Fibre batteries are pioneering as flexible power solutions because they can be woven into textiles. Textile batteries offer a convenient and seamless way to power future wearable electronics. However, current fibre batteries are only centimetres long because longer fibre batteries are difficult to produce and they are thought to have significantly higher internal resistances that compromise electrochemical performance. Here, we show our discovery that internal resistance of fibre battery has a hyperbolic cotangent function relationship with fibre length, where it firstly decreases before leveling off as length increases. Systematic studies confirm this is true for different fibre batteries. We thus produce metres of high-performing fibre lithium-ion batteries via an optimised industrial process. Our mass-produced fibre batteries had high certified energy density based on the total weight of a lithium-cobalt-oxide/graphite full battery including packaging. Over 80% capacity can be maintained after bending for 100,000 cycles, indicating high flexibility. We show fibre batteries woven into safe and washable textile by industrial rapier loom can wirelessly charge a cellphone or power a health management jacket integrated with fibre sensors and textile display.