Phone battery

[sprayer]

I decided to buy a spare battery for my phone just to use as a standby, having looked them up on the
net the genuine one (Nokia) was £14.99. + p&p coming to almost £20. however I came across a
''Compatible'' battery for just £2.95. which included p&p. Having received it I put it on charge, now with
my old original battery after about 90 mins. a message is displayed on the phone saying ''Battery Full Remove From Charge'' with the compatible one after almost 4 hours it shows that it's still charging,
however after removing the charger the battery meter on the phone shows Full and the charge seems to last about 3 days. So my question is what is in the genuine battery that is different to the compatible
one and is it safe to use if I remember to take it off charge after about 2 hours?