OT: Powerbank & air travel question

[Tim+]

I have a 12,000 mAh lithium ion power bank that we keep in the motorhome for emergency jump starts. We'd quite like to take it on a trekking holiday as a power supply. The problem is, BA lists restrictions in terms of Wh.

From what I remember of school physics, W = V x A. If I calculate based on its 12V output, that makes 144Wh. But what it I calculate it on its 5V output? Does it now become a 60Wh device?

BA allow devices up to 150Wh to be carried in hand luggage so am I okay? Just a bit confused.

I fully get why airlines are so wary and will be phoning BA before we travel but Wh just seems an odd way of setting a limit without specifying a voltage. Surely the mAh figure is more useful for rating stored energy?

Tim

[Scott]

Is a watt-hour not like a kilowatt-hour (ie, a unit of electricity) but smaller so they are seeking to limit the number of units of electricity you can carry? Seems logical enough.

[Andrew Gabriel]

You need to know how many lithium cells it is. Each cell is 3.4V, so it's 12 x 3.4 x number of cells. I guess it will be a minimum of 4 cells.

12 x 3.4 x 4 = 163Wh.

Yes, I would say the energy stored inside a jumpstarter lithium battery is beyond what's reasonable to carry as a consumer grade product (i.e. made cheaply in China) on a plane.

No, Wh is energy storage, but in converting mAH to Wh, you need to know that a lithium cell is 3.4V, and how many there are.

[Harry Bloomfield]

Tim+ explained on 07/03/2017 :

12000mh x your stated 5v = 60Wh

Watt/hour is a measure of the amount of energy stored in the device.

[Tim+]

Well I'm not opening it up to count them so I'm left with it's rating of

12,000mAh at some voltage or other. As it turns out, the rated DC output is either 5 or 15V so I suspect it won't be allowed. Shame that the voltage that the mAh rating is based on isn't stated anywhere.

It's one of these and fits the description of "cheap Chinese" product. ;-)

FLOUREON Multi Function Car Battery Jump Starter

Tim

[Tim+]

Well yes, but how do I know what voltage it's calculated on when the device has two voltage outputs (5 & 15 as it turns out).

Tim

[newshound]

I think this is *probably* right, provided it isn't actually a 12 Ah battery at 12 volts (which as OP says would be 144 Ah).

Correct, and it is energy which matters in terms of its destructive potential.

Has anyone taken one apart? I'd always guessed they contain four cells at 3.4 volts, which should be fine for starting a car without any voltage processing. I've always assumed they run a little switched mode power supply to generate the regulated 5 volts.

[The Natural Philosopher]

the lithium cells I use are 4.2v fully charged and 3.7V absolutely flattened.

[The Natural Philosopher]

It will be a 3 cell unit of around 11,1 v 'rating' but in reality over

12.5V fully charged

[The Natural Philosopher]

Lithium cells are not 3,4v

which should be fine for starting a car without any

[Tim+]

But again, I have to ask, how does knowing the number of cells help you workout the stored energy unless you know the value for each cell. Car batteries all have 6 cells but stored energy varies enormously so I'm not sure how opening it up and counting the cells helps.

Tim

[Tim+]

Okay, curiosity got the better of me and I opened it up.

The battery pack is all shrink wrapped and *appears* to be just three slim flat cells (like a triple decker mobile phone battery).

On my voltmeter the output is just 12, not 15V (as stated in the Amazon ad) although the pack has not been charged for weeks. Maybe it will rise after charging? I suspect not though as it claims to be good for jump starting even when the charge indicator LEDs drop to 3 out of 6, and it's showing 6 currently.

I suspect only the manufacturers have the vaguest idea of its capacity. Not sure that there's more I can do without unwrapping the batteries but it wouldn't surprise me to find no marks or branding in this kind of product. :-(

Tim

[harry]

Multiply the output voltage by twelve. So if it's 12 volt output, it's 144 VAh energy stored.

As it's DC, VAh is the same thing as Wh.

[Tim+]

Which output voltage? The 5 or the 12? I can't see any way of establishing which figure the 12,000 mAh is based on. If it was *just* a USB charging device there would be no issue. If it was *just* a 12V car jump starting pack, again no problem.

If I was a manufacturer trying to impress with my dual voltage product, I might be tempted to just quote the mAh figure with no specified voltage and hope that my customers assume the bigger storage value.

I suspect I'll just have to quote the brand and model to BA and see what they say. I just wanted to be able to say with confidence what its capacity was before phoning them. Looks like I won't be able to.

Tim

[harry]

Ampere hours is really a meaningless traditional term that should be abandoned.

Large batteries these days are quoted in (K)Wh capacity along with the output voltage.

It becomes even more meaningless when there are two voltage outputs as there will be two possible (m)ah "capacities"

[Dave Plowman (News)]

My Lidl version has different mA.hr marked on it according to each voltage output.

8000 mAh at 3.2-3.4v 4600 mAh at 5v 2000 mAh at 12v

So the watt.hrs are near enough constant, at 24 ish. 150w.hr is a pretty large battery, I'd say. Not sure I'd want to lug that around on a walking holiday.

[Harry Bloomfield]

harry was thinking very hard :

..and the larger Ah rating will be the 5v one rather than the 12v, simply because 12,000aH sounds much better in the advertising than does

5,000aH.

[Bob Minchin]

In order to make these things sound as impressive as possible, they quote mAh in terms of a single cell. If you have 3 cells then they will almost certainly 4Ah each.

So the capacity when flatish will 12 x 3.7Wh = just under 45Wh

[Caecilius]

I'm pretty sure manufacturers will try to stick the biggest possible number on the label that they can without being untruthful.

Working from this marketing-based assumption, rather than earlier engineering-based assumptions, they will use the lowest voltage they can get away with to make that number look big.

I would guess that's a single cell. If they're feeling generous, it'll be at the fully-charged voltage, but it's probably at the nominal voltage

So my guess would be something like:

12Ah x 3600s x 3.7v = about 160,000 joules.

That's only about 45Wh isn't it? (I hate these silly units line mAh and Wh; why can't we use the normal units for energy?).

[Tim+]

It's what I would do. ;-)

Dunno, but I feel a bit better about phoning the airline and quoting the lower figure which I think is the more probably than 144 Wh.

Tim