That's a good point Paul, and should deter most.i always try and take my battery indoors, even carrying it from the bike shed into work, my thinking being that without the battery, it is far less likely to be stolen,and of little value.
There was one cheeky one on ebay though, claimed as the bike being laid up in a garage for a while and they were unable to find the battery and charger.
A bit like all the VCRs in the past and TVs/DVD recorders/players currently offered with remote missing. Burglars don't have the time to hunt for remotes of course.
And I've recently been asked by a neighbour if I could find a power supply for a specific laptop. It turned out to be a £1200 latest business model bought at a boot sale for a fraction of that price. Need I say more!
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