• Modern batteries.
  • Owners commonly replace the long-dead Oldham battery with a car or leisure battery.
  • A LiFePO4 pack is a popular upgrade. A 12 V 20 Ah golf-trolley pack weighs about 2.5 kg against about 15 kg for lead-acid, and owners report much better acceleration and climbing. One reported about 7 miles of motor-heavy use from 20 Ah.
• C5Alive “QR” Quick-Release Power Wiring kit (v2, Feb 2006). This replaces the key-switch power block with:
  • quick-release battery clamps;
  • a relay;
  • a replacement dual-pole light switch that switches both main power and the lights. The lights are therefore always on when the C5 is on, so fit LED bulbs.

Fitting outline:

1. Remove the boot and the control box cover. Use 7 mm and 8 mm long-reach sockets on the two 7 mm and two 8 mm power nuts, and unplug the brown +12 V from X5.
2. Swap the light switch, wiring it exactly as the guide shows. Wrong wiring damages the control box and pod.
3. Run the new red/black leads through the seat grommet: red where brown went, black where blue went.

Don't drop nuts or washers into the control box.

• 3D-printed key-switch adapter (c5owners.com). This lets the original key switch sit on any battery using quick-release clamps. The original charger can still be used on a lead-acid car battery.

Use this to get a C5 running when the pod or control box is dead or missing, or after a reverse-polarity disaster.

• 12 V twin-relay circuit:
  • Use two 70 A automotive relays.
  • The handlebar switch, through a 5 A fuse, energises relay 1, which energises relay 2, which switches the motor.
  • Fit a 5 A diode across the circuit as shown, with the silver band towards relay pin 85.
  • Use 6 mm² cable for the motor circuit. Cheap 100 A jump leads work, or reuse the original brown (= +) and blue (= −) leads.
  • Keep the original orange-wire thermal switch in the circuit as the only motor protection, and add an 80 mm 12 V PC fan on the motor.
  • Optionally add a kill switch and a key switch.
• Keeping the original lights, indicators and horn: feed 12 V to the original control-box terminals just inside its inspection cover. The box is already fused.
• Instrumentation: add a shunt with a combined volt/ammeter in a modified pod housing.
  • Current guide: about 29–30 A on the flat; about 40 A climbing while pedalling; under 5 A means coast.
  • Voltage guide: about 13.5–14.4 V full. Stop at 10.5 V. The voltage recovers to about 11.5 V after a rest, enough for a mile or two home.
• Optional extras: hazard-warning wiring with 5 A diodes, an LED-compatible flasher, and a relay-driven motorcycle or car horn.
• A 24 V version uses a 4QD Porter 10 speed controller.

• 24 V conversions have been done since soon after launch. They make the C5 noticeably faster, so it is no longer an EAPC on the road.
• Usual approach: two 12 V batteries in series (one in each footwell), a PWM controller, a throttle in place of the push button, and a motor cooling fan. The original pod and control box are removed or bypassed.
  • Controllers: the 4QD Uni-8, later discontinued; its replacement the 4QD Porter 10, rated 12–48 V; and others.
  • Owners report 36 V is possible with good motor ventilation.
• The Sinclair designers' own notes (Unofficial Service Manual) on running the original electronics at 24 V:
  • pod: R17 doubled to 94 kΩ and C12 uprated;
  • control box: C2 uprated, a bigger heatsink on IC1, and a resistor in series with the relay coil.
• C5Alive 24 V kit (v2, Feb 2006). Instruction sheets W1–W6 and S1–S8, plus a cooling-fan bracket sheet:
  • battery 1 positive goes through a relay (switched by the C5 light switch) to the controller +B and the fan;
  • a link wire runs from battery 1 negative to battery 2 positive;
  • battery 2 negative goes to controller B−;
  • controller M+ and M− go to the motor brushes A and B;
  • the throttle cable runs down the steering column.

Notes from the kit:

• The original thermistor (purple/grey) and thermal lock-out (orange) become redundant, so tie them back.
• The lighting circuit is lost with this kit.
• Some early “2-battery” C5s already have a square cut-out in the right-hand moulding for the second battery's wiring.
• Cooling fan: C5Alive's kit clamps a 12 V fan to the motor with 3–4 brackets and a jubilee clip, set diagonally to clear the seat. 3D-printed fan connectors for 90 mm and 120 mm fans are available from C5owners.

• Metal gearbox insert: an alloy insert with a toothed ring and the large bearing fitted. You reuse the original 3 cogs and posts. It was offered by “Sinclair C5 Chas” (about £65 in 2013) and used on 24 V machines.
• Reproduction plastic gearbox cogs: moulded or 3D-printed.
• Disc brakes: front and rear conversions are documented in forum threads.
• Wider or upgraded tyres: Schwalbe Big Apple or City Jet on the rear.

Firmware and PCB files were published on c5owners.com and its 3D-model download site.

• Original-pattern parts:
  • Hi-Viz mast caps; pod caps holder; pod washer;
  • rear wheel centre caps;
  • chain-tensioner chassis clip;
  • light-switch bracket; handlebar switch bracket; handlebar grommet;
  • boot clips; rear frame inserts;
  • indicator template;
  • motor heat-sensor holder;
  • bulb access housing and top;
  • toolkit cap;
  • side-panel clips;
  • wing-mirror brackets;
  • battery stop bar.
• New designs:
  • cooling-fan connectors for 90 mm, 90 mm extra-long and 120 mm fans;
  • X-POD case and control box;
  • mudflaps (X-POD style);
  • a replacement boot lock;
  • rear-brake bearing bracket;
  • key-switch and battery adapter.
• If you sell prints, C5owners asks for credit in your listing.

• Adam Harper's record C5s reached up to 150 mph, with 0–60 mph in about 5 s, and drove through a 70 ft tunnel of fire.
• Other owners have fitted jet engines, bigger wheels and high-powered motors. These are for track and private-land use only.

• c5owners.com: “Simplified Relay Controls” (John Wilson); projects pages (POD ULA Replacement, C5duino, X-POD, Pedelec Mode, 24V Conversion); 3D printing pages; forum threads 23, 31, 53, 271, 317.
• C5Alive.co.uk downloads: 24V kit instructions (v2, 06-02-06); QR Power Wiring Fitting Guide (v2, 24-02-06); cooling fan mounting instructions (v1.01, 30-9-05).
• The Unofficial Sinclair C5 Service Manual, “Application of C5 instrumentation system to other vehicles”.
• Twist & Go magazine, p. 50 (Adam Harper).