Battery swelling can be caused by faulty batteries, however, if a battery or pack of batteries is faulty in a way which would cause it to swell then it should swell right away after being installed and not a year later. Swelling of the batteries a year after installation usually points towards a faulty battery charger which is outputting too much Voltage or not going into float mode when it is supposed to. We would test the output Voltage of the charger to help confirm its condition. For a 60 Volt lead acid battery charger, its output Voltage should be around 69 Volts. Please let us know how it goes.
I was wondering if the battery is swollen if you could still use the bike? i am scared it will explode.