All three expressions are very common in the U.S. when referring to the amount of battery left on your mobile phone. The most popular of the three is the last one "My phone is about to die". By saying this most people will understand that the battery on your phone is most likely at 5% or less. The first expression is the most accurate because you are letting the person you are talking to know exactly at what percentage your battery is at.
3例とも、携帯の残りの充電について言うときに、アメリカでよく使われる表現です。
三つの中で最も一般的なのは、最後の "My phone is about to die"（携帯の充電が切れそう）です。このように言うと、たいていの人は残りの充電が5％以下なのだろうと受け止めます。
一つ目の例は充電の残りが何％なのかを明確に伝えていて、最も正確です。
To explain to someone that your phone battery is about to die you can use these three expressions and add the percentage of battery you have left.
-I've only got 5% battery left.
-My phone is about to die, I've just 5% left.
-My phone only has 5% left.
携帯の充電が間もなくなくなることは、これらの表現に充電の残量（％）を加えることで説明できます。
-I've only got 5% battery left.（充電が5％しか残っていない）
-My phone is about to die, I've just 5% left.（充電が切れそう。あと5％しか残っていない）
-My phone only has 5% left.（充電が5％しか残っていない）
When your phone battery is low, and you want to say that you only have (number) percent left, you can make a statement using one of the above exampled sentences.
My phone battery is at 6% capacity, and I need to charge it.
With 3% battery power left, my phone could turn off at any moment.
I need to hang up now because my battery is at 5%.
"6% capacity" is another way of saying that the battery has a remaining charge of 6%.
"With 3% battery power left" is an expression meaning that the phone battery has a remaining 3% charge. If the charge goes below 3%, it may not have enough power to operate, therefore, causing the phone to automatically switch off.
If a person is using his/her phone while the battery is low, it is best to advise the person he/she is talking to that the call must come to an end before it drops in the middle of a conversation.
"6% capacity" は充電の残りが6％であることを表す言い方です。
"With 3% battery power left" は、携帯の充電が残り3％であることを表します。充電が3％以下になると、電池不足で自動的に電源が切れることがあります。
もし充電の少ない携帯を使っているなら、充電が切れそうだから電話を切らないといけないと伝えた方がいいでしょう（→例3）。
Since the phenomenon of a low battery is pretty common, native English speakers will tell you what percentage their battery is at. It's good to tell someone if you're talking to someone that you'll communicate with them once you are able to charge your phone so that they don't worry about you. I hope that this helps. :)
英会話講師のKOGACHIです(^^)/
おっしゃられている内容は、
My battery is at ～%.
「電池が～パーセントだ」
My battery is down to ～%.
「電池が～パーセントまで落ちている」
I have ～ ％ left on my battery.
「電池に～パーセントが残されている」
などと表現できます(*^_^*)
以上ですm(_)m
少しでも参考になれば幸いです(#^^#)
お困りの時は、いつでも気軽にご質問ください(^0^)/
★★どうぞよろしくお願いもうしあげます★★
see you soon♪
アメリカ合衆国 
日本 
アイルランド 
イギリス 
