[Power] Charging XO-1.0 From 12V DC Battery

[Richard A. Smith]

The following is another question on DC charging that was sent to me 
recently.  With the author's permission I've re-routed it to power@ and 
added him to the list.

On 12/13/2010 11:19 PM, Andra DuPont wrote:
> Rich, We have 100 XO laptops in a remote location in Kenya. They are
> off the grid, so using solar panels to charge four 100ah 12V
> batteries. They are currently using a 240VAC inverter and plugging in
> standard AC chargers to recharge their XOs.
>
> This must not be the most efficient way to recharge the XOs, as
> neither the inverter nor the AC chargers are 100% efficient in
> converting from DC to AC and then back to DC.

Correct.  Depending on how efficient your DC-AC inverter is you are 
losing 30-40% in the conversion process.

> I would assume that the
> XOs could be charged directly from the 12V batteries, and have read
> that 11.5 VDC is probably the optimum recharge voltage.

Yes.  The input range for an XO-1 is 11V to 18V.  Anything in that zone 
will be approximately the same efficiency (at the XO).  It only changes 
a few % depending on the voltage and the ambient temperature.

> I would like to make a multi-station recharger that would connect
> directly to the 12V battery. My main question is.... what do the
> recharging amps look like as the XO battery goes from a low state of
> charge to fully charged, assuming it is being charged from a 12VDC
> source? I know that there is a controller in the XO that manages this
> to some extent.

The charging curve for the battery will be similar to this:

http://dev.laptop.org/~rsmith/XO1-chg.png

That's the power input as seen at the battery for a bunch of different 
batteries on running XO-1s.  Its not a peak draw of the XO because it 
does not include the power consumed by the XO.

Based on the average of the curves in the above graph and the efficiency 
of the XO-1's DC/DC converter I estimate the power needed to charge a 
battery from full till empty as 26 WattHours.

One user on this list has also measured his charging energy as
24.2 Wh. [1]  So I think the 26Wh number is a good across the board 
number to use.

The maximum power input of the XO at the DC jack is 17 Watts and the 
lowest voltage the XO would draw that from would be 11V so each XO-1 
should never draw more than 1.55A.  That's your worse case.

A typical scenario would be a brief spike of up to 17W and then a roll 
off of down to 10W in around 1 to 10 minutes. Then around 10W for about 
1.5 hours and then starting to taper off to 1W.  Each battery/XO is 
slightly different so there's room for a few Ws wiggle on each 
measurement (except for the max of 17W)  I've tried to be on the high 
side of things.

So you should probably prepare for a several minute spike of 1.5A/XO and 
then a constant draw of around 1A/XO for 1.5 hours. That should let you 
size your wires accordingly.

> I have a few XOs here at my home in the States, and will probably do
> some tests to see what kind of currents I get doing a recharge from a
> 12VDC source....

Please do and report back.  Remember that the true peak happens the 
moment the XO turns on the charger and does not last for very long.  So 
be ready to sample your peak draw as soon as you apply 12V.

  but I thought you might have some advice on how to
> proceed with the multi-station recharger. Can I do something as
> simple as connecting 10 jacks in parallel to the battery and let the
> XOs charge without any additional electronics added?
>
> Thanks, Andy DuPont

Yes.  Given 11-18VDC the XO will handle the rest.  I think your largest 
challenge will be acquiring the cable that plugs up to the XO.  The mate 
to the DC jack is difficult to come by.  I've received reports that's 
its easier to find in electronics shops in Asia but right now I don't 
know of any source other than the OLPC Association or a custom cable 
maker.  They have only recently become available to order via the OLPC 
Association.  I'll have to ask what the minimum order and price is.

Various models of the Acer Aspire One and the Classmate have a plug that 
is compatible with the jack but I'm not sure if that helps in finding a 
cable.

[1] http://lists.laptop.org/pipermail/power/2010-December/000053.html

-- 
Richard A. Smith  <richard at laptop.org>
One Laptop per Child