If your vehicle was built in the last decade or so, there’s a good chance a simple battery isolator won’t charge an auxiliary battery properly. Smart alternators change their output voltage to save fuel, and lithium batteries need a specific charging profile. A DC-DC charger solves both problems. It takes whatever the alternator gives it and turns it into the right charge for your second battery. Here’s how to choose one.
Why a simple isolator often isn’t enough
An isolator just connects the two batteries while the engine runs. That works when the alternator holds a steady, high voltage. Modern smart alternators don’t: they lower their voltage when the starter battery is full, so the auxiliary battery at the back of the vehicle, on a long cable, may never get a full charge. Lithium batteries add another problem, because they need a charge profile that an alternator alone doesn’t provide.
A DC-DC charger sits between the two batteries, boosts and regulates the voltage, and charges the auxiliary battery in stages like a proper battery charger. It also stops the auxiliary battery from draining your starter battery when the engine is off.
Step 1: Choose the charging current
DC-DC chargers are usually sold as 20A, 25A, 40A and 50A units. As a rough guide, the number tells you how many amp-hours it can put back for each hour of driving, as long as the battery can accept that current.
Start with your daily use. If your fridge, lights and devices use around 70Ah a day, here’s roughly how long each size needs to put that back:
| Charger | Driving time to replace 70Ah | Typical fit |
|---|---|---|
| 20A | About 3.5 hours | Small batteries, light loads |
| 25A | About 3 hours | A popular all-round size for touring setups |
| 40A | About 1.75 hours | Larger lithium banks, bigger daily loads |
| 50A | About 1.5 hours | Big setups, short driving days, campers and caravans |
If you drive several hours most days, 25A is often enough. If you drive an hour and then stay put for days, a bigger charger, or solar, matters more. Not sure what you use? Our LiFePO4 sizing guide shows how to add it up.
Step 2: Check the battery and alternator can keep up
- Battery: every battery has a maximum recommended charge current in its datasheet. Pick a charger at or below that figure. Large lithium banks can usually take more current than small AGM batteries.
- Alternator: a 50A charger draws more than 50A from the alternator, because it has to make up for losses. On a small alternator that’s already powering lights, fans and a winch, that’s a big ask. Check your alternator’s rating before going big.
Step 3: Match it to your alternator type
Many DC-DC chargers need to know if your vehicle has a smart, variable-voltage alternator. Some detect it automatically; others use a setting or an ignition-sense wire so the charger knows when the engine is running even when the alternator voltage is low. This is the most common reason a new system “doesn’t charge”, so follow the installation manual for your exact vehicle.
Step 4: Decide where it lives
Chargers come in two broad types:
- Under-bonnet (engine bay) units are sealed against water and dust and rated for engine heat. Mounting near the starter battery keeps the high-current cable short.
- In-cabin units mount in the cargo area or under a seat, close to the auxiliary battery. They’re often cheaper, but some are not rated for engine bay installation at all. REDARC, for example, lists its BCDC Core range as unsuitable for the engine bay.
Most chargers reduce their output when they get hot, so give them airflow wherever they’re mounted.
Step 5: Solar input
Many DC-DC chargers include an MPPT solar regulator, so the same box charges from the alternator while driving and from solar when parked. If you plan to add panels, this saves buying a separate regulator. Some models prioritise solar when both are available to reduce the load on the alternator.
Step 6: Wire it properly
- Use the cable size the manufacturer specifies for your cable length. Long runs to the back of a vehicle need thick cable.
- Fuse the cable at both battery ends, close to each battery.
- Get a solid earth connection.
- Fit a battery monitor so you can see whether it’s actually charging.
If wiring isn’t your thing, a pre-wired power panel keeps the charger, fuses and monitor together. We covered one option in our look at REDARC’s REDWorks panels. For the bigger picture, see our complete 12V power guide.
Quick answers
Do I need a DC-DC charger with a lithium battery?
In most cases, yes. It gives the lithium battery the right charge profile and works with smart alternators.
Is a bigger DC-DC charger always better?
No. It has to suit your battery’s maximum charge current and your alternator’s spare capacity. Bigger only helps if both can handle it.
Can I mount any DC-DC charger in the engine bay?
No. Only units rated for engine bay installation should go there. Check the manufacturer’s specifications.
Sources
Always follow the installation manual for your charger and vehicle, and use a qualified auto electrician if you’re unsure.
Featured photo: Finding Dan | Dan Grinwis on Unsplash.
