Eric loves the 2 X 18v = 36V system from Makita.  It is a great way to double your power without going to a new battery platform.  Everyone hates getting a new battery platform.  The DLM380 from Makita is an electric lawn mower that uses the same batteries that you use for your 18V tools.  If you are a Makita user and need an electric lawn mower this is a no brainer.  The only bad thing is that it is only available in the UK.  Lucky Brits!