why does the battery on my honda inova 125 keep dying?

[GaryLDN]

I bought the bike second hand with 8000 miles on clock done a further 2000 and battery went i bought a new one and now a month maybe 2 max this one seems to be going flat on me too (electric start gone, at a set of lights with indicator on all goes dead no fuel guage milage gear lights nothing !) do i need to keep charging it ? im doing the london knowledge so im doing a minimum of 50 miles a day 4/5 days a week. it wasnt a cheap battery i bought cost over 60 pound . any help appreciated