I have a 20000 mAh battery pack but I can select a range of voltage output of 5V/9V/15V/19V.

With 5V I have 100 Wh, which is safe to deliver alongside, but with 19V it will be a whooping 380 Wh, which is way too over the limit and therefore strictly prohibited.

Here's the question: So I can bring my battery pack if I locked the voltage to 5V?

  • 3
    Most likely they will look at the 20000 mAh value, which is fairly common for phone add-on batteries and judge from there. – user13044 Oct 9 '16 at 3:19
  • What are you trying jumpstart with a 20A battery? – Karlson Oct 9 '16 at 5:30
  • @Karlson Most of the time my laptop, sometimes gadgets with lesser voltage like cameras. – Steve Fan Oct 9 '16 at 6:03
  • @Karlson What is a '20A battery'? – Tor-Einar Jarnbjo Oct 9 '16 at 9:10
  • @toreinarjambjo The current battery is capable of of. Car battery produces 40 , USB is about .5A. So I am curious about the possible use. – Karlson Oct 9 '16 at 13:14

With 19V it would be ... what? To get Watt hours you need a voltage yes but (almost always) the ampere-hour figure needs to be paired with the cell voltage to get the watt-hour figure. It's extremely likely to be a lithium-ion cell which is 3.7V so that'll be a 20 * 3.7 * VAh = 74 Wh battery. This is very crude because the voltage drops as the battery discharges but let's use this as an upper limit. And so the answer is yes, because the battery is under 100 Wh it is allowed in unlimited quantities. You can bring as many as you can carry. Actually, the weight allows us to counter check ourselves.

Countercheck: one Wh is .0036 MJ, Lithium-Ion batteries roughly store .4-.8 MJ per kg so to store .2664 MJ you need a battery weighing .33-.67 kg. A randomly picked 20 000 mAh "mobile charger" on Amazon weighs 357 grams of course this includes the plastic shell, electronics but still, it's close enough -- and again, remember, the energy content is less than 74 Wh.

  • Thanks, now I could charge up my MBA while watching movies or coding programs without anxious about explosions. – Steve Fan Oct 9 '16 at 15:29
  • Umm, you can't charge up your MacBook Air with that kind of battery (if that's what you were thinking). – Burhan Khalid Oct 10 '16 at 2:53
  • @BurhanKhalid Well, I believe the Air needs 14.5V and OP said the battery is 15V capable. But I am not sure whether that works and I am unaware of any external batteries with Magsafe cords. There are external battery packs with AC outlets, those would work. – chx Oct 10 '16 at 3:31

Without wishing to detract from chx's excellent answer, I note that the idea that a battery can have a current-time capacity of 20Ah at both 5V and 19V is pretty counter-intuitive, because energy delivered is voltage * current * time (leaving aside the odd root-2, issues with voltage falling off as batteries discharge, and so on). Given that 20Ah is a measure of current * time, to have a given current-time capacity independent of voltage suggests that, however much energy the batteries hold when the converter is running in 5V mode, they mysteriously have access to nearly four times as much (19/5) in 19V mode.

You don't say what brand of power brick this is, but I went and looked at the specs for an Anker Pro2 20000mAh 5V-12V-16V-19V power brick. The capacity is quoted as 20000mAh, but also as 74Wh. Unlike the current-time measure, Wh really are a measure of stored energy (they're more likely measuring deliverable energy, but those two should pretty closely match unless you've got either a perpetual motion machine or a melted brick).

When I put my cynical hat on, I guess that manufacturers of multivoltage power bricks will quote their current-time capacity at the most advantageous voltage (ie, the lowest). Leaving aside the root-2s and falloffs again, we find that 74Wh at 5V is about 15000mAh, so I suspect this is what they've done (or, as chx suggests, they've slightly naughtily quoted energy stored in the cells). At 19V, assuming comparable conversion efficiencies, the current-time capacity would be 5/19 of the quoted figure, ie about 5300mAh.

I also note that the FAA's FAQ on batteries applies a limit in Wh (being 100Wh for carry-on), suggesting that some physicists were involved in the writing of the rules. I suspect if you dig out your power brick's instructions you will find that the energy storage capacity (rather than the current-time capacity) is within spec for carry-on, output voltage notwithstanding.

  • I have yet to see a rechargeable "power brick" specification which didn't give its Ah rating at the internal battery voltage - they all seem to lie like that and get away with it. They should all give the Wh rating so we can compare apples with apples .... – brhans Oct 10 '16 at 14:22
  • @brhans couldn't agree more! – MadHatter Oct 10 '16 at 15:04

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