I'm intrigued. I had thought of building my own battery as I've seen several YouTubes where people harvest type 18650 cells from various sources (e.g. laptop batteries) and then build up batteries from the good cells. Typically, a low number of bad cells bring a battery down and the remainder tend to have better than 80% of original capacity. However, I haven't found a UK source of 'salvageable' cells or of the nickel strips and spot welder needed to build the batteries. The USA has several specialist suppliers for these, but by the time you've paid shipping and taxes they don't seem viable for UK. FWIW, a 200Ah Lithium pack would give me all the power I need -- and I know I'd need to swap out the current mains charger for one suitable for lithium and also replace the current split-charge relay with a B2B system.