How to choose a dual battery setup for Australian touring
A dual battery system does 3 jobs, and only 1 of them is storage. It keeps the battery that starts your vehicle separate from the one that runs your fridge, it holds enough charge to cover the gap between charging opportunities, and it puts charge back fast enough that the gap never grows. Get the separation and the charging right and a modest battery is plenty. Get them wrong and no amount of capacity saves you. This page is about choosing that setup. It is not about installing it, and it says so plainly below.
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 this page will not tell you
Nothing here is installation guidance, and none of it should be read as any. Sizing and protecting the circuit between a vehicle’s battery and a charger depends on the vehicle, the run, the charger and the standard that applies to the build, and this page will not guess any of it for you. That is an auto electrician’s job, and the charger manufacturer’s own manual outranks anything written here.
Earlier versions of this page carried cable, fuse, connector and alternator figures. Checking them found errors often enough that removing them is the honest fix rather than correcting them a third time. What is left is the part this site can stand behind: what each option is, who it suits, what it costs and what it gives up.
Why a second battery exists at all
The usual explanation is that a starter battery will go flat if you run a fridge off it. That is true and it is the least interesting part of the answer. The real reason is construction. A cranking battery is built to deliver an enormous current for a few seconds and then be immediately refilled, so it uses many thin plates to maximise surface area. A deep cycle battery is built to deliver a modest current for hours and be refilled slowly, so it uses fewer, thicker plates that survive being emptied and filled repeatedly. Those are different products wearing the same box shape. Running a fridge off a cranking battery does not just risk 1 flat morning: it takes life out of a battery that was never designed for the duty, and it does so quietly, so the failure arrives later and looks like bad luck.
So the second battery is not really a capacity upgrade. It is a separation of 2 incompatible jobs, and the separation itself is the product. Judge a dual battery setup on how reliably it keeps those 2 jobs apart, and on how much charge it can put back per hour of driving. Do not judge it on the number printed on the case.
VSR or DC-DC: the decision smart alternators changed
This is the choice that has genuinely moved in the last decade, and a lot of advice still in circulation predates the change.
What a voltage sensitive relay does
A VSR is a relay that watches voltage. When it sees the system voltage rise above a set point, it reads that as the alternator charging and closes, connecting the second battery to the same circuit as the crank battery. When voltage falls below a lower set point, it reads that as the engine off and opens, separating the 2 batteries so the fridge can only drain the second one.
It is elegant, it is cheap, it has no settings to get wrong, and for a long time it was the default fit on Australian 4WDs. It also has a structural limitation that has nothing to do with quality: a battery connected in parallel receives whatever voltage is present on the circuit and nothing more. It never receives a proper multi-stage charge, and it never receives a profile matched to its chemistry. On a lead acid second battery that usually means it lives permanently somewhere short of full, which is exactly the condition that shortens a lead acid battery’s life.
What a variable-voltage alternator does to it
Modern vehicles are built to a fuel consumption target, and the alternator is a parasitic load on the engine, so manufacturers manage it: the vehicle varies the alternator output, dropping the charging voltage during steady cruise and raising it when the crank battery genuinely needs charge. That is sensible for the vehicle and it is a problem for a relay whose entire logic is that voltage means the alternator is charging. There are 3 common outcomes and none of them is good:
The relay never closes. Cruise voltage sits below the connect threshold, so the second battery is never charged while you drive, and the first you know about it is a fridge that shuts down on the second night.
The relay chatters. Voltage hovers around the threshold, so the relay opens and closes repeatedly. That is hard on the relay, and charging arrives in fragments.
The relay closes and delivers nothing useful. It connects at a cruise voltage low enough that very little actually flows into a battery that is already partly charged, so the driving time you assumed was recharging you was not.
That last outcome is the one that catches lithium owners. A managed alternator commonly cruises at or below the voltage a 12V LiFePO4 pack rests at across most of its usable capacity, and 2 batteries sitting at the same voltage exchange very little charge. The honest failure mode of a relay on a modern vehicle with a lithium second battery is not dramatic. It is chronic undercharging that never announces itself.
What a DC-DC charger does instead
A DC-DC charger takes whatever the vehicle gives it, including a voltage lower than the battery it is charging, and converts it up to the voltage that battery actually needs. That single capability is the whole point, and it is why the smart alternator problem disappears. Around it sit 3 things a relay cannot do:
A multi-stage charge profile. Bulk, absorption and float, run to a schedule matched to the chemistry rather than to whatever the vehicle happens to be producing.
A chemistry selection. AGM, gel, calcium and LiFePO4 want different voltages. A charger set to the wrong profile will either undercharge the battery for its whole life or overcharge it.
Genuine separation. The 2 batteries are never directly connected to each other, so the second battery cannot pull the crank battery down under any condition.
Most current units also include a solar input with its own regulator, which removes a separate device from the shopping list and means the panel and the vehicle feed the same charge profile rather than arguing with each other. Many include an ignition trigger input, which is the correct way to tell a charger to start on a vehicle whose voltage no longer announces it. Both are worth asking about, because they change what else you have to buy.
It is not obsolete, and pretending otherwise would be dishonest. A relay is still a reasonable fit on an older vehicle with a conventional fixed-output alternator, feeding a lead acid second battery, where you drive for hours at a time and never take that battery deep. It is cheap, it draws almost nothing, and there is very little in it to fail. What it will not do is recover a deeply discharged battery quickly, charge lithium correctly, or behave predictably behind a managed alternator. If your vehicle or your trip has any of those 3 characteristics, the relay is the wrong device and adding capacity will not compensate.
AGM or lithium, honestly
This is where the touring internet is least useful, because lithium is genuinely better on several measures and that has hardened into an assumption that it is better on all of them. It is not, and the cases where AGM is still the right call are real and specific.
What lithium actually buys you
Usable depth. A lead acid deep cycle battery should not be taken below about half its rated capacity, so a 120Ah AGM is a 60Ah battery in practice. LiFePO4 realistically delivers 80% to 90%. That is close to double the working energy from the same number on the label, and it is why a lithium bank can be physically smaller for the same job.
Charge acceptance, which matters more than depth on a touring build. Past roughly 80% state of charge a lead acid battery simply will not take charge quickly, no matter how big the charger is, and the last part of the charge takes hours of engine running you do not have. LiFePO4 accepts close to full current until it is nearly full. On a trip where you drive an hour between camps, that difference decides whether you recover or slowly slide backwards, and it is invisible on any specification sheet.
Weight, and therefore payload. A lead acid battery is substantially heavier per usable amp hour. On a touring 4WD already carrying a canopy, water, recovery gear, a fridge and 2 spare tyres, mass is a budget with a legal limit attached to it, and the battery is one of the few items where you can buy the weight back with money.
What AGM still wins
Price, and by a wide margin. The gap is large enough to fund a DC-DC charger and a panel, which on a modest usage pattern is a better system than a lithium battery fed badly.
Heat and position. AGM tolerates an under-bonnet environment that most LiFePO4 batteries publish themselves out of. If the only practical place for the second battery is the engine bay, that is close to a decision on its own.
Cold charging, and the fact that you may have no protection at all. Charging LiFePO4 below about 0C plates metallic lithium onto the anode, and the capacity loss is permanent, cumulative and completely silent. Whether the battery stops you is a feature of that particular battery rather than a property of the chemistry. Several drop-in packs sold in Australia publish short circuit, over-voltage, low-voltage and high-temperature protection and no low temperature charge cut-out at all, and a published charge range such as 0C to 55C is an instruction to the owner rather than proof the battery enforces it. AGM has no equivalent trap, which is a real argument for it if you tour the high country or the southern inland in winter.
Replaceability. A deep cycle lead acid battery can be bought in almost any regional town in Australia. A drop-in lithium usually cannot. The further you go from a capital city, the more that matters, and it is a failure mode worth pricing rather than hoping about.
Unmanaged charging. If the setup is a relay and a cable, AGM is the chemistry that survives it. Feeding lithium from an unmanaged source is the wrong build.
The 2 questions that decide it
Strip out the brand argument and it comes down to how often you drive and where the battery can physically live. Drive most days and put the battery somewhere temperate, and lithium converts that driving into stored charge far more effectively than lead acid can, which is the whole case for it. Camp occasionally, park up with the vehicle idle, or have nowhere for the battery except the engine bay, and AGM does the job for a fraction of the money.
Cheapest usable storage, with a real catch
KickAss 12V 120Ah Deep Cycle AGM Battery
Type Deep cycle batteryChemistry Deep cycle AGM lead acid$250 to $300 as at July 2026
At roughly $270 for 120Ah this looks like the bargain of the page, and it is worth understanding why it is not quite that. Snowys states that a lead acid deep cycle battery should not be run below about 50% of charge, so the working figure is about 60Ah, not 120Ah. Against a 40L to 60L fridge drawing roughly 27 to 36 Ah on a hot Australian day, that is comfortably 1 night and marginal over 2 without charging.
Strength
Lowest cost per rated amp hour on this page, and it will run a fridge properly if you charge it every day and never take it below half.
Weakness
Half of what you paid for is unusable, and there is a placement restriction that matters on this exact page: KickAss states that using this battery under your bonnet voids the warranty, because it is not a CCA-rated battery. It is a rear-mounted or battery-box proposition, not an under-bonnet second battery. Rested voltage at the 50% point is roughly 12.2V to 12.3V, and do not rely on a fridge cut-out to protect it: those typically do not trigger until well under 12V.
This link is not a tracked affiliate link. It goes straight to the retailer and earns us nothing.
Read that card against the load model below rather than on its own, because they count different things. The card weighs 60 Ah of usable capacity against the fridge on its own, at 27 to 36 Ah a day, which is the same published band this page’s 30 Ah comes from. Add the 10 Ah a day of house loads this page also plans for and the honest answer is 1 night, not 2. Nothing about that makes it a bad battery. It makes it a battery you charge every day.
There is a gap in that recommendation and it is worth naming rather than papering over. No lithium battery pick appears on this page, because the retailer listings captured on 25 July 2026 did not include an observed price for the relevant LiFePO4 units, and a price band cannot be published without an observed price. Recommending a chemistry while linking only the alternative would be a strange kind of honesty, so the position is stated plainly instead: nothing gets published here that could not be sourced on a stated date, including when the missing figure is inconvenient.
Usable capacity, and sizing a bank against a real load
Rated capacity is the number on the case. Usable capacity is what you can take out without damaging the battery, and it is the only figure worth planning with. Hold lead acid to about 50% and LiFePO4 to about 85%, and every sum below follows from that. The chemistry, depth of discharge and cycle life evidence is set out in detail on the portable power page, so this section does the part that page does not: turning a daily load into a bank size, with the charging counted in.
The load model, stated once
Everything below assumes a fridge using 30 Ah a day, which sits inside the 27 to 36 Ah a day that Australian vendor guidance clusters around for a 40L to 60L compressor fridge in hot conditions, plus 10 Ah a day for lights, phone charging, a water pump and a camp light. That is 40 Ah a day, or about 512 Wh at 12.8V. That figure is used everywhere on this page, including in the frequently asked questions, and nothing here uses a different one.
What you actually have to buy, over 2, 5 and 7 days
The sum is deliberately simple. Take the daily load, subtract what goes back in during a day, multiply the remainder by the number of days, then divide by the usable fraction of the chemistry. What makes the result surprising is not the arithmetic. It is how fast the answer collapses once anything at all is going back in.
LiFePO4 bank size required over 2, 5 and 7 days, by how much charge goes back in per day
What goes back in each day
Net use per day
2 days
5 days
7 days
Nothing goes back in
40 Ah
about 95 Ah
about 235 Ah
about 330 Ah
About half the day's use goes back in
20 Ah
about 45 Ah
about 120 Ah
about 165 Ah
The whole day's use goes back in
0 Ah
about 45 Ah
about 45 Ah
about 45 Ah
Calculated for the 40 Ah per day load stated above and a LiFePO4 bank held to 85% usable, rounded to the nearest 5 Ah. The last row breaks even on paper, so it is sized as 1 day of buffer for the day nothing goes back in rather than as zero. Rerun every figure at the 50% lead acid floor and it becomes 1.7 times larger. How much actually goes back in a day depends on how long you drive, what charger is fitted and how much sun a panel sees, which is what the DC-DC charger page and the solar page work out. This page will not assume it for you.
Read the first row and the last row against each other. Carrying 7 days of this load with nothing going back in needs roughly 330 Ahof lithium, which is a bank most people will never buy and could not fit if they did. Replace the day’s use and the same 7 days needs about 45 Ah, because the system is barely running down. Autonomy is the expensive way to solve this problem. Recharge is the cheap way, and that is the single most useful thing on this page.
There are 2 honest cautions on that table. The middle row assumes half the day’s use comes back, and on a lead acid battery that is harder than it sounds, because the last part of the charge is where lead acid stops accepting current quickly. That limitation is the strongest practical argument for lithium on a build where the driving is short. And a trip is never the average, which is why the buffer row exists at all.
The battery box route, for people who do not want an install
Not everyone should wire a vehicle. If you camp a handful of times a year, if the vehicle is leased or about to be sold, if you move gear between 2 vehicles, or if you simply do not want anything cut into a loom, a battery box does most of the job with none of the commitment.
What you keep is the important part: the second battery is still separate from the vehicle, the fridge still runs from a dedicated source, and the box gives you sockets and a carry handle instead of terminals. What you give up is permanence and, on the cheaper boxes, active charge management. A box without a charger is storage. A box with an integrated DC-DC charger behaves like a fitted system, because it manages the alternator the same way, in a case you can lift out.
The one carried here is the KickAss Battery Box with integrated 20A DC-DC charger and accessory bundle, $500 to $630 as at July 2026↗, and it comes with a caution rather than a clean recommendation. The 20A version has been discontinued and KickAss now starts its range at 25A, so what sits on a shelf at this price is run-out stock. Checked in July 2026, the current 25A box listed below the run-out bundle at both street price and full recommended retail, so the older unit is worth taking for the accessories bundled with it rather than on price. Check which version you are holding before you order, and note that neither includes a battery.
Do not assume the component route is the cheap route either. A box plus a battery is not automatically cheaper than a ready-made all-in-one station of comparable usable energy, and the comparison has to be made on usable energy rather than rated capacity, because a 120Ah AGM held to its 50% floor is about 60Ah in practice. The portable power comparison runs that argument dimension by dimension, on observed prices with the date attached.
The upgrade path is the reason the box route is a first stage rather than a compromise. The battery inside the box can be replaced with a different chemistry later without replacing the box, and if you eventually have the vehicle wired properly, the box becomes the portable second bank you take to a campsite the vehicle cannot reach. Very little of the money is wasted.
What to ask before you commit
These are the questions that decide whether a quote is a good one, and none of them requires you to know anything about wiring. Ask them of an auto electrician, a 4WD outfitter or a retailer, and treat a vague answer as information in itself.
Does this vehicle have a variable-voltage alternator, and does the manufacturer specify where an auxiliary circuit may be connected on it?
What is the total fitted price, including the parts not on the quote yet? The charger and the battery are rarely the whole bill.
Where will the battery physically live, and what temperature does that space reach in February?
Does this battery publish a charging temperature range, and does the battery itself act on that range or is it printed as an instruction to me? Ask to be shown where it is stated.
Who repairs this brand in Australia, where is the nearest agent to me, what is the warranty term in writing, and does that warranty depend on who installs it? On several products it does.
If this fails 800 km from a capital city, what is the actual path back to a working fridge? A brand with a wide dealer network is worth real money on that question alone.
Frequently asked questions
Do I need a fitted dual battery, or will a battery box do?
A box is the right answer if you camp a few nights at a time, drive between camps, and do not want to modify the vehicle. A fitted system earns its cost when the battery is heavy enough that you do not want to lift it, when you want the charging permanently connected so it works without you thinking about it, or when the same battery has to feed a fridge, lights, a water pump and device charging from fixed circuits. The upgrade path runs one way, so nothing you learn from running a box is wasted: a box with an integrated DC-DC charger is the same charging behaviour as a fitted system, in a case you can lift out.
Can I still use a VSR if my 4WD has a smart alternator?
Usually not well. A voltage sensitive relay decides when to connect by watching voltage, and a variable-voltage alternator deliberately moves that voltage around to save fuel. The relay can then sit in a band where it opens and closes repeatedly, or never closes at all, so the second battery ends up chronically undercharged. On a lithium second battery the problem is sharper again, because a managed alternator commonly cruises at or below the voltage a LiFePO4 pack rests at, and 2 batteries connected at the same voltage exchange very little charge no matter how far you drive. Forcing the connection with an ignition trigger fixes the switching but not the underlying problem, because a battery connected in parallel only ever receives whatever the vehicle happens to be producing, with no charge profile of its own. On a smart alternator vehicle the honest answer is a DC-DC charger.
How big should my second battery be?
Size it against the gap between what you use and what goes back in, not against days of autonomy. On the load model used on this page (30 Ah a day for a 40L to 60L fridge in hot conditions plus 10 Ah of house loads, so 40 Ah a day), 5 days with nothing going back in needs about 235 Ah of LiFePO4. The same 5 days with about half the day's use going back in needs about 120 Ah, and a system that replaces the whole day's use needs about 45 Ah as a buffer for the day nothing goes back in. On lead acid every one of those figures is 1.7 times larger, because you can only use half of it. The bank is not the expensive part of the answer. The charging is.
Is lithium worth it for someone who camps a few weekends a year?
Often not. Lithium earns its price on 3 things: usable depth, charge acceptance and weight. If you camp 6 weekends a year, drive between camps, and never take the battery below half, you are buying capacity and cycle life you will not consume. AGM is also the chemistry you can replace in a regional town on a Sunday, which matters more the further you go. Where lithium is genuinely worth it is high use, long stretches without mains power, short drives that have to recover a lot of charge quickly, or a build where payload is already tight.
Will a lithium battery stop itself charging in the cold?
Only if that specific battery says it will. Charging LiFePO4 below about 0C plates metallic lithium onto the anode, and the capacity loss is permanent, cumulative and silent. But a low temperature charge cut-out is a feature of the individual battery, not a property of the chemistry. Several drop-in batteries sold in Australia publish only short circuit, over-voltage, low-voltage and high-temperature protection, and a published charge range such as 0C to 55C is an instruction to the owner rather than evidence the battery enforces it. Find the low temperature charge cut-out in the specification sheet for the exact battery you are buying, and if it is not stated, ask the seller to point to where it is. If nobody can, assume there is none.
What this page is built from, and what it does not know
Outback Rated does not physically test the products it compares, and nothing here should be read as a claim that any battery, box or charger has been trialled by us. The first-hand material on this page is about living with solar and a second battery in a touring vehicle in Australia, which is a system rather than a product, and the distinction is kept visible.
Product names, price bands and retailer listings come from an Australian retailer capture on 25 July 2026. Prices are published as bands with the month attached, never as fixed figures, and they move.
Every number worked out on this page is calculated from an assumption printed beside it. There is exactly 1 daily load model, stated above at 40 Ah a day, and every bank figure, every table cell and every answer in the questions above is derived from it.
This page does not carry install guidance, and that is a decision rather than an omission. Circuit sizing and protection belong to a licensed auto electrician and to the manufacturer’s own manual for the exact charger.
Where a figure could not be sourced, it is absent rather than estimated. There is no lithium battery pick above for exactly that reason, and the output ratings of the battery box were not captured either.