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While writing a forum post on proper lithium-ion battery care, I started wondering whether the proper term for recharging them while still fairly full is called topping up or topping off.
Perhaps both are accepted?

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3  
Top off is my usage. I don't think I've ever heard top up. –  user21497 Oct 21 '12 at 7:37
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Mine is the exact opposite. BrE v. AmE again? –  Barrie England Oct 21 '12 at 8:00
    
I would say 'recharge', but if I had to use one of the OPs suggestions I would use top up. Top off sounds like adding to a problem - "I forgot my phone, then the car broke down, and to top it off it started raining". –  Roaring Fish Oct 21 '12 at 8:52
    
@RoaringFish: Well, recharging might indicate a low charge level at the start of a charging cycle, whereas topping up/off indicates that the battery already had quite a high charge level to begin with. (E.g.: NiMH batteries should generally be recharged when nearly depleted, not be topped up/off, while lithium-ion batteries can be topped up/off safely.) –  oKtosiTe Oct 21 '12 at 12:09
    
@oKtosiTe - Fully cycling either battery chemistry is bad for it. –  Fake Name Oct 21 '12 at 13:00
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2 Answers

up vote 3 down vote accepted

Either one is acceptable.

Both are metaphors from filling containers with liquids -- whatever you may have been taught, electricity is not a liquid, but it often seems to behave in similar ways, and of course there were no native words for electric phenomena until it was discovered, so metaphors are inevitable. (Much the same thing is true of computers, for the same reasons.)

Both are phrasal verbs. One uses the common completive up particle (burn up 'burn completely', fix up 'fix completely').

The other has a usage of off that refers to the fact that open containers of liquids can overflow their top when overfilled -- this is the source of the causative verb to top 'cause (a vessel) to become full (of liquid)' -- plus the off that occurs in run off 'overflow (of liquid)'.

So top up means 'fill completely' and top off means the same thing; both indicate at least a chance of overflow in an open vessel. And which one gets used is largely a matter of personal or occasionally local taste. There's no semantic difference.

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No real semantic difference, but I have the distinct impression there's a significant UK/US split favouring top up/off respectively. I think Brits normally only use top off where there's an element of flamboyantly finishing some elaborate/drawn-out procedure. –  FumbleFingers Oct 21 '12 at 12:28
    
@FumbleFingers ...like the topping-off ceremony to mark the end of construction of a building. –  DaveP Oct 21 '12 at 13:04
    
Yes, Americans, too. To top it all off, we will have fireworks after dinner. But never "To top it all up". –  John Lawler Oct 21 '12 at 13:28
    
@DaveP,John: Well, I won't bother posting a "UK-specific" answer, but assuming other answers/comments are correct in saying it's unremarkable for Americans to top off fuel tanks/batteries/etc. I'm increasingly feeling there's a significant difference there. In BrE, top off almost always implies end/complete something with a "flourish". –  FumbleFingers Oct 21 '12 at 13:39
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@FumbleFingers: What, fireworks isn't enough flourish? –  John Lawler Oct 21 '12 at 13:57
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When you fill your car with gasoline it is common to "top off" the tank by adding a little more after the pump automatically shuts off. People use the same language when referring to a charge on a battery.

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