The Dual-Battery Setup for Overlanding: Wire the Truck Once (DC-DC Charger + Lithium, Sized Right)

A dual-battery system does one job: it lets the fridge, the lights and the dish run for days without touching the battery that starts the truck. Simple idea. The reason so many builds disappoint is that people buy the two expensive boxes first and treat the wire, the fuses and the maths as an afterthought. The wire is the build. Get the five numbers right and any decent battery and charger will do the job for a decade.

So that’s the order here. What the fridge actually draws. How big the bank needs to be. How many amps the charger should push, and why bigger is often wrong. What gauge the cable has to be, measured the way nobody measures it. Where the fuses go. Then the picks, the layout, and the list of ways this goes wrong that I pulled from the owner threads. And before any of it, the honest question: whether you should build this at all.

First: should you build this at all?

If you already own a decent power station and you camp a handful of weekends a year, the honest answer is probably no. An alternator charger like the EcoFlow 800W pushes up to 800W into a power station while you drive, which refills a 1kWh unit in about an hour and twenty minutes. The old objection to power stations, that they trickled at 100W off the cigarette socket and took all day, is dead. The station is the battery, the BMS, the inverter and the solar controller in one box, and it comes out of the truck for a power cut at home. Our car-charging guide covers that route in full.

Build the wired system instead when you want 12V outlets distributed through the vehicle rather than everything cabled back to one box, when you need sustained high current (a winch, an inverter that runs a real appliance), when you want to replace a failed part instead of a whole unit, or when this is a permanent camper rather than a daily driver. Note that the alternator charger still needs a fused positive run, a ground and correctly sized cable. It skips the distribution build, not the wiring. Everything in the cable and fuse sections below applies to it too.

Not sure you need any of it yet? Can you run a 12V fridge off your car battery overnight works out how far the starter battery alone gets you: fine for one night, not for a parked camp day.

Number one: what the fridge actually draws

The fridge is 70-80% of a camping rig’s daily load, so everything downstream is sized from it. Ignore the nameplate. The number that matters is the metered daily average, compressor cycling included. The best test I found ran three Dometic units on three shunts for 48 hours at a 34F setpoint: a CFX3 75DZ used 31 Ah a day, a 55IM 25 Ah, a CFX50 22 Ah. Owner meters on other fridges land in the same band: an Engel MT45 at 17.5 Ah, an EdgeStar at 26 Ah in a summer van, an ARB 47L at under 10 Ah on a mild fridge setting but 21 Ah run as a freezer. The outlier is an Indel B in freezer mode at over 100F: 40-65 Ah a day. Our amp-draw guide has the full set.

Fifty amp-hours a day is the planning number for a normal weekend rig. Eighty if you work from it or heat it. If someone tells you their 100Ah battery runs the fridge for a week, the fridge wasn’t running.

Number two: how big the bank needs to be

Rated capacity is not usable capacity. Lead-acid and AGM should not go below 50%, so a 100Ah AGM gives you 50Ah. Lithium iron phosphate (LiFePO4) is happy to 80% depth of discharge every day and most makers say 100% is fine for lifespan; design at 80% and you have margin for a cold morning. So: a 100Ah LiFePO4 is 80Ah usable, which is about two nights of a weekend rig with no driving and no solar. A 200-230Ah bank is a long weekend. That is the whole sizing rule.

It also settles the AGM-or-lithium question for anything that lives inside the vehicle. A 100Ah lithium in a Group 24 case weighs about 21 lb and gives ~90Ah. The best pure-lead AGM in Group 31 weighs 76 lb and gives ~51Ah. Roughly three and a half times the weight per usable amp-hour, with a third of the cycle life. AGM keeps exactly one job in this build, and it’s in the layout section.

Number three: how many amps the charger should push

A DC-DC (battery-to-battery) charger sits between the starter battery and the house battery. It takes whatever the alternator gives, boosts it to the 14.4-14.6V lithium needs to finish, caps the current so a hungry lithium bank can’t cook the alternator, and keeps going through the 12.4-12.8V eco-mode dip that every 2015-plus vehicle drops into. A plain isolator or voltage-sensing relay does none of those three things, which is why Victron’s own engineers have a blog post about alternators smoking on lithium. For lithium, or for any modern vehicle, the charger is not optional.

Size it by the lower of two ceilings. Battery ceiling: no more than 50% of the lithium bank’s amp-hours (a 100Ah battery tolerates 50A). Alternator ceiling: no more than 50% of the alternator’s rated output, continuously, because the rest of the truck still needs it. A 120A alternator caps you at 60A, a 90A alternator at 45A. Take the smaller number, then check the battery’s own maximum charge current, which on a couple of brands is only 50A. Practical floor is about 20A per 100Ah; below that you’re charging through a straw.

Those are driving hours at rated output, with 10% added for losses. Now discount them. The only long-run field log I found of a 50A charger shows 45-48A at highway speed but 30A at idle. Town driving, traffic and a long cable all push you toward the idle number, so add 20-30% for a real trip. Even so, look at the fridge row: a 25-30A charger puts a whole weekend of fridge back in the drive home. The case for 40-50A is Starlink, a diesel heater, or short hops between camps, not a fridge. Most weekend rigs are overbuilt here.

Number four: the cable, measured the way nobody measures it

Here’s where builds quietly fail. Redarc’s own support page lists undersized cable as the first reason a charger isn’t charging from the vehicle; Renogy’s troubleshooting guide says the same in different words. A charger fed at 13.1V after a long thin run does less than one fed at 14.2V, and the gauge you need depends on the length of wire the current travels, which is positive plus return, added together. A charger ten feet of wire away from the starter battery is a twenty-foot run. Most online charts quietly assume one-way and double it for you; some don’t say. Get it wrong and you are about three wire sizes off.

That table is the marine (ABYC) formula at a 3% voltage drop. The manufacturers’ own tables run a touch looser: Redarc calls for 8 AWG on a 25A unit up to 5m and 6 AWG beyond, 6 AWG on the 40A up to 5m and 4 AWG to 9m; Victron wants 6 AWG under 5m and 4 AWG to 10m on its 50A. If the manufacturer’s table says bigger, follow it. One more trap: a charger draws more on the input side than it puts out, because it’s boosting voltage at about 90% efficiency. A 40A Renogy metered at 39-40A out was pulling 49A in, and spiked near 80A at engine start. Size the input cable and the input fuse for that, not for the number on the box. Buy fine-stranded welding cable rather than stiff automotive cable; it survives the firewall bend and the vibration.

Number five: the fuses, and where they go

One at each end of the run between the two batteries, within seven inches of each battery terminal. That’s the marine standard and it exists because a single Group 31 battery can deliver around 5,000A into a short. An unfused run from the starter battery is the one mistake in this build that burns the vehicle. Redarc specifies a 40A fuse each side of its 25A charger and 60A for the 40A and 50A units; Victron says 60-70A on its 50A. Bolt-down MIDI or MRBF terminal fuses, which mount straight on the battery post and satisfy the seven-inch rule by construction. Redarc is blunt about the alternative: blade fuse holders and circuit breakers are not recommended. One tester documented a cheap breaker labelled 250A tripping every time the alternator hit 85A, and 20 of 22 no-name ANL fuses failing unsafely on the bench.

Ground is the other half of the circuit and the part that fails slowly. Bare metal, paint removed, star washer, sealed, and on the frame or engine block rather than a body panel. On a fresh install a good chassis ground measures better than the supply wire. Five years of corrosion later, it’s the reason the charger reads low. Full-time builders run a dedicated negative back to the starter battery for exactly that reason. For a weekend truck a properly prepared chassis ground is fine.

The wire people forget: ignition sense on a smart alternator

If the vehicle is 2015 or newer, has start-stop, or is a Jeep with eTorque, assume it has a variable-voltage alternator that sits at 12.4-12.8V while cruising to save fuel. A charger that decides the engine is running by watching voltage never sees the 13.2V it’s waiting for, so it never turns on. One caravan owner metered his relay-based system with the engine running and found 4-5A flowing out of the leisure battery into the car. The fix is a wire. Redarc’s blue wire to a switched ignition feed drops its turn-on threshold from 13.2V to 12.0V. Renogy ships an IGN wire in the box. Victron does it by voltage with a smart-alternator preset, which works on most vehicles and fails on the Euro vans that sit at 12.3V, where owners either retune the thresholds or feed the H terminal a switched positive. Find the switched source with a meter at the fuse box: 12V in ON, zero in ACC, tap it with a fuse tap. And the reverse trap: on an older, conventional alternator, don’t connect the Redarc blue wire, or the charger keeps running with the engine off and flattens the starter.

Where it all goes (not under the hood)

The factory second-battery tray in the engine bay is tempting: shortest cable, tray already there. It is the wrong place for lithium and for the charger. Redarc states flatly that manufacturers will not warrant a lithium battery mounted under the hood, its manual restricts the lithium charge profile to cabin installs, and the charger itself derates above 55C and shuts off at 80C. Under-hood air runs 122-140F in normal driving and past 158F near the exhaust, and it keeps climbing for a minute or two after you park. A lithium pack is rated to about 135-140F. Park it there and the BMS disconnects mid-trip with the pack nominally full, and the fridge goes dark.

• In the cab or behind the seat. Cool, dry, damped, where Redarc points you. The default answer. The input run gets long (20-30 ft round trip), so the cable table above does the work. Keep the charger within a metre of the house battery on the output side.
• A drawer system in the bed or cargo area. Battery, charger, shunt and a fused distribution block all in one serviceable place, each branch (fridge, lights, pump, USB) on its own blade fuse. The longest input run of all, so budget for 4 AWG. Best for a permanent build.
• A removable battery box on Anderson plugs. Lifts out for camp. Fine on SB50 connectors at 25-30A. At 40-50A the input current exceeds the plug’s rating; go SB120 or hard-wire.
• The engine bay. Only for an AGM, and only if that’s the only place it fits. The Odyssey below is the one battery in this post rated to live at 176F.

The house battery

One warning before the picks. The DIY battery world’s most-cited tester, Will Prowse, published a series in late 2025 alleging a terminal design defect in Battle Born’s 100Ah pack; Battle Born’s parent company sued him for trade libel in June 2026, alleging his tests were rigged and that he had taken affiliate income from them for years. It is in court and unresolved. I’m not recommending Battle Born until it settles, and I’m not condemning them either. What the episode does prove is that no single YouTube verdict is settled fact, which is why every number in this post is cross-checked against at least two sources.

The DC-DC charger

The parts that make it not catch fire

None of this is glamorous and all of it is where the money should go before you upsize the battery. Tinned copper lugs with adhesive-lined heat shrink on every termination. A hydraulic crimper if you’ll build more than one rig; for a single install, an auto electrician will crimp four lugs for pocket change and that is a perfectly good answer. A DC-capable clamp meter, because the only way to know the charger is delivering rated amps is to measure it; many cheap clamps are AC-only. And a shunt, because every Bluetooth battery app is a coulomb count that drifts; the fridge numbers at the top of this post came from three of them.

• Blue Sea 5026 ST Blade fuse block – 12 circuits, negative bus and cover; the distribution block a drawer build is organised around. Size each branch fuse at 125% of its load and round up (a fridge gets 7.5A for the compressor start).
• Blue Sea 2151 MRBF terminal fuse block with a 60A MRBF fuse – bolts to the battery post, which satisfies the seven-inch rule with nothing dangling.
• Victron SmartShunt – the instrument behind the fridge test above. Tells you what the rig actually draws instead of what you guessed.
• 4 AWG welding cable, 25 ft red + 25 ft black – covers the worst case in the table and bends round a firewall.
• 4 AWG tinned copper lugs with adhesive heat shrink – corrosion is what kills grounds and terminals; tinned and sealed is the fix.
• 16-ton hydraulic lug crimper – a hammer crimp is one swing from a cold joint, and a cold joint on the input run is the failure mode owners mistake for a dead charger.
• DC clamp meter, 400A – must say DC, not just AC.

How this goes wrong (from the owner threads)

• Input cable sized to the output number. A 40A charger pulls up to 60A in and near 80A at start-up. Fuse and cable the input.
• No ignition wire on a smart alternator. The single most common no-charge complaint in the forums. The charger is waiting for a voltage it will never see.
• Ignition wire connected on a conventional alternator. The opposite mistake: the charger runs with the engine off and the starter battery is flat by morning.
• Lithium in the engine bay. Warranty gone, and the BMS disconnects the fridge on the first hot afternoon.
• Ground on a painted panel. Works on day one. Then it doesn’t. Block or frame, bare metal, sealed.
• No fuse at the starter-battery end. Five thousand amps looking for a way out. This is the one that burns the truck.
• A cheap resettable breaker instead of a bolt-down fuse. Trips at half its rating, or doesn’t trip at all.
• Charger mounted where it can’t breathe. Redarc derates above 55C, Victron 1.5% per degree above 40C. It still shows a charging light. It just isn’t doing much.
• A melted terminal blamed on the charger. One owner’s dead Renogy read 14V unloaded and 2V under load; the fault was a cooked negative lug. Check the lugs before the box.
• Buying a Renogy DCC50S for 50A of alternator charging with solar connected. You’ll get 25.
• A base LiTime charged on a frosty morning. No low-temp cutoff means every sub-freezing drive-charge takes a little capacity for good.

What to skip

• A plain isolator or VSR with lithium. It can’t reach 14.4V, can’t cap the current, and drops out in eco mode. It’s a relay, not a charger.
• 50A and 200Ah for a fridge-only rig. A 100Ah bank and a 25-30A charger replaces a weekend’s fridge in the drive home. Spend the difference on cable and lugs.
• The Sterling BB1230 if you read it as 30A. It’s 30A in, about 22A out. Honest engineering, misleading name.
• Bluetooth as a reason to buy. Every app is a drifting coulomb counter with a 15-20 ft range. A shunt is more accurate than all of them.
• Marine-grade Class T fusing on a camping rig. MRBF or MIDI at the right value is what Redarc and Victron specify and it satisfies the standard. The non-negotiable is a fuse at both ends within seven inches, not which exotic fuse.
• Battle Born, for now. Not because the allegation is proven; because it isn’t, and there are equivalent packs without an open lawsuit attached.

Build the rest of the rig: the 12V fridge this battery is for · how long a battery runs a fridge · charging a power station while driving · what Starlink Mini adds to the budget · the air compressor that runs off it · recovery boards for when the power isn’t the problem.

The other half of setting up the truck is what gets it unstuck. The recovery kit checklist sizes the rope and shackles the same way this guide sizes the wiring: from the door sticker, with the failure named.

FAQ

What size DC-DC charger do I need for a 100Ah lithium battery?

Twenty-five to forty amps. The battery tolerates up to 50A, but the alternator shouldn’t give more than half its rated output continuously, and a 25A charger already puts a weekend of fridge use back in about two and a half hours of driving. Go to 40-50A when the bank is 200Ah or more, or when a Starlink or a diesel heater is in the daily load.

Can I just use a battery isolator instead of a DC-DC charger?

Not with lithium, and not on a vehicle built after about 2015. An isolator passes whatever the alternator gives: too low a voltage to finish charging lithium, no current limit to protect the alternator, and it drops out entirely when a smart alternator falls into 12.5V eco mode. The DC-DC charger fixes all three.

Can a lithium battery go under the hood?

No. Lithium packs are rated to around 135-140F and the engine bay routinely passes that; manufacturers void the warranty for under-hood mounting and Redarc restricts its lithium charge profile to cabin installs. If the engine bay is your only option, use a heat-rated AGM like the Odyssey and accept half the usable capacity.

What gauge wire do I need for a DC-DC charger?

Depends on the amps and the round-trip length (positive plus return added together). As a starting point: 8 AWG for 25-30A up to 20 ft round trip, 6 AWG for 40A, 4 AWG for 50A or any run past 20 ft. Then check the charger’s own manual, which may call for a size bigger, and size for the input current, which runs 15-50% above the output on most units.

Do I need the ignition wire?

On any vehicle with a smart or variable-voltage alternator, which is most of them from 2015 onward and everything with start-stop, yes. Without it the charger waits for a voltage the alternator never reaches while cruising. On an older vehicle with a conventional alternator, leave the Redarc blue wire disconnected or the charger will run with the engine off.

Is a power station a better option than a dual-battery system?

For a lot of weekend campers, yes. With an alternator charger a power station refills at up to 800W while you drive, it needs no distribution build, and it leaves the vehicle for use at home. The wired system wins when you want outlets all over the vehicle, sustained high current for a winch or inverter, or a permanent camper you can service part by part.