"(they only last c.2 years reliably)", must be a 'one man band' putting forward the idea that batts. need to be replaced that often as 'the norm'
He meant to say replace every 5yrs, (the recommended length of time as per the NSI) could last 7yrs though, still change at 5 though, always better to be safe than sorry
It really depends how the battery is tested before replacement though, using the right equipment it could have to be replaced in 4,3,2 or even 1yr.
b, October 2015