12V power for overlanding fails for one reason: it was sized from a product list rather than from an energy budget. A fridge, two lights and a phone charger do not need 200 Ah — but they do need a battery, a charging path and cable that can carry the current without losing voltage. Here is the arithmetic, in the order you should do it.
Work in amp-hours (Ah) at 12 V. Every appliance has a rated current; multiply by hours used; sum; add 30% margin for inverter losses if you run mains equipment.
12V power for overlanding, step 1: build the daily load table
| Load | Current | Hours/day | Ah/day |
|---|---|---|---|
| 60 L compressor fridge (cycling) | 4.5 A running | 10–14 h effective | 45–60 |
| LED interior / camp lights | 0.5 A | 4 | 2 |
| Phone + tablet + camera charging | 2 A | 1.5 | 3 |
| Water pump (shower) | 3 A | 0.25 | 0.75 |
| Diesel heater (winter only) | 1.5–10 A | 2–6 | 8–25 |
| Laptop via inverter | 5 A at 12 V | 2 | 10 (+15% loss) |
A realistic summer figure for two people: 50–65 Ah/day. Winter with a heater: 70–90 Ah/day. That is the number everything else is designed around.
Step 2: size the battery on usable capacity
Lead-acid and AGM batteries should not be discharged below 50% if you want more than 300 cycles. LiFePO4 can go to 90–100% routinely without meaningful capacity loss.
| Chemistry | Rated | Usable (to 50%) | Weight | Cycle life |
|---|---|---|---|---|
| AGM 100 Ah | 100 Ah | 50 Ah | ~30 kg | 300–500 |
| Gel 100 Ah | 100 Ah | 50 Ah | ~32 kg | 500 |
| LiFePO4 100 Ah | 100 Ah | 90 Ah | ~12 kg | 2,000–4,000 |
So a 100 Ah LiFePO4 replaces roughly a 180 Ah AGM bank, at a third of the weight. For a 65 Ah/day load with one day of autonomy, 100 Ah LiFePO4 is the minimum sensible size; 150 Ah gives you two cloudy days without panic.
Two cautions: LiFePO4 must not be charged below 0 °C (most good BMS units block it), and it needs a charger with a lithium profile — an old alternator-only setup will only fill it to 70–80%.
Step 3: size the solar to the load
The usable output of a flat-mounted panel is roughly:
Ah/day ≈ panel watts × 0.9 ÷ 13 V × effective sun hours × 0.75
The 0.75 covers heat derating, angle losses, dust and controller efficiency. Effective sun hours vary from about 5.5 (summer, low latitude) to 2.5 (winter, high latitude).
| Panel | Summer (5 h) | Winter (2.5 h) |
|---|---|---|
| 100 W | ~26 Ah | ~13 Ah |
| 200 W | ~52 Ah | ~26 Ah |
| 300 W | ~78 Ah | ~39 Ah |
| 400 W | ~104 Ah | ~52 Ah |
Against a 65 Ah/day load, 200 W of solar covers you in summer and leaves a deficit in winter — which is why most serious setups pair 200–300 W of solar with a DC-DC charger from the alternator for driving days.
Step 4: MPPT vs PWM
MPPT controllers convert excess panel voltage into extra current. The gain is typically 15–30% in cool or low-light conditions, less in hot full sun. For a 12 V system with a “12 V nominal” panel, PWM is acceptable; with a 30–40 V house panel, MPPT is essential. Price difference is small relative to the battery it protects.
Step 5: cable, fusing and voltage drop
Voltage drop is the hidden killer. A 3% drop on a 12 V system is 0.36 V, which is enough to make a smart charger read the battery as fuller than it is and cut back charge current.
| Current | Cable length (round trip) | Minimum cross-section |
|---|---|---|
| 20 A | up to 4 m | 6 mm² |
| 30 A | up to 5 m | 10 mm² |
| 40 A | up to 5 m | 16 mm² |
| 60 A | up to 3 m | 25 mm² |
Fuse at the battery, not at the load. A 60 A ANL fuse on the main feed, then individual blade fuses on each branch (15 A for the fridge circuit, 5 A for lights). A fuse protects the cable, so it must be rated below the cable’s ampacity and as close to the source as possible.
Step 6: DC-DC charging from the alternator
Direct alternator charging through a relay works on older vehicles with simple charging systems. On modern vehicles with smart alternators, the voltage drops to 12.4–13.2 V for long periods and a relay will barely charge anything. A DC-DC charger takes whatever voltage is available and outputs a stable 14.2–14.6 V charge profile.
Typical sizing: 25–40 A for a 100–150 Ah bank. Remember that this load adds to the alternator’s existing demand — check the alternator rating before adding a 40 A charger.
The practical build for a 4WD
- 100–150 Ah LiFePO4 in the rear, close to the loads, with a 40 A DC-DC charger and 200–300 W solar through an MPPT controller.
- One main 60 A fuse at the battery positive, a battery isolator switch, and a shunt-based monitor so you know the real state of charge — voltage alone is useless on lithium.
- Separate the charging path from the load path so a fault on one does not take out the other.
- Keep the battery above 0 °C for charging; if you camp in real winter, insulate the battery box or use a self-heated battery.
If you are kitting out a fleet or a dealership demo vehicle, our OEM/ODM team can advise on 12 V accessory specifications that pair with our camping equipment range.
Related guides: Camping water filtration and storage · Camping shower setup
