Electrical basics for van builders

The four numbers

Voltage is electrical pressure. Current (amps) is flow. Power (watts) is what a device uses right now: W = V x A . Energy (watt-hours) is power over time: Wh = W x hours . A 60 W fridge averaging 8 hours of compressor time uses about 480 Wh/day. Amp-hours only make sense when voltage is stated: 100 Ah at 12.8 V is about 1,280 Wh; 100 Ah at 25.6 V is 2,560 Wh.

AC, DC, series and parallel

Batteries and most van loads use DC. Household outlets use AC. An inverter changes DC to AC; a charger changes AC to controlled DC. In series, voltages add and current stays the same. In parallel, voltage stays the same and available current/capacity adds. Only combine components that are approved to be combined.

Why system voltage matters

At the same power, doubling voltage halves current. A 2,400 W inverter can pull roughly 220 A from a loaded 12 V bank after losses, 110 A at 24 V, or 55 A at 48 V. Lower current means smaller cable and less heat, but 24/48 V require compatible equipment or converters for 12 V loads.

Surge, drop and loss

Motors and compressors may start at several times running power. Voltage drop is pressure lost in wire and connections. Loss becomes heat: P = I²R, so high current punishes bad connections. Design important DC circuits around 3% drop and other circuits around 10% only where the equipment and applicable rules allow; for battery/inverter links, lower is better.

Ohm's Law you need

Use V = I x R to understand trouble. If a connection adds only 0.01 ohm at 200 A, it drops 2 V and makes 400 W of heat. That is why clean surfaces, correct lugs, specified torque and short high-current runs matter.

Worked example

A 1,200 W induction hob on 120 V uses 10 A AC. Through a 90% efficient inverter from a 12.0 V battery, it needs about 111 A DC: 1,200 / (12 x 0.90). Ten minutes uses about 222 Wh from the battery.

CHAPTER CHECK

• I can explain the purpose of every component covered here. • I can identify the exact manufacturer limit that controls my design. • I have added the decision, rating or measurement to my schematic/worksheet.