It does not appear to be a very good translation of this word.
1473 (εγώ) is the personal pronoun, "I", so it tells us that Jesus was talking about Himself.
1510.2.1 (ειμι) is the real core of the question. 1510 is the infinitive "to be, exist". The following numbers (".2.1") tell you more about the nuances of meaning - tense, voice, etc. Some lexicons will give another code here instead - in this case, my intralinear has G5748. Either of these codes tell you that this word is present tense, indicative mood.
Based on these findings, the most direct way to express this in English is "I am".
Contrast this with the word He used to describe Abraham. In my intralinear, 1096 is also decorated with G5635; we'll come back to that. 1096 (γίνομαι) is the infinitive "to become, come into existence, arise, be made." Note that He chose not to use the same word, even though some translations will translate this "was". The majority of translations say either "was born" or "existed".
G5635 indicates the second-aorist tense, middle-deponent (active) voice, infinitive. Aorist tense is normally translated as past tense, though strictly speaking it is outside the concept of time. In this case, the second (punctiliar) aorist implies that the statement was, is or will be true at some point without a concrete definition of that point. Active voice ties the subject to the action - in a generic sense, he did it, it was not inflicted upon him. "Abraham came into existence".
Coupling this with our discussion above, one could amplify this as follows: "At whatever time Abraham came into existence, at that time I already was and still am."
However, there is another aspect that we have to consider as well. The Jews were quite aware that God used the name or term "I am" to refer to Himself. (c.f Exodus 3:14) The Septuagint even translates this with the exact same Greek words: εγώ (G1473) ειμι (G1510). (In this case, my copy of LXX uses Robinson's Morphological Analysis Codes - for 1510 in this verse, the code is V-PAPNS, which means "verb", "present tense", "active voice", "participle", "nominative", "singular" - effectively, exactly the same as above.) This explains their immediate response - they grabbed stones to kill Him, because they and He both knew that He had just claimed to be God and they couldn't or wouldn't accept that as truth.
The issue with this translation, then, is that it is rather weak linguistically. "have been" could be taken in a number of different ways - Jesus had previously existed, but subsequently did not, Jesus merely came into existence before Abraham, etc. None of these carry the weight of His claim - to be a member of the Godhead.
