I went down Screwfix and bought some really thick nylon wire or whatever it's called for my strimmer, the thin stuff would break about 5 times when I was cutting the edges of the garden.
Looks similar to mine which I've had for a couple of years - the battery lasts for about 25-30 minutes but takes a few hours to charge up. It doesn't use a spool which is good but uses plastic "keys" instead. These cut quite well