Christians, just how powerful and intelligent are angels? And what is their lifespan?

Angels are eternal, heavenly, beings that were created by God before the earthly creation; but unlike God they are not omniscient, omnipresent, and omnipotent. They are most often depicted as messengers and spiritual warriors who sometimes serve the role of intermediaries between God and man. In the heavenly order, angels occupy the lowest rank (under archangels) but are above humans. Given 1/3rd of them chose to side with Satan, it can also be said that angels have free will. In terms of power, they are most often compared to stars, burning fire, and whirlwinds. They can manifest in whatever shape, size, and intensity they please - whether physically, in visions, or in dreams. Given that angels are privy to a vantage point above the material realm, and have existed for an unknown number of ages, it can be assumed that they are fairly intelligent.

Angels are, were, or will be human beings, except for the spirits that followed Satan in the war in heaven.

They are generally more intelligent than we are.  They are definitely more powerful than we are.  Their many wings and eyes represent power and understanding, respectively.  Most angels are immortal.

very light-bright and holy and powerful - they also have supernatural powers.

They likely have unlimited lifespan - The Archangel Gabriel talked to Daniel and then around 600 years later talked to Zechariah John the Baptist's father.

i don't know if they can die of other causes such as in battle.

Let me start with your last question. Well, it´s not the case that they have a lifespan. On one hand, the Scriptures tell us that God has always been an immortal Spirit. His Son Jesus was endowed with immortality after his test on the earth, after his resurrection. Joining them, we read about 144,000 humans bought from the earth, who, on being resurrected to heaven, become also immortal, incorruptible spirits. - Psalm 90:2; 1 Tim 1:17; Ro 6:9; Re 1:18; 1 Cor 15:42-54; Revelation 5:9-10.

On the other hand, the Scriptures tell us that, after Christ´s Thousand-Year Reign, after mankind´s final test, Satan and by extension, his demons, will be "thrown into the lake of fire." In symbolic language, that means their destruction/annihilation. Being symbolically "tormented day and night" guarantees that they will remain inactive in death/nothingness "forever." - Revelation 20:1-10.

From the above, we can infer that God´s angelic spirit sons are not endowed naturally with immortality, and incorruption. Otherwise, how could they be destroyed/annihilated as in the case of Satan and his demons? As stated before, it is not the case that they have a lifespan. However, just like humans who will live forever on the earth, their living eternally depends on ther continued loyalty, and obedience to God. Their eternity is conditional, and prospective. - Psalm 37:10, 29; 104:35; Proverbs 2:20-22; 10:7.

Therefore, since neither humans nor spirit creatures are inherently immortal, or incorruptible, it means that, every now and then, they must replenish their "fuel," using an outside source for maintenance. The immortal/incorruptible spirits aforementioned don´t. They must be self-sustained in themselves, completely independent spirit beings.

In a nutshell, spirits are a higher form of life. They “are greater in strength and power.” And, according to the Scriptures, man was made “a little lower than angels.” - (2 Pet. 2:11; Heb. 2:6, 7) They are classified in different ranks. Overall, they must definitely have higher mental capacity than humans, move with supernatural speed, and high degree of freedom, speak different human languages with ease on top of their own, and appear and disappear at will in our environment. (Ezekiel 1:14; Ge 19:15-16; Luke 2:15; 1 Cor 13:1) Talking about power and strength a single angel killed in one night 185,000 Assyrian soldiers. - 2 Kings 19:35.

Since spirits don´t depend on air molecules, in other words, an atmosphere to communicate, they must use some kind of "telepathy" for the latter. Spirits who are God´s enemies, just like the faithful spirits, can apparently cause humans to see visions, and what they see may appear to be real. At Jesus´ temptation in the desert, the Devil evidently produced a vision in order to show Jesus “all the kingdoms of the world and their glory.” (Matt. 4:8) Most certainly, wicked spirits must be behind the "virtual-reality" scenarios, and "missing time," reported in UFO "abducton" cases. - Re 12:9, 12; Ephesians 6:12.

it is irrelevant how intelligent they are, they are merely messengers to Do as they are instructed to do. Or perform a certain thing which they are instructed to do.

They are IMMORTAL.

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Both good or bad they re very powerful One was used by God to wipe out an army of 185000  that was menacing His people 2nd Kings 19;32-35 Those many loyal ones used to spread the good news Rev 14;6  2Thess 1;6-10  and clean the earth at armeggedon will live eternally Those thrown from heaven Rev 12;7-12 will be destroyed at end of the thousand year reign of Christ Rev 20;7-10 Gen 3;15

i think theyre pretty intelligent, not sure how long they live

Angels exist in the hereafter and are not subject to death. God's angels are capable of doing all that he allows them to do, so their power is the same as God's, but limited by God as well.