Probably blowed up batery or smartphone with multimeter First, I turned off my LG-D280N and removed battery. Set the multimeter into 20VDC (next is 2VDC) and correctly checked battery pins and checked the voltage.
Next, I checked  pins on case, set &quot;diode test mode&quot; and touched two case diode pins (NOT touched battery!).
Then on phone I did the same, but I was  scared and after second or less stopped touching this with multimeter cables. When I turned on, I saw on notification bar a symbol like this ( this is not the exact symbol but looks similar with this difference - yellow was transparent, red was white and black was also white)

Did I damaged my battery or the phone?
Diode test mode is used to:



  .... display the voltage drop measured across the diode under test, as a small current is passed through it.



Diodes in battery circuits are used to prevent accidental reverse polarity of battery
I suspect in your case, you had reversed the polarity while measuring it causing the Diode to blow up ( and saved the internal circuitry).
This has caused the system alert  ( see lg-icons, symbol looks very similar to what you have described in words ) to be flashed as the circuitry is damaged and battery / mains cannot be used to power the device


I am not an expert in these matters but have tinkered with circuits way back

In any case, since you see the system alert and probably can't use the phone, only option is to service center for repair and if possible oversee the repair to see the actual damage. Remember, mobile battery circuits have tons of safety features built in so it shouldn't be a major damage. If it is only a Diode, normally it isn't a challenge to replace it

All the best