Heavy battery drain six months after upgrading to denim?

[Windows Central Question]

Hi.
I had bought my Lumia 520 back in 2013. It's been wonderful.
I upgraded it to Denim back in January 2015. For the first three weeks, it was working properly. Then, I noticed that my battery was draining faster while doing basic things like using internet for 30 minutes at a stretch. I thought it was calibrating and didn't bother much about it.
Now, after 6 months of usage using this denim update, I found that my battery is not only draining faster but the back of the phone is also heating up. For example, if I WhatsApp for 2 hours continuously, my battery drains to 44% from 100%. Even as I'm typing this question, my battery drained from 99% to 94% and I charged it about 50 minutes ago. It's irritating me and I'm worried. Is my battery gone to hell? Or is it the update?
PS. There were no problems during the update itself.

 

[LockOnTech]

Are you getting a proper cellular signal strength in your area? Is it like that while using wifi? Try a soft reset and monitor you battery usage to see which application is using the most power.

 

[RumoredNow]

I'd try a new battery. It sounds like the one you have is fading due to age and use combined.

 

[AtaraxisOfDoom]

The cellular signal fluctuates and I use mobile data 90% of the time. Also, my background tasks except WhatsApp are off, my brightness level is low and then set to auto. Bluetooth is off. Location is off. Basically, every extra function is off.

 

[AtaraxisOfDoom]

Can I know the cost of a new battery? Is it more than 500INR? Or should I just send my phone off for servicing? :-(

 

[RumoredNow]

The cellular signal fluctuates and I use mobile data 90% of the time. Also, my background tasks except WhatsApp are off, my brightness level is low and then set to auto. Bluetooth is off. Location is off. Basically, every extra function is off.

Are these the same conditions as prior to the update? I don't think there is anything inherent in Denim that has it wait months and then suddenly start degrading battery life...

I have no idea what batteries can be priced at in India. Search your local sources for Nokia BL-5J Battery.