Any ideas what this is? It keeps popping up randomly on my device. I can be completely ignoring my browser, and going through my gallery and apps and it pops up.
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Definitely, do NOT press the "update now"!!! That for sure introduces malware. Though batteries do have some kind of firmware (mainly for charging control), it is never ever updated this way (in fact, it's not updated at all to my knowledge).
Having made clear what you should not do leaves the question: What should you do about it? You must find out what produces this popup. In 90%+ of all cases, it's a "bad app". So check all apps you've installed shortly1 before this started. Especially check those apps you've installed from "other sources" (see: side-loading). The risk catching such a "bad app" is highest with sources you don't know you can trust. Remove those and see if the issue goes away. If not, as a last-resort you might need to do a factory-reset.
If those popups only occur in your browser (the screenshot looks like such an example), it might also be a "bad website". Avoid it then, see if it happens on other sites as well.
For one thing, the popup tells the truth: pressing "update now" has a "high risk level" – you will get worse stuff installed on your device …
1: "shortly" is a relative term here; some malware waits up to 30 days before acting such
Generally, the possibility of it to be a malware is high, because normal update notifications are too ugly to notify users and this one looks pretty, doesn't it?
Batteries don't have firmware. That should be enough to dissuade you from falling for this obvious phishing scam.
It's just an unwanted pop-up that you should avoid clicking on. Bad websites mostly with extensions .mobi, .biz, (sometimes .com also) etc and all torrent and file hosting websites use these pop-ups for installing malware apps or redirects you to some app the Play Store.
1. Use Firefox for Mobile along with the extension AdBlock. Chrome still doesn't support extensions.
2. Use Maxthon Browser which comes with AdBlock preinstalled.
3. If the device is rooted just install AdBlock from here.