Dual battery and 12V systems: DC-DC, VSR, AGM and lithium
The question is usually some version of: my mate put a second battery in his old Hilux with a $90 solenoid and it's been fine for a decade, so why is everyone telling me I need a $400 DC-DC charger?
The answer is that your mate's old Hilux had a simple alternator that put out a steady voltage whenever the engine was running. Yours probably doesn't. Vehicles built to modern emissions and efficiency standards commonly run variable-voltage or "smart" charging — the vehicle's computer decides what the alternator does, and it will deliberately drop output when it thinks the starting battery is full, in order to save fuel. Many also have stop-start.
That breaks the old approach, and it's the single reason the market moved.
VSR versus DC-DC
A VSR (voltage sensitive relay, sometimes a smart solenoid or dual battery isolator) is a switch. It watches the voltage at the starting battery. When the voltage rises above a set point — indicating the engine is running and charging — it connects the second battery in parallel. When the voltage drops, it disconnects, so you can flatten the auxiliary battery without stranding yourself.
That works beautifully when the alternator holds a steady, healthy charging voltage. On a variable-voltage vehicle it can misbehave: the alternator may sit at a voltage too low to ever trip the relay, or it may hover around the threshold and cause the relay to connect and disconnect repeatedly. And even when it does connect, the auxiliary battery is only getting whatever voltage the vehicle decided to supply, which may be well below what's needed to actually fill it.
A DC-DC charger is a converter, not a switch. It takes whatever the vehicle gives it, over a fairly wide input range, and produces a proper multi-stage charge output at the voltage the auxiliary battery needs. It also limits the current it draws, which protects the alternator, and it isolates the two batteries.
Practical guidance:
- Modern vehicle with variable-voltage charging or stop-start — DC-DC. This isn't upselling, it's what the vehicle requires.
- Older vehicle, simple alternator, AGM auxiliary, modest needs — a VSR can still be a legitimate, cheap, reliable choice.
- Lithium auxiliary battery — DC-DC, effectively always. Lithium needs a specific charge profile, and its low internal resistance means it will pull as much current as you let it, which can cook an alternator that wasn't designed for it.
Most DC-DC units also take a solar input, which is worth having even if solar comes later.
AGM versus lithium
Both work. They suit different budgets and different uses.
AGM is a sealed lead-acid battery. Cheaper up front, tolerant of heat, widely available, and it doesn't care much about being charged in the cold. The trade-offs are weight and usable capacity: as a general planning rule you only draw down about half of a lead-acid battery's rated capacity before you start shortening its life badly. So a 100 Ah AGM realistically gives you something like 50 Ah of usable energy, at roughly 25–30 kg.
Lithium (LiFePO4 in this context) is lighter, and you can use a much larger share of its rated capacity — commonly 80% or more, depending on the manufacturer's guidance. So a 100 Ah lithium delivers meaningfully more usable energy than a 100 Ah AGM at roughly a third to a half the weight. It also holds voltage up better as it discharges, which matters for fridges and inverters. It costs considerably more up front, though cycle life is much longer, so the cost per cycle can favour it.
Two lithium caveats that matter in Australia:
- Heat. Most LiFePO4 batteries are not rated for the temperatures found under a bonnet. Check the manufacturer's temperature range before considering that location.
- Cold. Charging LiFePO4 below freezing damages it. Most quality batteries have a battery management system that blocks charging when it's too cold, but if you're in the alpine country or the desert in winter, check that yours does.
Under bonnet or in the cabin
Where the auxiliary battery goes drives a lot of the rest of the design.
Under the bonnet is convenient, keeps the cable run to the alternator short, and uses space you weren't otherwise using. But it's hot, it vibrates, and space is genuinely tight on modern engine bays. It suits AGM far better than lithium. You'll also need a proper tray designed for your vehicle — battery trays are among the most series-specific parts you'll buy, because engine bay layouts change between generations.
In the cabin, canopy or tub is cooler, kinder to the battery, safer for lithium, and generally where a serious system ends up — close to where the fridge and the outlets are. The cost is a long cable run from the alternator, and that's where people get it wrong.
Over a long run, voltage drop is the constraint. Undersized cable over five or six metres wastes voltage as heat, which means the DC-DC charger sees less than it should and everything charges slowly. Size the cable for the current and the round-trip length, use proper crimped terminations, fuse it at both ends and at the battery, and protect it wherever it passes through a panel. If you're not confident doing this, this is the part worth paying an auto electrician for — undersized or unfused battery cable is a genuine fire risk.
Sealed lead-acid and lithium batteries are generally acceptable inside a cabin, unlike flooded batteries which vent gas. If you're mounting a battery in an occupied space, check the manufacturer's guidance and mount it so it cannot move in a crash. An unrestrained battery is a projectile.
Sizing it for a fridge
The most common actual requirement: run a fridge for a few days without driving.
A typical 12V compressor fridge draws a few amps while the compressor is running, but it only runs part of the time. What matters is the average over 24 hours, and that depends heavily on ambient temperature, how cold you've set it, how often you open it and whether it's in the shade. In mild conditions a modest fridge might average somewhere around 1 to 1.5 amps over the day; in a hot Australian summer with the fridge in a sunlit canopy, considerably more.
That gives you a rough planning figure of roughly 25–35 Ah per day for a typical fridge in reasonable conditions, and meaningfully more in heat. Treat that as a starting estimate only — the honest way to size a system is to measure your own fridge with a shunt-based battery monitor over a couple of days in the conditions you actually camp in.
Then work backwards:
- Multiply your daily draw by the number of days you want to sit still.
- Add anything else — lights, charging phones, a water pump, an inverter for something bigger.
- Divide by your usable share of capacity: about 50% for AGM, more for lithium per the manufacturer.
If the answer is a bigger battery than you want to carry, solar is usually the cheaper and lighter fix than more battery, provided you're parked somewhere sunny. A battery monitor is the single best value item in any 12V system — guessing state of charge from voltage alone is unreliable, especially with lithium, whose voltage barely moves across most of its range.
Before you buy
- Find out whether your vehicle has variable-voltage or smart charging before choosing between VSR and DC-DC. Ask the retailer or your dealer, and quote your VIN.
- Confirm the battery tray or mounting kit is listed for your series and build date — engine bays change between generations.
- Check the battery's temperature rating against where you plan to mount it, especially for lithium under a bonnet.
- Size cable for the actual run length, and fuse every circuit close to the source.
- Measure or estimate your daily amp-hour draw before choosing capacity.
- Budget for a battery monitor. It pays for itself in avoided guesswork.
- If you're not confident with high-current wiring, get an auto electrician to do the install.
This guide is general information, not fitting or engineering advice. Fitment, compliance and safety depend on your specific vehicle — confirm with the retailer and a qualified fitter before you buy.