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Forum Index : Microcontroller and PC projects : Let the buyer beware of USB charger pack

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MMAndy
Regular Member

Joined: 16/07/2015
Location: United States
Posts: 91
Posted: 02:46am 17 Aug 2015
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Let the buyer beware of USB charger Lithium power banks specs.

Our specs for a portable USB power charger Lithium pack/bank to power our Embedded BASIC Laptop:
USB 5V battery/charger Lithium power pack - high current with LCD status display with 0-100% indication.
USB 5V power cannot drop below 5 V during a charging cycle.

We chose a 50,000 mAh power pack from an offshore supplier for about $30. ($USD)
It takes about 8 hours at 1 amp per hour to fully charge this bad boy but that's only 8 amps. yet this power pack is rated at 50 amps. You cannot remove more amps from the Lithium power bank than it takes to charge it!
The USB charger power bank rating of 50,000 mAh is very suspect and deceiving.

Note: USB (Lithium) battery/charger packs are quoted in mAh but that's a "marketing"
number which is very deceiving. The mAh is the current available from the nominal
voltage of the Lithium pack around 3.7 V. The USB pack will also need to step-up the voltage to 5V.
A crude calculation, is most USB packs, are at least 65-70% efficient but some we have seen this value is < 65%.
So 20,000 mAh pack will only produce about 14,000 mAh. At 50,000 mAh ~ 32,500 mAh.

Vendor specs:
2014 listed top brand 50000mah Power bank with LED diaplay external battery For iphone Samsung phones charger

*Input Voltage: DC 5.0V
*Input Current: 1000mA (Max)
*Dual USB output, charge 2 devices simultaneously
*Output Voltage (Port A): 5.0V
*Output Current (Port A): 1A
*Output Voltage (Port B): 5.0V
*Output Current (Port B): 2A
*IC Protection for charging / overcharge / short circuit / discharge
*Circle Time USE
*Dimension: 128.5*72.5*22.5mm
*Net weight: 295g
*LED indication for Battery Status
color : silver,golden
Package include:
*1 x 50000mAh power bank
*1 x USB charge cable


http://www.ebay.com/itm/50000mAh-Dual-USB-LCD-Power-Bank-External-Backup-Battery-Charger/301701820712?_trksid=p2047675.c 100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D32938%26meid%3D6a15f5fee9ed4026ac1a2c959f05b72e%26 pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D321832032914



 
robert.rozee
Guru

Joined: 31/12/2012
Location: New Zealand
Posts: 1456
Posted: 03:39am 17 Aug 2015
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it is most likely the rating is 50,000 mWh (milli Watt hour), and was quoted in mAh largely because the seller (a) doesn't speak a word of english and (b) has around zero knowledge about what he/she is selling.

if there are four 18650 cells in the bank, at 3.7v each, this translates to 50,000mWh / (3.7v x 4) = 3378mAh capacity for each cell. a quick check with google shows that 3.4Ah or thereabouts is a typical capacity of an 18650 li-ion cell.

the specifications in the ebay auction you link to give:
"Battery Capacity(mAh): 40001-50000"

i'm picking the 40,000 is the effective mWh rating given an 80% efficiency to the step-down converter used. it is most common for li-ion cells in packs to be connected in series (either 4 in series for 14.8v or 2 parallel/series for 7.4v) and to employ a step down-down converter. the higher the voltage you start out with, the easier the converter is to implement cheaply. and, as you note, these things are cheap.

for charging, 8 hours x 5v x 1A -> 40,000mWh. i'd put that number down to wishful thinking!


cheers,
rob :-)
 
twofingers
Guru

Joined: 02/06/2014
Location: Germany
Posts: 674
Posted: 03:51am 17 Aug 2015
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@rob

  Quote  is a typical capacity of an 18650 li-ion cell.


A very good cell has 3400mAh (Samsung, Sanyo, Sony, ...). Most of the "chinese" Li-Ion cells have only 400-1200mAh (AFAIK). They are often (typically) labeled as 4000mAh, 4500mAh or more.

Regards
Michael
"The trick, William Potter, is not minding that it hurts." Lawrence of Arabia?
 
WhiteWizzard
Guru

Joined: 05/04/2013
Location: United Kingdom
Posts: 2745
Posted: 05:48am 17 Aug 2015
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There are many things to be wary about regarding these kind of numbers.

The unit may well be 50,000mAh BUT probably not (makes good marketing though!). And you say it takes 8hours to fully charge - but what device is saying it is fully charged?
These 'cheap' packs normally include a 1A charger as these are cheap - above 1A charging becomes a lot more costly (relatively speaking). And then who says it is charging at 1A anyway?

As for the displays showing SOC (state of charge) - this is actually quite difficult to achieve 'under load'.

Note that a lot of charger chips have a timer cut-off (programmable up to around 6-8hrs). So maybe it is a 50,000mAh pack, with a timer cut-off which then incorrectly reports it is fully charged after this amount of time.

I have spent many hours trying to maximise power from a LiPo/LiIon pack to run the School Computer project I am nearing finishing. Even as we speak I am testing a high-spec 8Ah pack from which the circuit is drawing between 700mA and 850mA (700mA when fully charged, rising to 850 as vBatt drops). 10hrs theoretically maximum expected, with 4hrs observed (when cutting off at 3v5). 2A charging is taking up to 4hours which is correct!

So another important aspect is how low a voltage you discharge to. I would not recommend going much below 3v5 - but obviously 'runtime' is increased if you go down to near 3v0 (which is the voltage of most cut-off circuits). And if you do go below 3v0 then exactly what safety cut-off circuit is there in place? (do not rely on the inbuilt one in a 'cheap battery pack' (it probably doesn't even exist even though the 'advert' says it does!)

For anyone thinking I am 'negative' against these things then NO - it is just many years experience dealing with 'cheap' things that claim to be of a certain specification and turn out to be the total opposite of what you were expecting.

But much more importantly, when it comes to LiPo/LiIon battery packs then there is a very serious safety issue to observe - when things go wrong they literally can explode.

If you incorporate one of these battery packs into a 'gadget' for others to use then please ensure you know exactly what is happening with regards to:

1: Actual charging current (needs to be no more than rated. If above 1A then needs a safety cut-off in the form of a timer and/or temperature. You need to be in control of these settings

2: Actual charging voltage (needs to be no more than 4.2v at charge termination). Being less then 4v2 can increase the life of the pack.

3: Short circuit protection. Even if the end-user has no contact with the battery pack, what about any 'exposed' header pins that have voltage on them? Also what if someone shorts out general I/O pins - current can find strange paths back to your power source. I have had to add no end of current limiting protection and also current paths to ground just to avoid a potential 'short circuit'.

4: Avoid draining the battery too much - you definitely need a cut-off circuit that you know the parameters of; even if your standby current is 1mA you ideally need to isolate your circuit totally (unless you don't mind replacing battery on regular basis)


Be interesting to hear about you run-time and charge-time once you find a suitable 'pack'.

WW
Edited by WhiteWizzard 2015-08-18
For everything Micromite visit micromite.org

Direct Email: whitewizzard@micromite.org
 
MMAndy
Regular Member

Joined: 16/07/2015
Location: United States
Posts: 91
Posted: 09:01am 17 Aug 2015
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FYIO ...

For air travel/transport it can't be more than 32,000 mAh. ~ 118 wH (watt-hours @ 3.7V x 32)

Very few USB battery banks allow to charge the internal battery at the same time
of charging external devices.

I think we are going to go with two very fast input charging packs with enough energy
to last for 8 hours to power our Embedded BASIC Laptop.
~ 30 wH or 8,000 mAh.

More research ongoing ... Edited by MMAndy 2015-08-18
 
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