My phone is lowbat and not charging

Generally in diagnosing a phone that won't charge, start where the power begins and work your way into the phone. What I mean by that is first thing is to make sure your electrical outlet is working; switch to a different outlet or plug in a lamp to verify you've got power there. Next is the charger itself; if you have a voltmeter you can verify that it's putting out power, or you can try a different charger or try the charger on a different phone or device to make sure it's working. After that, the cord itself is next; again, try a different cord or try the cord on another phone if you can't test it with a voltmeter.

Now we've made it to the phone itself. Take some isopropyl alcohol (90% or higher concentration; don't use the 70% stuff we all have around the house), a microfiber eyeglass cloth and a tooth pick and clean out the charging port. Check your work with a magnifying glass and a bright light, looking for any kind of dirt, lint or anything else that's plugging up the port. Also look for bent or broken pins inside the port; bent ones can sometimes be straightened with fine tip tweezers.

That's all that can be done without opening up the phone. Does the phone indicate that it's charging when you plug it in? If not, you may have a broken charging port. Fixing that requires you to replace the lightning port assembly inside the phone. If it shows it's charging but the battery isn't holding a charge, that tends to point to the battery being at fault; an iPhone 6S is old enough that if the battery hasn't been replaced yet, it's probably due for a new one.

Instructions and parts can be found by clicking on the "iPhone 6s" link in the upper right hand corner of this page.

Good luck; let us know what you find!