[Peripherals] Charging the XO off a bicycle dynamo

[Nick Foster]

I am going to run with Justin's suggestion of using a battery, since I
believe it to be the most practical solution to your charging woes. So
everything below assumes you can find a battery.

It can be dangerous (and certainly damaging to the battery) to charge a
lead-acid battery without a charging system protecting it, as Justin hinted
above, unless you can be sure that your generator when unloaded will never
output more than 14.5V. I understand that the generator will provide quite a
bit more voltage when unloaded. That said, a battery would provide a good
source for the OLPC to charge from, and you should not need inductors
filtering anything out, since the impedance of the battery itself is pretty
dang low. But they wouldn't hurt, either. If you were to create a simple
battery-protection circuit (undervoltage and overvoltage) and ride with the
generator charging the battery at all times, you wouldn't have to cart the
OLPC around while developing the power to charge it later; for instance,
ride to school, ride home, plug in, charge. Practically speaking, this might
be the most feasible solution to powering the laptop in the field since it
doesn't require carting the OLPC with you everywhere and allows you to
charge at your leisure. Additionally, a circuit similar to the voltage
regulator on your car would cut out the generator when the battery is
charged, reducing the pedaling effort of the rider since the generator would
then be unloaded. My old college electronics prof would say such a circuit
would be WICKED CHEEP to implement. It's just an ON-OFF regulator.

Since, as mentioned earlier in the discussion, it takes a heck of a lot of
pedaling to charge the OLPC completely, you would eventually develop an
energy deficit in the lead-acid battery (more charging than pedaling) and it
would go dead on you. LA batteries do not like to be stored empty for any
length of time (the plates will sulfate and the battery will eventually fail
to charge and discharge). Thus, some sort of indication of battery life
might be useful. This might be a chicken-and-egg problem, but Holden, you
have been using the oscilloscope feature of the OLPC to measure voltages;
this facility could be very easily harnessed to determine the approximate
charge state of a lead-acid battery. Of course, this presumes that the
laptop isn't already dead!

I don't know how cheap motorcycle batteries (or other small LA batteries)
are in other countries. The OLPC batteries are specified as 16.5W-h (NiMH)
or 22W-h (LiFeP); I don't know which one yours has. To fully charge an OLPC
from a fully-charged LA battery, given reasonable conversion losses and
safety margins, you would want a battery of at least 4 amp-hours; motorcycle
batteries are typically 12 amp-hours, so you could get away with something
significantly smaller, if available locally. UPS backup batteries and home
alarm system batteries are the first things that come to mind, sources which
probably aren't too common in rural areas of developing countries! But if
you know of cheap local sources or surplus batteries, this solution might
work.

Richard's point about using a full-wave bridge rectifier is spot-on; doing
so will double the power output of your generator but will not increase the
voltage output. That would be a good first step to take.

--n

On Wed, Apr 30, 2008 at 9:43 AM, Justin McAllister <mcallister at avinc.com>
wrote:

>  Some ideas:
>
> If you can't get a bigger dynamo, you may be able to make what you have
> work.  You'll need 3x rectifiers (for 3 dynamos) and probably some more
> filtering (the more the better for this type of thing).  Feed each of the
> dynamos through the rectifier and combine at the capacitors.  That way each
> you'll be combining rectified AC (all positive voltages) and if the signals
> are out of phase they'll just help to decrease your ripple.  Also if the
> 8712 is getting really hot it may be going into thermal shutdown.  You can
> usually combine them in parallel to spread the thermal load, and, of course,
> you can add heat sinks.  However, if your 12V dynamos are only putting out
> 12V, once you rectify you're dangerously close to the 11V lower end of the
> XO requirement, so this *may* never work.  Enter idea #2:
>
> Now that I'm reading the specs, another thought might be to use 2 dynamos
> in series.  Rectify the individual outputs, then put the negative terminal
> of one as your ground, connect +/- of the other, and use the + of the the
> other as +V.  Since the OLPC can take up to 25V, you may be able to feed it
> a higher voltage easier than you can up the current. This will also help to
> make sure you're above the 11V threshold. If you do this you'll want a
> higher voltage regulator or you'll still just be wasting that extra power.
> Also if you do this be careful with the polarity of the electrolytic caps.
>
> One final thought is to run this whole system from a 12V
> (car/truck/rv/boat) battery, charged by the dynamos.  This is equivalent to
> just putting super-mega-ultracaps on your rectifier, but the point is that
> the average current draw will come from the battery mainly, then you'll
> mostly be charging the battery from the dynamo(s).  There are some extra
> losses involved here and I hope Mr. Foster will correct me if this is a
> stupid idea, but I think it will provide a more consistent source for the XO
> charging.  If you go this route you'll probably want to put a pair of
> inductors on the leads going to the XO, with a cap on the XO side of the
> inductors.  The idea behind that is the XO proabably wants a fairly constant
> charging current, which the inductors will help maintain, even as the
> voltage on the battery has noise on it. You may be able to go without the
> regulator, since the XO can handle 25V the system should never get close to
> that and the battery probably won't care unless you're charging with more
> than 14.5V. Safety note: anytime you charge a lead-acid battery that's not
> sealed make sure you do so with adequate ventilation, since the battery will
> off-gas if slightly overcharged.
>
> That's about $0.40 more than you proabably wanted, but good luck.
>
> Justin
>
>
> Holden Bonwit wrote:
>
> Updates will now occur on the wiki at the following section:
> http://wiki.laptop.org/go/Battery_and_power#Practical_trials
>
> Please contribute if you can, by posting there, or replying to me.
>
> And please let me know if my wiki updates are not proper in any way,
>
> -Holden
>
> ==
> Holden Bonwit
> hbonwit at alumni.nd.edu
>
>
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