Do you need a DC-DC charger, and which size class suits you
This page answers 2 questions and deliberately stops there. Whether your vehicle needs a DC-DC charger at all, which is mostly decided by what its alternator does rather than by what you are charging. And which size class suits the way you travel, which is decided by how many hours the engine actually runs on the days you camp. What it will not do is hand you an amp rating, a cable size or a fuse, because those depend on your specific vehicle and battery and they belong to somebody who can see both.
These picks come from published manufacturer specifications, Australian retailer listings checked on the date above, and owner-reported experience treated as a pattern rather than as proof. We do not physically test the products we compare. Read the method in full.
What a DC-DC charger actually does
A DC-DC charger sits between the vehicle’s starting circuit and your auxiliary battery and does 3 things a plain connection cannot.
It charges at a voltage that would otherwise do nothing.It takes whatever input sits inside its accepted range and produces its own regulated output above the battery’s resting voltage, which is the only condition under which current flows into a battery at all.
It bounds the load. The auxiliary battery draws what the charger is prepared to give it rather than whatever the circuit will allow, so a flat battery becomes a predictable load on the vehicle rather than an open-ended one.
It runs a proper charge profile. Bulk at constant current, then a held voltage while current tapers, then whatever the chemistry needs at the top. An alternator does not do stages. It produces a voltage and gets on with running the vehicle.
It also keeps the 2 batteries separate, so the fridge cannot flatten the battery that starts the engine. That is worth stating plainly, because it is the failure people are actually frightened of and it is the one thing every option in this category is sold on.
Why a modern alternator usually makes one necessary
Vehicle manufacturers regulate alternator output to reduce the engine load that charging represents, because that load costs fuel and therefore emissions. The practical consequence is that alternator voltage on a modern vehicle is a managed variable rather than a constant. Once the vehicle judges the starter battery satisfied, output can be pulled down towards the battery’s own resting voltage. Some systems also push voltage sharply higher on deceleration to recover energy that would otherwise be lost as brake heat.
Either behaviour breaks a voltage-sensitive relay, in 2 different ways. A relay that opens and closes on voltage will chatter when the source swings across its threshold. Worse, when it is closed and the alternator has backed off, it is joining your auxiliary battery to a source that is not high enough to charge it. Current flows from higher potential to lower, so there is simply nowhere for it to go. That is not a brand problem or a wiring fault, it is the circuit doing what the physics says it will do.
A DC-DC charger is indifferent to all of it. It accepts a wide input band and produces its own regulated output, so a sagging or swinging source is a nuisance rather than a failure. Whether your vehicle behaves this way is the single fact that decides whether a relay is a legitimate cheap option or a wasted weekend, and it is worth confirming on your own model rather than on a general rule.
Lithium changes the argument again
If the battery you are charging is LiFePO4, there is a second reason the charger earns its place, and the popular explanation of it is wrong. It is not that a lithium pack has unusually low resistance: a drop-in 12V pack, once its management system is counted, often measures higher at the terminals than the AGM it replaced. What matters is the flat voltage curve. A lead acid battery’s terminal voltage climbs as it charges, so current tapers away within minutes and the alternator gets a break. A lithium pack holds its voltage nearly flat for most of its charge, so whatever current starts flowing keeps flowing. The load is a question of duration rather than peak.
How much that matters depends on the size of the bank and how the vehicle is wired, and it is a question an auto electrician can answer for your vehicle in a conversation. On a recent vehicle with a managed alternator there is often the opposite problem waiting as well: the supply can sit too low to ever fill a lithium pack, no matter how long you drive. A DC-DC charger solves both cases at once, which is why it has effectively become standard equipment alongside a lithium auxiliary battery.
Where this page stops
The rating you can actually run, the conductors that feed it and the protection that circuit needs are not general knowledge. They depend on your vehicle’s alternator, the length and route of the run, the battery you are charging and where the unit will live. Get them specified by a licensed auto electrician or by your battery supplier, and follow the manufacturer’s own installation documentation for the unit you buy. This page will not guess a number for you, and it is worth being suspicious of any page that does without asking what you drive.
Size classes, and how your driving decides
Units sold into the Australian touring market are commonly in the 20A to 50A range, with larger specialist units available above that. The useful way to read those classes is not against the size of your battery. It is against how many hours your engine runs on the days you are actually camping.
You move camp most days, with a real drive between stops. Your engine hours are the plentiful part of the system. A charger from the lower end of the range will usually put the day back into the battery well before you arrive, and the extra amps you paid for sit idle.
You move every few days, with short hops. The same daily consumption now has far fewer engine hours to land in, so the case for a larger class strengthens, up to whatever your battery and your vehicle will support. This is the band where the choice is a genuine choice.
You sit still for days at a time. Size is the wrong question. The charger is not the constraint and buying a bigger one changes nothing.
The default advice in this category is to buy the biggest unit you can afford. That only holds if nothing else in the system caps what it can deliver, and usually something does.
The battery has its own maximum. Every battery worth buying publishes a maximum charge current, and several manufacturers also publish a lower figure they recommend if cycle life is what you care about. Exceeding it does not charge the battery faster. It ages it, and it can trip the protection in a lithium pack. Those figures belong to your battery, so get them from its datasheet or its supplier rather than from a general rule.
The alternator has a finite surplus. A rating is a peak figure, and a large part of it is already committed to the vehicle. What is left for your charger is smallest exactly when you need it most: idling in traffic on a hot day with the air conditioning working. What your alternator can spare is another question for the auto electrician.
Heat. Conversion loss leaves as heat, a larger unit makes more of it, and many converters reduce their own output when they get hot. In an Australian summer, in a canopy that probably has a fridge in it as well, that means the amps you paid for are least available on the day you most wanted them.
The charger is rarely the whole cost. Stepping up a size can pull the rest of the installation up with it. Ask for the installed price of each option rather than comparing the price of the boxes.
Plenty of units in this category take solar as a second input, regulating the panel internally as well as charging from the vehicle. It is a genuinely good idea, with 2 catches that are rarely mentioned.
What it buys you. Having 1 box instead of 2 means a single install, a single set of terminations and a single place to look when something is wrong. Both charge sources agree on the same charge profile, which removes the real and surprisingly common fault of a solar regulator set for a different chemistry to the vehicle charger. Where the unit prioritises solar, it uses the free energy first and reduces the load on the vehicle automatically.
The first catch.On many dual-input units the 2 inputs share a single output budget. Where that is true, solar and the alternator together still cannot exceed the unit’s rated output, so you cannot stack the 2 sources to recharge faster. That is defensible engineering and often the right behaviour, but it matters if you bought the unit expecting the opposite. Confirm it on the exact model rather than assuming in either direction.
The second catch. A single device now carries both charging paths. When it fails you have lost the vehicle charging and the solar at the same time, potentially a long way from a shop. A separate regulator and a separate charger is 2 boxes and 2 sets of wiring, and it is also 2 failures that do not take each other out. Which trade-off is right depends on how remote you travel more than on the specification sheet.
The most integrated version of this idea in the set this site tracks is the REDARC GoBlock PPS12100, and it is routinely mis-described, so it is worth being precise. It is not a charger. It is a complete portable dual battery system with the battery included: LiFePO4 storage alongside DC-DC charging, solar regulation, a mains charger, battery management and Bluetooth monitoring in 1 housing. Compare it against a battery plus a charger plus a regulator, or against a complete power station, never against a bare DC-DC unit. It is widely stocked in Australia rather than sold direct, and street pricing varies enough between stockists that no band is published here: check the current price at 2 or 3 sellers. There is no link to it on this page and we earn nothing if you buy one. The portable power comparison covers it and the all-in-one alternatives in full.
If you simply do not drive far enough
This is the answer the category avoids giving, and it applies to a large share of the people asking the question. Base camp touring, a week at the same beach, a fortnight at a station stay, a caravan parked while you day-trip in a second vehicle: in all of those the engine barely runs, and a DC-DC charger only works while it does. No size fixes that.
What actually closes the gap, in the order worth trying:
Shore power. A single night on 240V at a powered site resets the whole system, and on a trip with a resupply stop in it that is often the cheapest solution available. It uses a different charger to the one this page is about, and many integrated units include one.
More storage, last. A bigger battery does not generate anything. It buys days before the problem arrives, which is genuinely useful on a short trip and useless on a long one. Buy capacity to bridge a known gap rather than to avoid answering the charging question.
The clean version of the rule: if you move most days, the DC-DC charger is your primary charge source and solar is the top-up. If you sit still most days, solar is primary and the charger is the top-up. A great many touring setups are built the wrong way round, with an expensive charger and a token panel, on a travel pattern that needed the reverse.
Where the charger shows up in an actual purchase
For most Australian tourers the charger is not bought on its own. It arrives either as a component wired into a fitted system, which is the dual battery route, or built into a portable battery box you drop a battery into. The box route is where a lot of people start, because it puts a real charger in the system at a price that still leaves budget for a decent battery, and because it moves between vehicles instead of being wired into one.
The cheapest honest way to run a fridge off the vehicle
KickAss Battery Box with integrated 20A DC-DC charger and accessory bundle
Type Battery box, charger fittedBattery Not included$500 to $630 as at July 2026
This is the component-route answer, and for most Australian tourers it is the correct one: a box, a battery and a DC-DC charger that manages the alternator properly, rather than an all-in-one station whose 12V port is an afterthought. The integrated 20A DC-DC charger is the part that matters, because it is what turns driving time into recovered amp hours instead of hoping the alternator and a long cable run do the job. Add a battery and the total still lands below most all-in-one stations of comparable usable capacity.
Strength
Integrated DC-DC charging in a portable box, at a price that leaves budget for a decent battery, from a brand carried by a mainstream Australian retailer.
Weakness
The 20A version has been discontinued. KickAss now starts its range at 25A, so what you find at this price is run-out stock, and the 25A replacement lists higher at full retail. Check which version you are actually buying before you order, and note the battery is not included.
This link is not a tracked affiliate link. It goes straight to the retailer and earns us nothing.
Read the size on that box against the driving classes above rather than against the price, and read the weakness note carefully before you order: the 20A version is run-out stock, the current range starts higher, and the band shown is a clearance spread across Australian sellers rather than a going rate. Check which version a listing is actually selling you.
If you want the box without the charger, the Hardkorr Heavy Duty Battery Box PRO is sold as a plain enclosure with no charger and no battery, in the $225 to $275 band as at July 2026. Hardkorr also lists versions of the same box with a DC-DC charger fitted, and this site publishes a band only where it captured the figure itself, so for those the price here is Not published. What is worth knowing for budgeting is the direction and the size of the step: the charger-fitted versions run to a multiple of the empty-box price, so do not read the empty-box figure as the cost of a working setup. Hardkorr also sells a cheaper non-PRO heavy duty box, so confirm which enclosure a price refers to.
This category is full of imported hardware sold through several channels at once, and the things that decide whether you are happy in 3 years are not on the front of the box.
Who carries the warranty here. A named Australian entity standing behind the product is worth more than a longer term administered from overseas. Ask who you would actually be dealing with on a claim.
Where it gets serviced, and where parts come from. A unit that fails 900 km from a capital city is a logistics problem before it is a warranty problem.
Heat. Ask whether the manufacturer publishes what the unit does as it gets hot, and what clearance its own documentation requires. An Australian summer is the operating condition, not the edge case.
How it knows the engine is running. Some units use an ignition signal, others start and stop on input voltage alone, and on a vehicle with a managed alternator that distinction is worth asking about before you buy.
Whether it moves with you.A fitted unit stays with the vehicle when you sell it. A box goes into the next one, and it also goes into a mate’s ute for a weekend. That is a real difference in what your money buys.
What to ask the installer or the retailer
Everything below is a question rather than an answer, which is the correct division of labour: you know how you travel, and they can see the vehicle.
Given this vehicle and this battery, what rating do you recommend, and what is stopping it from being higher?
Does this vehicle have a managed or variable-voltage alternator?
What does the battery manufacturer publish as its maximum charge current, and does it publish a lower figure for cycle life?
What cabling and protection does that rating require on this vehicle, and is it included in the quote?
Where will the unit be mounted, and what clearance does its own documentation require?
Is the warranty affected if it is not installed by a licensed installer?
If it is a dual-input unit, do the solar and vehicle inputs share a single output budget?
What is the installed price, not the price of the box?
The limits of this page
Outback Rated does not physically test the products it compares, and nothing here should be read as a claim that any charger has been handled, wired or trialled by us. Product names and price bands come from the generated product data, observed at Australian retailers on the date at the top of this page and published as bands rather than fixed prices, because street pricing in this category moves.
The bigger limit is deliberate. An earlier version of this page carried a sizing formula, charge-rate tables, alternator arithmetic and cable and fuse figures. Verification found enough of that wrong that the honest response was to remove the category of claim rather than correct it again, so this page no longer produces a number you could act on with a tool in your hand. It can tell you what these things do, who they suit, what the trade-offs are and what to ask. The circuit itself belongs to someone qualified to specify it. The full method, including what gets excluded and why, is published separately.
Frequently asked questions
What does a DC-DC charger actually do?
It sits between the vehicle's charging system and your second battery and produces its own regulated output, rather than passing on whatever the alternator happens to be doing. That gives the second battery a proper multi-stage charge matched to its chemistry, keeps the load on the vehicle bounded rather than open-ended, and keeps the starter battery separate so a flat auxiliary battery cannot leave you stranded.
Do I need one, or will a voltage-sensitive relay do?
It depends almost entirely on what your vehicle's alternator does. A relay is a switch, not a charger: it joins the 2 batteries and hands the auxiliary battery whatever voltage is present. On an older fixed-voltage alternator charging an AGM over a long drive, that gets you most of the way there. On a vehicle that manages its alternator voltage, which covers a lot of what has been sold in the last decade, the relay can be closed onto a source that is too low to charge anything, and it can chatter as the voltage swings across its threshold. Confirming which type your vehicle has is the single cheapest thing you can do before spending anything.
What size DC-DC charger should I get?
This page will not give you a number, and you should be wary of any page that does without knowing your vehicle. What it can tell you is the shape of the answer. Units sold here are commonly in the 20A to 50A range, larger ones exist, and the useful question is how many hours the engine actually runs on the days you camp. Frequent, longer drives mean the smaller classes are usually enough. Short hops between longer stays push you up. The actual rating that your specific vehicle and battery can support, and the cabling and protection that go with it, is a job for an auto electrician or your battery supplier.
Is a bigger DC-DC charger better?
Not automatically, and it is the most common way to spend money in this category without getting anything for it. Your battery has a maximum charge current published by its maker and a charger cannot exceed it usefully. The alternator has a finite surplus, and the surplus is smallest exactly when you are idling in traffic on a hot day with the air conditioning running. A larger unit makes more heat and can reduce its own output when it gets hot. And the charger is rarely the whole cost of stepping up a size, because the rest of the installation scales with it.
Is a dual-input charger with a solar input worth it?
Often yes, for the tidiness rather than the arithmetic. Having 1 box means a single install, a single set of terminations and a single place to look when something is wrong, and both charge sources agree on the same charge profile, which removes a genuinely common source of confusion. There are 2 things worth checking on the exact model. First, whether the 2 inputs share a single output budget, because where they do, solar and the vehicle together still cannot exceed the unit's rated output. Second, that it is now a single device carrying both charging paths, so when it fails you lose both at once, which matters more the further you travel from a shop.
What if I do not drive far enough?
Then no charger fixes it, and that is worth knowing before you buy one. A DC-DC charger only works while the engine is running, so a week parked at the same camp is not a problem it can solve at any size. Solar is the charge source that works while the vehicle sits still, a night on mains at a powered site resets the whole system, and reducing the daily draw is cheaper than either. A bigger battery buys days rather than energy, which helps on a short trip and not on a long one.
Send a correction with the source attachedif something here does not match a manufacturer’s published specification. Corrections are checked against the source and the page records what changed.