Vehicle Power · Volume 2
Charging From a Modern Truck, Which Is Not What It Used to Be
Introduction
🔴 THIS VOLUME DESCRIBES AN ARCHITECTURE THAT WAS NOT BUILT. Corrected 2026-08-30. There is no DC-DC charger and no house battery in this rig. The fridge came out of the build, the CPAP became the only load that mattered, and the answer collapsed to an EcoFlow River 3 Max Plus (858 Wh, 24 lb) charged from the truck’s 12 V socket at 220 W and from a Renogy 200 W portable panel. ⭐ The volume is kept because the reasoning is sound and the decision is a real one a reader may face — but it is a road not taken. → Vehicle Power vol 1 and Weight and Balance vol 3.
The traditional dual-battery installation was a relay. It closed when the engine ran, paralleling a second battery onto the alternator, and it opened when the engine stopped so the starting battery could not be flattened. It was cheap, it was simple, and on a 2026 truck it does not work properly.
Understanding why is the whole of this volume, and it determines what gets bought.
2.1 The smart alternator problem
Modern vehicles do not run their alternators at a fixed voltage. To improve fuel economy, the engine controller varies alternator output continuously — raising it to recover energy during deceleration, and dropping it once the starting battery is satisfied, sometimes to little more than the battery’s resting voltage. On a vehicle with engine stop/start, this behaviour is more aggressive still, because the whole system is tuned around battery state of charge.
This truck has stop/start, which is a reliable indicator that it also has smart charging and a battery-monitoring sensor watching the starting battery’s state.
Two consequences follow, and both break the traditional approach:
A relay-based isolator cannot charge a house battery properly, because it simply passes through whatever the alternator happens to be producing. When the alternator drops to 12.8 V because the starting battery is full, the house battery receives 12.8 V — which will never fully charge anything, and will never charge lithium at all.
A LiFePO4 house battery makes this worse, not better. Lithium wants a specific charge voltage in the region of 14.2–14.6 V with a hard cutoff, and its flat discharge curve leaves very little margin. Below about 14.0 V it never reaches full; above roughly 14.8 V, cells are at risk. A variable-voltage alternator wandering between 12.8 and 15 V satisfies neither requirement. With an isolator, a lithium bank commonly sits permanently around 80% charged, which quietly costs a fifth of the capacity that was paid for.
There is a second, subtler hazard in the other direction: a large, deeply-discharged lithium bank connected straight to an alternator will accept enormous current, because lithium has very low internal resistance and no self-limiting behaviour. Alternators are cooled by assuming they will not run at full output indefinitely. This is a known way to destroy an alternator, and on a new truck it is an expensive way.
2.2 The DC-DC charger, and why it is not optional here
A DC-DC charger sits between the vehicle’s electrical system and the house battery and re-processes the input rather than passing it through. It takes whatever the alternator produces — 12.5 V, 14.8 V, anything in its input range — and produces a controlled, multi-stage charge profile matched to the house battery’s chemistry.
That solves all three problems at once:
- It boosts low alternator voltage to the 14.4–14.6 V a lithium bank needs, so the battery actually reaches 100%.
- It limits current to its own rating, which protects the alternator from a hungry lithium bank.
- It isolates, so the house loads can never flatten the starting battery.
For a modern vehicle with a smart alternator and a lithium house battery, this is not a refinement. It is the only architecture that works.
2.3 The gotcha that catches people at installation
DC-DC chargers normally decide the engine is running by watching input voltage — above a threshold, start charging. On a smart-alternator vehicle this detection fails, because the alternator may legitimately be producing a low voltage while the engine runs perfectly.
The fix is a switched ignition signal wire to the charger’s engine-detect input, telling it explicitly that the engine is running and charging is permitted regardless of what the voltage looks like. Most quality DC-DC chargers provide this input specifically for the purpose.
Plan for this wire at installation. Retrofitting it means going back into the dash for a switched source, and a “why is my battery not charging” investigation that ends here is a common and avoidable waste of a weekend.
2.4 Sizing the charger
The previous volume established that driving is the primary charging source, so the charger’s rating determines how quickly a day of camp is recovered.
Table 1 — Sizing the charger
| Charger | Recovery rate | Two hours of driving |
|---|---|---|
| 20 A | 20 Ah/hour | 40 Ah |
| 30 A | 30 Ah/hour | 60 Ah |
| 40 A | 40 Ah/hour | 80 Ah |
| 50 A | 50 Ah/hour | 100 Ah |
Against a daily consumption of 50–65 Ah, a 30 A unit restores a full day in about two hours of driving, which matches how this rig will be used. A 40 A unit buys margin for hot weather and longer camps; a 50 A unit starts to ask real questions of the truck’s alternator and its wiring, and is more charger than a single 100 Ah bank needs.
30 A is the right answer, 40 A if the budget allows and the wiring is sized for it.
A note on the alternator: a DC-DC charger draws its rated output plus conversion losses from the vehicle continuously while charging. A 30 A charger is a real and sustained additional load, and it is worth confirming the truck’s alternator capacity before adding it to headlights, a heated cab, and a winch. On a modern mid-size truck a 30 A continuous accessory load is generally comfortable; it is worth verifying rather than assuming.
2.5 Where the house battery goes
Three candidate locations, each with a real tradeoff.
Under the hood. Shortest cable run, simplest install — and the worst thermal environment on the vehicle. LiFePO4 must not be charged below freezing, and engine-bay temperatures in summer are hostile to cycle life at the other end. Most quality lithium batteries include low-temperature charge cutoff protection, which prevents damage but also means the battery simply refuses to charge on a cold Michigan morning. Under-hood is the least good option for lithium.
In the bed, under or beside the DECKED system. Reasonable access, good for weight placement — low and ahead of the rear axle — but it means a long, heavy cable run from the front, and the bed is not a sealed environment.
In the cab, behind or under a rear seat. The best thermal environment by a wide margin, which matters most for lithium and matters especially in a Michigan winter. It also puts the battery inside where a low-temperature cutoff is least likely to trigger. The cost is space, and the requirement that the installation be genuinely secure — an unrestrained 30 lb battery in a cab is a serious hazard in a collision.
For this build, in-cab or in the bed ahead of the axle, low, and properly restrained. The thermal argument for the cab is strong on a truck that will be used in cold weather; the bed is the compromise if space is unavailable.
2.6 Cable sizing, which is a safety item
The cable between the starting battery and the DC-DC charger carries the charger’s full rated current continuously, over a run that on a crew-cab truck may be fifteen to twenty feet each way.
Two rules that are not negotiable:
Size for voltage drop, not just for current. A cable adequate to carry 30 A without melting may still drop enough voltage over twenty feet that the charger sees less input than it needs. The usual target is under 3% drop. On a run of this length at 30 A, that means substantially heavier cable than intuition suggests.
Fuse at the source, at both ends. A fuse protects the cable, not the device, and it must sit at the end where the energy comes from — within a few inches of the battery terminal. An unfused cable from a starting battery is a fire waiting for a chafe point, and it will deliver hundreds of amps into a short without hesitation. Both ends get a fuse because the house battery is also a source.
Protect the cable everywhere it passes through metal. Grommets at every bulkhead, abrasion sleeve where it runs along a frame rail, and secured every few feet so it cannot move. Vehicle wiring fails by chafing, and chafing is a function of movement.
2.7 The stop/start interaction, which cuts both ways
The Cab and Cockpit dive covers the harness that permanently defeats the truck’s engine stop/start. It has an electrical consequence worth noting here.
With stop/start active, the engine shuts down at every halt — and the alternator stops with it, while the accessory loads keep drawing. A cab full of radios and a fridge in the bed will drain during those stops, and the vehicle’s own battery-monitoring will eventually inhibit stop/start because of it.
Defeating stop/start therefore also stabilises the charging system, keeping the alternator turning whenever the engine is running. That is a genuine secondary benefit of the harness and it is worth recording, because it is not why the harness was bought.
References
- DC-DC charger versus battery isolator: an isolator is a relay that passes alternator voltage through unregulated; a DC-DC charger re-processes it into a controlled multi-stage profile.
- Smart alternators reduce output once the starting battery is satisfied, which can leave a voltage-sensing DC-DC charger below its trigger threshold; a switched ignition signal wire to the charger’s engine-detect input is the standard remedy.
- LiFePO4 charge requirements: approximately 14.2–14.6 V depending on manufacturer, with incomplete charging below about 14.0 V and cell risk above about 14.8 V; a DC-DC charger is required rather than recommended for lithium on a modern vehicle.
- Lead-acid usable depth of discharge of approximately 50%, giving roughly a 2:1 capacity requirement against LiFePO4.
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