In the UK people can say
I need to top up my mobile phone (= pay more money so you can make more calls)
How do we say this sentence casually in the US or other countries?
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American English doesn't have a good idiom for this. "Top up" is used very rarely. It may sometimes be used to describe filling up a partially-full container of liquid ("I've drunk half of the coffee from my mug; please top it up."), though "top off" is more common. Neither is used to talk about putting money in accounts.
Apparently Virgin Mobile calls their cards "top-up cards," but TracPhone and Boost Mobile never use the phrase "top-up" anywhere on their sites. (Instead, they use "Add airtime" or "Refill.") Apparently Best Buy sometimes uses "top-up" to describe refill cards, but mostly the carriers themselves call them "Refill cards." (Consider this example from Verizon: the Best Buy website calls it a "top-up card" but the phrase "top-up card" does not appear anywhere on the product. Instead, it says "Refill card.")
Instead, Americans would probably use add minutes or add more minutes in conversation:
I need to add [more] minutes to my phone. I'm almost out of minutes.
Note that the British prefer "mobile" to talk about mobile phones, while American speakers prefer to say "cell" to talk about their cell phones.
In America we say to "top off" rather than to "top up".
Also, we generally say "cell phone" rather than "mobile phone", though if you said "mobile phone" people would know what you meant.
There may be a difference in meaning here, though. To "top off" something is to fill it to capacity. Often the connotation is that the container is already close to being full. Like if you had a 10 gallon container that presently contained 9 gallons, "topping if off" would mean adding an additional 1 gallon to bring it back up to its full 10 gallons. You probably wouldn't say that you were topping it off if it presently contained only 2 gallons. Then you'd say that you were "filling it up".
As cell phone accounts, in the U.S. anyway, don't normally have any upper limit, you wouldn't normally talk about topping one off. I think most Americans with prepaid cell phone accounts say "I am adding minutes to my cell phone" or "adding to my account".
I think the most common American English equivalent would be to "buy more minutes". For example,
I can't make the call right now. I need to buy more minutes for my (cell)phone.
Agree that in most areas of the US, "cell" is more commonly used as "mobile", but I think that may be changing. In technical documentation, "mobile" is typically used, but in conversation "cell" is more common. People would understand regardless of which term you used.
In American English, "topping up" and "topping off" are used to describe filling up containers of liquid, such as tanks of gasoline. Gasoline retailers specifically recommend against filling gas tanks to 100 percent full, because that can cause excess gasoline to spill out.
Statements like this are common and natural:
Thus, I intuitively understood the original poster's example:
I need to top up my mobile phone.
I have a pay-as-you-go cell phone. I try to keep $50 [or some other amount] of available balance on it. I am currently below that amount. The next time I am at Safeway [or another store or on-line site that sells cell phone minutes] I will buy some more minutes. That way, I can make a few long calls, without running out of minutes.
If this is what the original poster meant, then "I need to top up my cell phone" is a natural way of saying it in the United States.
To top up a mobile
is just a shortened (modern) version of what used to be said when mobiles first appeared en-mass in the UK, back in the 90's which was:
To top up the credit on a mobile
Here, as has already been noted, there is an assumed analogy between a mobile's credit [tank] and a petrol tank in a car (or topping up the oil in the engine, coolant in the radiator, etc.). However, nowadays "the credit on" is dropped as it is generally assumed that you are talking about the credit - although you could say
I'm just going to top up the charge on my mobile
thus clarifying that you are referring to the electrical charge in the battery... :-)