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Follow-up question to Are battery packs allowed in hand luggage?

I have a 26,000 mAh battery pack ("Intocircuit Power Castle") which I want to take on a plane (Royal Air Maroc). Airline battery capacity limits tend to be based on Watt hours (Wh), and I'm confused about the conversion formula - specifically, the voltage part.

The conversion formula is mAh divided by 1,000 (or, Ah) times V = Wh. From what I can find online, V means "operating voltage", and this is where I get confused.

I can't see any reference to operating voltage on my battery. As for output voltage, there are four possibilities:

  • USB at 5V
  • DC at 12V
  • DC at 15V
  • DC at 19V

...which could give anything from 130 to 494, if any of these even are the operating voltage.

How do I find out the operating voltage to make this calculation? Or, is it possible to check based on mAh alone?

Funnily enough, someone asked my exact question about my exact brand of battery at some forum. Unfortunately, in a perfect example of why forums are terrible, instead of answering the question, they just started arguing about whether the asker even needed a battery (trust me, I do), whether it's standard to have power sockets on airlines these days (trust me, where I'm flying, it isn't), and recommending solar charging (that's already how I charge this battery!).

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  • milliAmp-hours surely, not millihour-amps? Although I guess those units are equivilant, just the former is really odd sounding (who ever uses millihours?) – CMaster Dec 17 '15 at 9:30
  • What kind of output DOES the battery suppy (ie, how do you plug things in to charge off it)? Do you have a multimeter or voltmeter avalable to you (or could you go to the store and buy one?) – CMaster Dec 17 '15 at 9:32
  • Doesn't seem possible to figure that out on mAh alone. But my educated guess is that the battery will be allowed in carry-on baggage. – Egil Dec 17 '15 at 10:29
  • @cmaster output is USB, or DC similar to a DC adapter. There's one DC out socket and a switch to choose voltage. mHa was a typo... D'oh – user56reinstatemonica8 Dec 17 '15 at 10:41
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The maximum nominal voltage of a Lithium battery cell is 3.7V (according to Wikpedia). Thus, in order to get an upper bound on the energy stored in the battery in Wh, you need to take the mAh rating and multiply it with 3.7/1000. External battery packs that provide USB power (at 5V) use additional circuity to step up the voltage.

Having said that, there are batteries that consist of multiple cells. In such a case, the battery normally either has a Wh rating printed on it (e.g., for laptops) or a voltage rating printed on it.

They may of course be exceptions to the rule that it's either at most 3.7V or something else is printed on the device. However, this would make the battery look unnecessarily bad, so for marketing reasons, there should be few exceptions.

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  • This is a Lithium Polymer battery, I think that link only applies to disposable lithium batteries? – user56reinstatemonica8 Dec 17 '15 at 11:30
  • @user568458 The nominal cell voltage also 3.7V for Lithium Polymer batteries (according that their Wikipedia article). Also, "Lithium battery" is the superclass of batteries if I am not mistaken. – DCTLib Dec 17 '15 at 11:36
  • RavPower (a battery pack manufacturer whose products include the kind of mixed-voltage-output laptop chargers I'm asking about) published this article blog.ravpower.com/2017/04/flying-planes-power-banks which seems to confirm that the "nominal voltage" of a lithium polymer power bank is 3.7V as you say – user56reinstatemonica8 May 20 '18 at 21:30
  • They also give a useful rule of thumb - if the mAh is advertised as below 27,027 (/ 1000 x 3.7 = 100 Wh), they say you can probably take it on carry-on baggage without needing approval (unless the airline has a special policy). If it's above that but below 43,243 mAh (/ 1000 x 3.7 = 160 Wh), they say you will probably need to get prior approval from the airline. If it is above this, it probably won't be allowed, unless special permission is granted and it's packed by crew in a dangerous goods area. – user56reinstatemonica8 May 20 '18 at 21:46
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Broadly speaking you have the problem that if you can take it on or not is at the discretion of airport security. If you can't clearly show that it is within their rules, then good luck convincing them to beleive your calculations. On the other hand, they may not be aware that there are any restrictions, or choose not to inspect the battery too closely.

The current-time rating of a battery normally refers to the voltage of the actual battery itself. However it isn't entirely clear here what that is - in fact it is possible that changing to voltage output rewires how the cells are connected in the battery. Equally, there is a marketing reason to give bigger numbers throughout the product range, even though the smaller devices only provide 5V through USB.

This amazon page however does provide some useful hints. It says that a full charge takes 5-6 hours at 18.5V/2 Amps. So that's a maximum of:

6 * 2 * 18.5 = 222 WH

We know that there will be inefficencies there, so it will definitley be less than that.

As several other items in the range are USB only, and it's the only value from the 4 that fits under our calculated maximum, I'd guess that they are referring to the battery capactity at 5V, meaning 130WH.

But as we're making assumptions here based on limited information I'd suggest two things:

  1. Contact the manufactorer asking for the capacity in power-time units as opposed to current-time.
  2. Don't try to take it unless you are prepared to leave it behind in the event of an unusally thorough security check.
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