ELECTRIC buses in the city are failing to spark a green revolution in public transport, say critics, but officials said yesterday they aim to change this.

Ambitions of extending the network are being hampered by high vehicle and maintenance costs and the congestion buses cause while recharging.

Passengers have also complained about having to wait for four to five minutes while bus batteries are being charged. Once charged, a bus can travel 5 to 6 kilometers.

Operator Shanghai Baishi Co, which is attempting to increase the use of clean energy transport in suburban areas such as Chongming Island, said improvements have already been made. Four city routes - No. 11, No. 26, No. 825 and the Expo site - use the green buses.

Green buses include capacitor and more powerful supercapacitor buses produced in the city.

While these received praise in trials, question have been asked after a green bus caught fire last year and staff raised doubts about the batteries.

Last summer, a bus manager surnamed Lei said the capacitor bus technology "is still not mature enough."

A senior dispatcher surnamed Wu said crews have found that the batteries on the capacitors are hard to handle and repair.

An electric bus costs around 2 million yuan (US$300,000), with annual maintenance at around 500,000 yuan.

During the 2010 World Expo in Shanghai the buses proved a great success, but downtown road conditions are more challenging.

Some drivers have complained that other vehicles occupy the space in front of charging points.

And when drivers do access charging points, traffic jams often build up behind them.

Technicians are now improving the battery life.