Van leisure battery without sun: DC-DC, shore, generator

Winter yield does not drop a notch, it changes order of magnitude. Low sun, short days, a sky that is overcast more often than not: in December, a 200 Wp array returns roughly 0.1 to 0.3 kWh per day at our latitudes, against 0.8 to 1.2 kWh in June. The gap between the two seasons is counted in multiples, not in percentages.

Demand meanwhile moves the other way. Nights are long, so lighting runs more; a forced-air heater draws power for its blower and its glow plug; damp air forces you to ventilate and then to reheat. An already frugal winter van commonly burns 0.5 to 0.8 kWh per day excluding electric heating — several times what the roof is producing at that moment.

Hence the conclusion to hold on to before buying anything: adding surface area repairs nothing, because doubling a negligible output gives a negligible output. What changes the equation is a source that is decoupled from the weather.

You can put a number on that shortfall for your own van: our solar and battery calculator takes your appliances and your destination to show your winter shortfall month by month.

• • Alternator via a DC-DC charger — on a Euro 6 vehicle the alternator is smart: its voltage is regulated for the starter battery and swings roughly between 12.2 and 14.8 V. A plain split-charge relay can no longer cope — either it never reaches the absorption voltage of a LiFePO4 house bank, or it lets the house bank pull the starter battery down. The DC-DC charger takes whatever the alternator gives and delivers the right profile. At 50 A and about 13.5 V that is close to 0.68 kWh per hour of driving: one hour of road covers a frugal day, and charging while you drive becomes plannable instead of hopeful.
• • 230 V shore power — a 15 to 30 A charger refills a bank overnight. But the bollard is often limited to between 4 and 10 A, i.e. 900 to 2,300 W. An electric heater and a charger at the same time, and the pitch breaker decides for you — sometimes the breaker for the whole row.
• • Generator — a last resort, and it has to be owned as one. A small inverter unit burns roughly 0.5 to 0.9 L/h for about a kilowatt, and it is prohibited on most aires and in most parking situations. It buys energy at the price of everything that made the spot worth stopping at.

Two limits are not negotiable. The first: never size the DC-DC past the alternator. A continuous draw beyond roughly a quarter of its rated output, on top of the vehicle's own loads, is the classic way to cook an alternator. Engine-running detection and temperature protection are not comfort extras.

The second: cable section does the other half of the job. Apply the house formula S = (I × L × 2) ÷ (ΔV × 56), with I in amperes, L the one-way length in metres, ΔV the accepted drop in volts and 56 the conductivity of copper. For 50 A over 4 m with a 0.4 V drop: S = (50 × 4 × 2) ÷ (0.4 × 56) ≈ 18 mm², so 25 mm² off the shelf. An undersized run turns the charge into heat and the fuse into a suggestion.

None of the three sources is better in the abstract. It is the shape of the deficit — its cause, not its size — that names the answer.

And the rule that comes before all the others: no charging source is cheaper than a load you removed. A kWh you never consume needs no cable, no alternator, no fuel and no pitch with a bollard. It is the only lever that improves winter and summer at once.