In a nut shell you get what you pay for .
Buy an OEM battery and you wont have those troubles.
NMIH and LiPo batteries tend to have a higher self discharge rate than
the earlier NiCads.
On top of that, built-in protective circuitry can add to the discharge
rate. Most of the current crop of laptops should be placed on a charger
every few weeks or so. Regardless of what a user does, the batteries
will loose capacity over time, such that they will need replacing
between two and four years after initial purchase/use.
I have an HP laptop that was purchased in late 2007, and due to the luck
of the draw, has had it's battery recalled several times. So, the next
time around, I'm finally going to have to pay for a new battery.