All the android phones i have owned ,after installing a certain no of apps, start showing this behavior of random battery drainage.
The steps i take which has been very helpful are
1. Installing root app delete( Lets u delete all the bloatware that comes with phones. Especially with popular phones like S3, there are ready made lists at xda forums u can use to delete unwanted apps without tension) - Root Required
2. Installing greenify( Hibernates all the apps when not in use, dont greenify apps u use constantly like whatsapp)- Root Required
3. Installing Mobiwol ( Firewall which lets you block the apps from connecting to internet. I disable most of the apps). - Root not Required
4.Installing juice defender- This helps me to turn off 3G When screen is off. Most people wouldn't like this option, but i dont have a compulsion to check whatsapp with each new notification.
This has immensely helped my battery problems and am able to go over a day and a half with medium usage on my Lava iris 501. Sometimes, i face this issue randomly even after having applied all the above steps. But a simple reboot brings my phone back to normal.
I think android memory management itself has some flaws and we need to fine tune it to conserve battery. Ios has mastered this thing i feel as even on standby, my ipad mini drains only 1% every 2 days.