If you need to maintain charge over a long period and it's not practical to remove the battery, then get something like an Optimate. All my bikes are permanently on these when in storage over the winter. If you need to run an extension lead then you will find an ordinary black builders bucket fits nicely over it to keep the rain off. But agree that if it keeps going flat then there is something wrong. If you have an ordinary multimeter just set it to dc volts and measure the battery voltage, should be a little over 12 if it's good. Now start the engine and check that rises to a maximum of usually about 14 volts. If it doesn't get that high chances are your alternator is faulty. If you hold the revs at about 2000 and the voltage keeps on climbing to much more than 14 v then your regulator may be faulty. You should be able to find the regulator cut off value for your particular vehicle on the net, but it's commonly about 14.2.