The battery is the C5's fuel tank. Original Oldham batteries are now long dead, so almost every C5 on the road today runs on a replacement. This page covers the original system and the modern options. Part details are on Battery and charging.

• Battery: Oldham 12 V, 36 Ah (5-hour rate), 6-cell flat-plate semi-traction lead-acid, about 15 kg, 236 × 136 × 206 mm. It was designed to be discharged deeply (to about 80 % of capacity) for around 300 cycles. Hoover called it “essentially a fork-lift truck battery”. See Battery.
• Master security switch: clips into the battery cover. Positions are I (insert/remove), P (lights only) and D (drive). The red cut-out button is the emergency stop. See Master security switch.
• Charger: a matched two-stage lead-acid charger by Stellar Components. It plugs into the switch aperture. A red light means charging and green means full; a full charge from cut-out takes about 8 hours. See Battery charger.
• Battery gauge: the right-hand bar on the instrument pod. It counts down from a full battery and resets every time the switch is reconnected, so always reconnect a fully charged battery. On the last segment it warns for 20 seconds, then cuts the motor. See How the electrics work.

• Charge after every use, and at least monthly when the C5 is laid up. Never leave a lead-acid battery flat.
• Connect the charger to the battery before switching on at the mains. Switch off at the mains before disconnecting.
• Top up weekly (on original wet batteries) with distilled water, only when fully charged, to 6–10 mm below the lip. Shake out gas bubbles first.
• Smear the terminals with petroleum jelly, and fit the contact cover whenever the switch or charger isn't plugged in.
• Take the master switch off the battery if the C5 won't be used for a week or more. The instruments draw a small current all the time.
• Safety: charging gives off explosive gas, so no flames or sparks. Take off rings and watches. Neutralise acid spills with washing soda or bicarbonate of soda in warm water.

Sinclair claimed up to 20 miles from one battery. Pedalling away from rest and using “burn and coast” can roughly double these figures; cold weather can cut them by up to half. See Specifications.

Sinclair sold a spare battery fitting kit for a second battery in the right-hand footwell, used either:

1. as a spare, swapped into the left footwell when the first is flat; or
2. in parallel, with a second (right-hand) master switch. Switch the left one to D first, then the right; remove the right-hand key before driving; press both cut-out buttons to isolate the motor.

• Same-footprint lead-acid: Oldham types 015 and 038 fit the original battery top and key switch. A 3D-printed adapter lets the original key switch sit on other batteries.
• Leisure or car batteries: usually connected with quick-release clamps and a relay switched by the lamp switch or a separate key switch. Car batteries aren't designed for deep discharge and will wear out quickly.
• LiFePO4: a 12 V 20 Ah golf-trolley pack weighs about 2.5 kg instead of 15 kg. Owners report much better acceleration and hill climbing, and around 1,000+ cycles.

See Modifications for wiring options.

• Sinclair C5 Owner's Handbook (1985), pp. 6–9, 14–16, 32
• The Unofficial Sinclair C5 Service Manual, “Battery Protection” and “Range”
• Sinclair Vehicles, Using a Second Battery (part no. 100959/3/85) and spare battery kit fitting instructions (part no. 100906)
• Hoover C5 Service Engineer Training Notes (1984)
• c5owners.com: “What to Look for When Buying a Sinclair C5”; forum Light weight LiFePO4 batteries (Dan, May 2013)