Samsung Galaxy S5 — Slow S5 battery charge after update to Lollipop 5.0.2

I've been very impressed with my S5 so far, including its battery consumption and ability to charge from 0 to 100% much quicker than my old S3 - two hours or even less. However, I've just updated from Kitikat to Lollipop 5.0.2 and it's all gone wrong! The phone takes hours to charge. Also, if I switch it off at night with about 15%-20% battery remaining, it won't start in the morning as the remaining charge has somehow gone to 0% - it has to be plugged into a charger for at least 5 mins before it'll even start.

What's going on? I've read that a complete reset could help, as the problem may be related to installing the update over old firmware, but my firmware has also been updated within a month or so. I don't want to do a full reset if I can avoid it. My S5 is a SM-G901F model, if that makes any difference.

Any suggestions very welcome!

Thanks.