fit a fuse or breaker close to each battery. A fuse protects the cable, so never fit one larger than the cable can carry.
Size cables for the current and the run length, because longer runs cause more voltage drop.
See our MIDI fuse sizing guide and circuit breaker guide.
Follow the manufacturer's instructions. If you're unsure, ask an auto electrician.
Pick the Right Charging Setup
The DC-DC charger vs battery isolator choice comes down to your alternator, your battery and your budget for wiring.
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