Objective comparison between emergency usb chargers

Ed Kapitein ed at kapitein.org
Fri Aug 14 00:11:11 CEST 2009

Laszlo KREKACS wrote:
> Hi!
Hi Laszlo,

I am an electronics engineer and would like to help you.
> I have already bought 3 different usb chargers, and I can test them.
> I also just ordered two additional chargers from ebay.
> So I would like to know any details, how to make objective (neutral)
> comparison between them. Would like to summarize my test
> data on a dedicated wiki page for everyone's benefit.
Please do, i would like to have one for the holiday season
> Im on om2009t5, so maybe the sys paths are different, so
> would like some pointers where to look and what is exactly
> the differences between 2.6.28, 2.6.29 and 2.6.30.
>  How to measure charging data, how to automatically
> resume (from suspend) check the charging datas and
> suspend again?
I would insert a small foil between the battery terminal an the
freerunner itself.
The kind of foil you find for connecting displays, printer head's etc.
And i would do measurements with a normal multimeter. I have no idea how
accurate the software measurements are, it would be nice to compare them
with "real" measurements.
A plotter would be nice, but taking measurements every hour or so should
be good enough.
> How to measure voltages? (output voltage of
> one charger without load is 6.7V)
> How works the different charging modes? (100,500,1000mA)
> Is it fix? So if I set 500mA, and the charger can deliver only
> 340mA, can the charger overheat? Can I charge with 340mA
> or only with distinct 100, 500 and 1000mA?
> I also read on the wikipage, that If I set the charging current
> higher what my charger can deliver, I can destroy not only
> the charger but the freerunner itself too. (is it the case?
> some technical explanation?)
Highly unlikely, as far as i know you set the *limit* so it will not
charge with more than the set limit.
And if the charger can not provide more it will either supply what it
can, or shutdown with a self protect mechanism.

> How the current limitation works? So if I set 1000mA, and
> the charger can only deliver 500mA, what will exactly happen?
> (how the current limitation works inside the freerunner?)
> I hope we can collaborate, collect useful data and publicate
> them. Im very tired all these cheap half-finished electrical
> devices (I can only charge 27% from a 3500mAh 3.7V
> accumulator! It is insane!)
i think 3.7 V is just not enough to charge the freerunner, in need to be
5.5V  if i am not mistaken.
> I also want to collect the *efficiency* of the charger backed up
> with some measured data.
> For example MintyBoost site have some detailed measurement
> how the efficiency is of the Mintyboost (from 60-80% depending of
> the charging state of the battery).
> My goal is: having a usb charger what is capable to recharge
> two or three times the FreeRunner.
> Best regards,
>  Laszlo
> _______________________________________________
> Openmoko community mailing list
> community at lists.openmoko.org
> http://lists.openmoko.org/mailman/listinfo/community
If you like to test the chargers without risk of damaging  your
freerunner, you can make a small power drain, that will sink 100, 500,
or 1000mA.
this is easy enough with some cheap electronic components and will give
you a more "lab" way of measuring.
If you use the Freerunner battery as load, there are more variables,
like remaining capacity present in the battery, current will decrease as
the battery gets fuller etc.

to get an idea on how such a current sink could look like, please take a
look at http://www.discovercircuits.com/DJ-Circuits/10ampbattest.htm
(this is a 10 Amp sink, way to much for your tester, but it gives an idea)
Normally if you discharge a battery, you would do so at 10% of its
capacity, and the voltage should remain constant for +/- 8 hours.

I can write some schematics for the current sink if you like to give it
a try.\

Kind regards,


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