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.<br><br>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.<br>
<br>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!<br>
<br>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.<br>
<br>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.<br><br>
--n<br><br><div class="gmail_quote">On Wed, Apr 30, 2008 at 9:43 AM, Justin McAllister <<a href="mailto:mcallister@avinc.com">mcallister@avinc.com</a>> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000">
Some ideas:<br>
<br>
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:<br>
<br>
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.<br>
<br>
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.<br>
<br>
That's about $0.40 more than you proabably wanted, but good luck.<br><font color="#888888">
<br>
Justin</font><div><div></div><div class="Wj3C7c"><br>
<br>
Holden Bonwit wrote:
<blockquote type="cite">
<div style="font-family: times new roman,new york,times,serif; font-size: 12pt;">Updates
will now occur on the wiki at the following section:<br>
<span><a href="http://wiki.laptop.org/go/Battery_and_power#Practical_trials" target="_blank">http://wiki.laptop.org/go/Battery_and_power#Practical_trials</a></span><br>
<br>
Please contribute if you can, by posting there, or replying to me.<br>
<br>
And please let me know if my wiki updates are not proper in any way,<br>
<br>
-Holden<br>
<div> </div>
==<br>
Holden Bonwit<br>
<a href="mailto:hbonwit@alumni.nd.edu" target="_blank">hbonwit@alumni.nd.edu</a></div>
</blockquote>
</div></div></div>
</blockquote></div><br>