Verb, transitive or intransitive. The OED’s definition:
To bring (something) up to its full capacity; to fill to the top (a partly full container, spec. (the cells of) a motor vehicle’s battery). Used esp. with reference to a drinker’s glass, freq. with the person as object.
The first citation is from a 1937 article in The Times: “In order to help the owner-driver to look after his battery, a combined acid-level indicator, vent plug and filler cup has been introduced, thus enabling the cells to be ‘topped up’ accurately and visibly, without removing the vent plugs.”
Top up is subtly different from the similar fill or fill up, indicating the the real or metaphorical receptacle is not (or not yet) empty. Replenish would probably be the closest equivalent, a word that does not trip off the tongue. Its widest use in the U.K. came from pay-as-you-go mobile phone companies, such as Virgin, who, interestingly, slightly changed the meaning. That is, there is no such thing as being “full” of minutes; topping up your mobile means simply adding more money to your account.
Chris topped up the generator with gas, spilling it on the hot metal. Then he urinated on some paint cans in the alley and locked the door. (Dan Baum, The New Yorker, September 19, 2005)/ I’m sure that the Republicans will claim savings — but those savings will come entirely from limiting the vouchers to below the rate of rise in health care costs; in effect, they will come from denying medical care to those who can’t afford to top up their premiums.(Paul Krugman, New York Times, April 4, 2011)