Search for advice on DC-DC chargers and almost everything you find comes from someone who sells them. Retailers, brands, and the occasional workshop that fits them for a living. That’s not a conspiracy, it’s just how the internet works — but it does mean the answer to “do I actually need one of these” is usually written by someone with a commercial interest in you saying yes.
So here’s the version from someone who has no interest in selling you one. I run them, I’ve wired them, I’ve watched them fail on my own vehicle and on other people’s, and there are setups out there that genuinely don’t need one.
What a DC-DC charger actually does (and what an isolator doesn’t)
The short version: it takes whatever the alternator is producing and turns it into a proper charging profile for your second battery. It boosts the voltage when the alternator’s output drops, regulates it so it stays in the range your battery actually wants, and applies the charge in stages rather than dumping everything in at once.
Your alternator’s output was designed for one thing — a standard lead-acid starter battery sitting in the engine bay. Not a deep-cycle auxiliary battery in the back of the vehicle. An old-fashioned isolator, whether it’s a solenoid or a voltage-sensing relay (VSR), has no charging profile at all. It just connects the two batteries together when the engine’s running and hopes for the best.
That “hopes for the best” causes two problems.
Why isolators damage your starter battery
With an isolator, the two batteries are read together. So the system sees an average voltage across both of them.
Here’s the scenario. You’ve run your fridge and lights overnight. Your accessory battery is now lower than your starter, which is still full. The average of the two reads low. So the alternator keeps pushing charge into both batteries, with no regulation about how much goes where — and your starter battery, which was already full, gets overcharged.
The overcharging doesn’t happen overnight. It happens on your next driving day, when the alternator is actually charging and reading that averaged voltage. Not enough to boil your starter in one hit, but enough to shorten its life — and one day it might leave you stranded somewhere you really don’t want to be. The first system I ever wired up myself used an electromagnetic solenoid as the isolator and nothing else. It worked for a long time. It was also slowly cooking my starter battery the whole way.
Why isolators don’t work with smart alternators
Isolators and smart alternators don’t work together. At best they work some of the time — which, as a charging system, is another way of saying they don’t work. That’s the part of the story everyone talks about.
But it’s only part of it. The charging control problem existed long before smart alternators did: even with a traditional fixed-voltage alternator, an isolator was still dumping raw output into a battery it wasn’t designed for. Smart alternators made DC-DC chargers popular. They aren’t the only reason they’re necessary.
Smart alternators: why DC-DC chargers became necessary
Modern emissions standards are pushing manufacturers into technology they wouldn’t otherwise choose. A smart — variable voltage — alternator drops its load at idle, when it isn’t needed, which reduces the engine load and therefore reduces fuel consumption and CO2.
To be precise about what it does and doesn’t do: this is a CO2 and fuel-efficiency story, not a noxious-emissions one. It doesn’t clean up your exhaust. What it does is help a manufacturer hit fuel consumption and CO2 targets — and in Australia that’s now a regulated thing, with the New Vehicle Efficiency Standard starting on 1 January 2025 and compliance requirements from 1 July 2025.
What the emissions rules actually changed
The point is that all of this happens in a test. A stock vehicle, no accessories fitted, being measured under controlled conditions. Dropping the alternator’s load while the vehicle sits at idle during a test cycle moves the measured number. That’s the mechanism, and it’s real — when Europe moved from NEDC to WLTP, measured CO2 rose by around 10% and the benefit of regenerative charging finally showed up properly.
What that has to do with your vehicle is a different question.
And this isn’t the first time that Euro-derived emissions regulations have made four wheel driving worse in Australia. Just look at what Diesel Particulate Filter (DPF) regulations are doing to us.
Why the “real world” saving is overstated
Here’s the illustration I keep coming back to. You’ve got a V8 Landcruiser, loaded with two tonnes of accessories, roof rack full, pushing all of that through the air at 100km/h with the drag of a garden shed. Or you’re in low range, flogging the tits off it through a boggy river bed. Or anything in between.
That vehicle is not going to notice, in any practical way, whether the alternator is charging at 12.2 volts or 12.8.
The official “real world” figures say otherwise, and that’s rather the point: the gap between what passes for real-world testing and what actually happens to a loaded touring vehicle is enormous. Fuel savings from smart charging are real in the test, and on a bare vehicle driving around town. On a fully loaded tourer, they’re a rounding error.
Which is why I say the emissions benefit is the only thing these alternators are really good for — because from the driver’s seat of a loaded 4WD, that’s the honest reading.
What matters for you is the practical consequence: your alternator now spends a lot of time deliberately not charging. Voltage drops to save fuel, and an isolator or relay has no way to force it back up. A DC-DC charger does, which is precisely why the modern advice exists.
If you can’t imagine what a fully-loaded 4WD will look like because you don’t have on yet, we do have a guide to choosing the best 4WD for real world conditions.
When you don’t need a DC-DC charger
The honest answer is narrower than the industry would like: a setup doesn’t need a DC-DC charger only if nothing is running while the vehicle is off.
No fridge. No overnight lights. No charging devices while you sleep. If you’re a day-tripper who goes home every night, your second battery isn’t doing much of anything, and a simple isolator will keep it topped up harmlessly. If you’re leaving the vehicle parked for a week with a fridge running, you need proper charging, and you need it to be reliable.
Most people reading this are in the second group. Which is why the rest of this article is about doing it properly.
What’s in my vehicle, and why I’m changing it
I’m running a Projecta 25A DC-DC. The honest reason is that it was available at Bunnings and I still had a credit account from my old pressure cleaning business. I was in a rush to get a new Cruiser ready before a tour and I’d already spent a fortune on everything else the build needed. Convenience won.
It was sufficient for about two years. Over the long term it hasn’t worked out. With a voltmeter on it, it’s simply not charging a lot of the time. Yes, these units interrupt charging intermittently to measure battery voltage — that’s normal. But this was off at least half the time and not keeping up with the load while driving. The auxiliary battery was getting a bit imperfect, sure, but not bad enough to explain it.
I don’t know whether the unit was faulty or whether it was overheating in the Pilbara. But the load I had on it was well within what a 25A charger should handle.
The behaviour was remarkably similar to what I’ve seen from a DC-DC installed on a smart-alternator vehicle without the ignition-switched feed wired up — in that case it only runs when the alternator happens to be under higher load and putting out higher voltage. I have seen exactly that a few times, from bad installations, broken feeds, or a blown fuse. I checked mine and the switched input was correct, so it may be that part of the circuit inside the charger stopped working.
Either way, I’m changing it. I’ll move to a 40A RedArc BCDC for headroom, at the same time as I move from a 200Ah AGM to a 300Ah lithium. I’ve seen a lot of RedArc chargers — on my own fit-outs and on customer vehicles — and they are never the ones I’ve seen problems with. At least in this case, that’s what the premium bought. It doesn’t automatically follow that every expensive 4WD accessory is worth the money — plenty of them are marketing hype. This one earned it.
What actually fails: DC-DC charger installation faults
The majority of DC-DC failures I’ve come across were really installation failures. The list, roughly in order of how often I see it:
No ignition-switched feed
By far the most common. The charger either doesn’t run, or only runs occasionally, depending on alternator load and voltage. It looks like a broken charger. It usually isn’t.
Ignition feed wiring that breaks
The ignition feed carries almost no current, so people reach for the thinnest wire they’ve got. Current isn’t the problem — durability is. A wire with fewer, thicker cores is far more prone to breaking with off-road vibration, particularly when it’s cable-tied to something that moves, and a broken feed looks exactly like a failed charger.
I use speaker wire for my own ignition feed, because it’s very flexible, has lots of fine cores that can move over each other, and survives being cable-tied to a vibrating vehicle far better than stiff single-core cable. I have no idea whether that meets the relevant standards, so don’t take it as advice.
Main cables too thin for the distance
This is a separate problem from the ignition feed, and it catches people because the current looks manageable on paper so the gauge seems adequate. Length is what ruins it. A feed from the engine bay to a second battery at the rear, under load, drops enough voltage to cause real trouble: the cable gets hot and risks a fire, or the charger has to work harder from a lower input voltage and overheats or misbehaves.
The cable run from the charger to the battery
Just as common, and often missed: the run out of the charger to the auxiliary battery is too long or too thin. By the time you account for voltage drop, the battery never quite reaches a full charge. It looks like a battery problem. It’s a wiring problem.
Poor ventilation and the wrong mounting location
Chargers mounted where they can’t breathe, or exposed to the elements when they aren’t a model designed to be engine-bay rated — waterproof and built for much higher operating temperatures.
Undersized fuses and the bigger-fuse trap
Undersized fuses aren’t usually a fire risk in themselves, because the fuse does its job and blows. The problem is what happens next: the owner gets sick of it blowing and fits a bigger fuse, which may then exceed what the fuse holder is rated for. Start with the overpriced oversized fuse holder on day one. You’ll only buy it once.
Low-voltage cut-outs: cheap protection for your auxiliary battery
This one isn’t strictly a DC-DC thing, but it’s relevant because batteries and chargers normally get upgraded together.
Fit a low-voltage cut-out between your auxiliary battery and your accessories.
That way, if your DC-DC is set up wrong, has failed, or simply can’t keep up with the load, the cut-out protects the battery from being run flat. It’s a small cost next to what you’ve already spent, and it saves the battery from a mistake that isn’t yours — a bad install, a bad unit, or a load you underestimated.
Sizing a DC-DC charger: what actually needs power
The load list is shorter than the industry pretends: a fridge, lights, and maybe some music.
On music — this is less of a factor than it used to be, because portable Bluetooth speakers have gotten genuinely good. But while we’re here: don’t be the person who drives out to get away from it all and then spends the whole trip listening to the same sounds you always hear. There’s a whole argument in that, and I’ve made it elsewhere.
Some people run a small drawer fridge as well as a main one. Induction cooktops are becoming popular, and I’ll probably move to one myself once the new battery is in — mostly so I don’t have to think about which way the wind’s blowing when I pull up on tour and boil the billy for smoko.
Then there’s the part where people lose the plot.
The coffee machine problem
If you’re carrying a portable oven — the 12-volt kind that sits in the back, not the cast-iron pot you bury in the coals — you should be planning ahead and cooking while you’re driving. Building a battery big enough to run an oven while parked is well and truly overkill.
And if you’re thinking about a 300Ah lithium battery, a 40A charger and a 3,000W inverter to run a coffee machine, you’re a city-slicker pompous snob who has bought into the idea that you need “good coffee” in the bush. It’s ridiculous. You can survive on instant coffee — and if you can’t, get an AeroPress. It makes better coffee than any machine that’s realistically coming with you, costs a fraction of that setup and takes up no space. Pod machines just fill your rubbish bag faster while you’re at it.
Solar and DC-DC chargers: where they overlap
Two different things, with one overlap: most DC-DC chargers these days have a built-in MPPT solar regulator, so the charger becomes part of your solar setup rather than a competitor to it.
The decision is about how you travel.
If you park up in one spot for multiple days at a time, get solar. If you only do that occasionally, consider a solar blanket instead of a permanently mounted panel — cheaper, packable, and it doesn’t commit you to a fixed installation you’ll rarely use.
If you’re moving every day, the charger is doing the work. That’s the whole point of it.
Lithium or AGM for a touring 4WD
My own view has shifted, and it’s a personal one rather than general advice. I run more gear than the average person because I’m operating tours, and the amount of usable amp hours I need is the problem — 100Ah of AGM is not comparable to 100Ah of lithium in what you can actually take out of it. Getting that capacity in AGM means carrying weight I can’t afford, because I’m already carrying plenty of everything else.
For the average person, though, you can probably stay on AGM — save the money, or avoid the fire risk of cheap lithium. If you can comfortably afford lithium, the advantages are real. Just make sure you get one with a good charge-control chip. Rate of charge and discharge is what causes lithium to overheat, and the cheaper chips — even where they’re safe — sometimes go to sleep at lower voltages, which makes a perfectly healthy battery look dead.
What to spend money on (and what to skip)
Save on features. Spend on a brand.
I’d rather have a basic unit from a good brand than a feature-stuffed one from a nobody. I like RedArc for exactly this reason — their simple models, without a wall of buttons and settings I’ll never use. You can buy the RedArc charger and completely skip the control panels, battery managers and monitoring displays. Almost nobody needs them. Stop the 12v pissing contest.
If you’re working out the whole setup rather than just the charger, start with our dual battery guide and the twelve-volt basics — this piece assumes you’ve got the broader picture sorted.
And if you’re weighing up a good battery with a cheap charger against a cheap battery with a good charger: spend the money on the charger. Get the charging right first, and when you’re ready to upgrade the battery it becomes a simple drop-in job — provided the charger is lithium compatible. Nearly all of the ones I’ve seen in recent years are, but check.
The brands, named
Since people always ask, and since I’ve had my hands in enough engine bays to have an opinion:
- RedArc — what I use, and what I’d buy.
- Enerdrive / Dometic — I don’t mind them at all.
- Projecta — I’ve got one and I’m getting rid of it.
- iTechWorld — good value if your budget is genuinely tighter.
- Kings / Supacentre — if that’s all you can afford, fine, do it if you must. But you can usually do better by buying less: a basic unit from a better brand rather than a loaded one from a cheaper one.
Closely related: what the industry would rather not mention is that the upsell is the product. Retailers push the models with touch screens and Bluetooth connectivity, because those are the ones with the margin. None of it makes your battery charge any better.
The standard advice, challenged
“You must run a DC-DC charger with a smart alternator” — that’s pretty much true, and I’d say it’s becoming more true every year as more vehicles get variable-voltage charging.
What gets left out is that the old way still works, mostly. A solenoid or relay with a switching wire from the ignition will charge a second battery perfectly acceptably, with one exception: the charging regulation between the two batteries that we covered earlier. It degrades your starter battery slowly. Mine did, for years, before I knew any better.
DC-DC chargers are cheaper and more abundant now than they were when I wired that first solenoid up, which is why the old approach has largely had its day.
And if money is genuinely tight: buy a cheap one, but buy one with a decent warranty — and don’t put a good battery behind a bad charger. You’ll ruin the more expensive of the two components while saving money on the cheaper one.
In short
A DC-DC charger exists because your auxiliary battery needs a charging profile your alternator was never designed to provide — and because your alternator is no longer a fixed-voltage power supply, so a plain isolator can’t force it to charge when the vehicle decides not to. The charging control problem came first; smart alternators are what made everyone notice.
If you run a fridge overnight, you want one, and you want the installation done properly — because that’s where the majority of the failures actually live.
If you don’t run anything while the vehicle is off, you probably don’t need one at all, and nobody selling them is going to tell you that.
