The original master security switch clips into a battery cover that only fits Oldham-pattern batteries (types 015 and 038, the latter with pole adapters). Every original battery is now dead, so most C5s are rewired to run from a modern car, leisure or LiFePO4 battery. Other reasons:

• Lost keys. The switch keys were individually coded, and replacements can no longer be ordered.
• Charging. The original charger plugs into the switch aperture in the battery cover. Modern batteries need their own charger on ordinary terminals.
• Quick battery swaps. Quick-release clamps make it easy to take the battery out for charging.
• Conversions. 24 V kits and replacement control boxes bring their own power switching.

The master security switch did two jobs besides connecting the battery:

• Emergency stop. Its red cut-out button isolates the motor if the motor relay welds shut, which would otherwise make the C5 run away. Always fit a kill switch within easy reach.
• Security. Without a key, anyone who finds the switch can drive the C5 away. Fit a key switch (some owners hide it, for example under the instrument pod) and lock the C5 to something solid. Sinclair recommended a cable lock.

The clearest published wiring is John Wilson's “Simplified Relay Controls” on c5owners.com. It runs the original 12 V motor with or without the original electronics.

1. Battery + → 5 A fuse → kill switch (toggle) → key switch → 5 A diode → relay 1 coil (pin 85). Pin 86 goes to battery −.
2. Battery + (6 mm² cable) → relay 1 pin 30. When the key is on, relay 1 connects pin 30 to pin 87, which becomes the switched +12 V supply.
3. Relay 1 pin 87 → 5 A fuse → handlebar power switch → (optional) thermal trip → relay 2 coil (pin 85). Pin 86 goes to battery −.
4. Relay 1 pin 87 (6 mm²) → relay 2 pin 30; relay 2 pin 87 (6 mm²) → motor +. Motor − (6 mm²) goes to battery −.
5. Optional 80 mm 12 V cooling fan across the motor feed, so it runs whenever the motor does. Wire it to relay 1 pin 87 instead to keep it running all the time the C5 is on.

Parts:

• two 70 A 12 V automotive relays (4-pin);
• a 5 A, 100 V diode;
• two 5 A fuses in automotive holders;
• a toggle switch (kill switch) and a 2-pin key switch;
• 6 mm² cable for the battery and motor circuit;
• 8 A cable for the switching circuits;
• a quick-release battery terminal set;
• 9.5 mm (35 A) spade connectors for the relays and motor;
• 6.3 mm (15 A) spade connectors for the switches.

Cheap 100 A jump leads are a source of heavy cable. The original wiring can also be reused: brown = +, blue = −.

The diode: it sits in series with relay 1's coil, with its silver band towards pin 85. If the battery is ever connected the wrong way round, relay 1 can't energise, so nothing else is powered. This is the “safety” in safety wiring kits; connecting a battery backwards is a common way to destroy an original control box.

Feed relay 1's switched +12 V (pin 87) to the original control box at X5 (the brown +12 V auxiliary connection), and connect X7 to battery −. The control box is fused internally, so the original lights, indicators and horn work as Sinclair intended.

Put a 200 A shunt in the negative lead between battery and motor, and fit a combined volt/ammeter (100 V / 200 A type), fed through a 1 A fuse from relay 1 pin 87. It can be mounted in a modified pod housing.

These match the original motor-load display, which shows about 15 A per segment and 30 A as the maximum continuous current. See How the electrics work.

John Wilson's set also covers:

• indicators from scratch with an on-off-on switch and flasher relay (with LED bulbs, use an LED-compatible flasher relay and a smaller fuse);
• hazard warning lights using 5 A diodes;
• a horn relay (30 A relay, 15 A fuse for a car horn; a 1.5 A motorcycle horn needs only 5 A);
• a 24 V version with a 4QD Porter 10 speed controller.

• Chas's safety wiring kit (“SINCLAIR C5 CHAS”, sold on eBay from about 2013). It connects to the control box battery terminals in place of the key switch leads, so the original pod and control box keep working. It switches power through a relay and switch, clamps onto a car or leisure battery, and protects against reverse polarity. No schematic has been published.
• C5Alive QR Power Wiring kit (v2, February 2006). Quick-release clamps and a relay switched by a replacement dual-pole lamp switch, wired to the control box where the brown and blue leads went. The lamp switch becomes the power switch, so the lights are on whenever the C5 is; fit LED bulbs. See Modifications.

• Relay contact wear. The original control box has a commutation diode (D1) across the motor to suppress arcing at the relay contacts when the motor switches off. The Unofficial Service Manual calls contact arc suppression vital. The basic twin-relay circuit doesn't include one, so relay 2's contacts take the arc every time. Consider adding a heavy-duty rectifier diode across the motor, as Sinclair did, connected so it does not conduct in normal running (cathode to motor +).
• Lights always on with the QR kit's dual-function lamp switch. Use LED bulbs.
• Battery gauge. Without the original pod there's no low-battery cut-out. Watch the voltmeter, and stop at 10.5 V on lead-acid. LiFePO4 packs need their own battery management.
• Don't push the C5 backwards quickly with any wiring. The motor acts as a generator.

Master security switch · Battery · Control box · Power switch · Batteries and charging · Modifications

• John Wilson, Sinclair C5: Simplified Relay Controls, c5owners.com (diagrams and parts lists)
• C5Alive.co.uk, QR Power Wiring Fitting Guide (v2, 24 Feb 2006)
• The Unofficial Sinclair C5 Service Manual, “Control Box” and “Known Problems”
• Hoover C5 Service Engineer Training Notes (1984): motor load display calibration
• c5owners.com forum (closed to new posts, still readable):
  • Next restoration for Tony: SINCLAIR C5 CHAS, Feb 2015 (how the safety cable connects)
  • Car batteries, May 2013
  • C5 for sale: C5mick, 2015 (safety wiring harness fitted for car or leisure batteries)