[Peripherals] Charging the XO off a bicycle dynamo

Thu May 1 10:03:19 EDT 2008

Couple comments here:

1) Can I just mention how much more daunting it is to think about recharging a car battery with a little bike dynamo, than it is to charge the XO laptop battery that is just 6 little cells?  It makes me not want to pedal.

2) The less hardware the better.  This keeps cost and complexity down, both of which are important, in my opinion for this type of project.  Not having a battery was one of my soft requirements (the guys down at the shop already recommended this -- probably because it's the right answer), both for computer speakers to be powered (mp3 or walkman audio input, but cycle power) and for the XO charger.

3) Everyone's doubting that charging by cycle makes sense, but here's how I see it: The laptop (currently) consumes in the neighborhood of 6 Watts.  My bicycle 
alternator puts out 6 Watts.  I think a 1:1 ratio is pretty good.  If we put 2 dynamos on that maybe goes to 2:1 and I'm even happier.  But I've got to think that someone would want a simple solution like this.  I'm living in a town of millions right now (Meerut, UP, India) and we lose power daily (12 hours or so).  Power is an issue, and with the growing population here in India, and additional consumption per-person average (cell phones, anyone?), it is only going to become a bigger issue.  The state is clearly behind on this, and so there are two options: private enterprise power solutions and personal / self-ingenuity solutions.  For the less wealthy, these personal solutions are going to become more appealing.  </rant>

Turns out the larger bicycle dynamo / alternator is not to be found, I'm considering it word-of-mouth vaporware.

I leave on Sunday and so I hope someone else can try the multiple-dynamo with multiple-rectifier setup that has been discussed.  In India, your local electronics shop should be able to make this for 100 or 200 Rupees.  I'll put some schematics on the wiki page.

-Holden

==
Holden Bonwit
hbonwit at alumni.nd.edu

----- Original Message ----
From: Nick Foster <bistromath at gmail.com>
To: Justin McAllister <mcallister at avinc.com>
Cc: Holden Bonwit <hbonwit at alumni.nd.edu>; richard at laptop.org; arjunsarwal at gmail.com; peripherals at lists.laptop.org; Paul Yarin <yarin at blackdust.com>; Tara Chklovski <tara.chklovski at gmail.com>; Mike Simpson <mikebsimpson at yahoo.com>
Sent: Thursday, May 1, 2008 12:05:37 AM
Subject: Re: Charging the XO off a bicycle dynamo

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 (undervoltageand overvoltage) and ride with the generator charging the battery atall times, you wouldn't have to cart the OLPC around while developingthe 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 havework.  You'll need 3x rectifiers (for 3 dynamos) and probably some morefiltering (the more the better for this type of thing).  Feed each ofthe dynamos through the rectifier and combine at the capacitors.  Thatway each you'll be combining rectified AC (all positive voltages) andif the signals are out of phase they'll just help to decrease yourripple.  Also if the 8712 is getting really hot it may be going intothermal shutdown.  You can usually combine them in parallel to spreadthe thermal load, and, of course, you can add heat sinks.  However, ifyour 12V dynamos are only putting out 12V, once you rectify you'redangerously 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 2dynamos in series.  Rectify the individual outputs, then put thenegative terminal of one as your ground, connect +/- of the other, anduse 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 thecurrent. This will also help to make sure you're above the 11Vthreshold. If you do this you'll want a higher voltage regulator oryou'll still just be wasting that extra power.  Also if you do this becareful 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 isequivalent to just putting super-mega-ultracaps on your rectifier, butthe point is that the average current draw will come from the batterymainly, then you'll mostly be charging the battery from the dynamo(s). There are some extra losses involved here and I hope Mr. Foster willcorrect me if this is a stupid idea, but I think it will provide a moreconsistent source for the XO charging.  If you go this route you'llprobably 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 isthe XO proabably wants a fairly constant charging current, which theinductors will help maintain, even as the voltage on the battery hasnoise on it. You may be able to go without the regulator, since the XOcan handle25V the system should never get close to that and the battery probablywon'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 youdo so with adequate ventilation, since the battery will off-gas ifslightly overcharged.

That's about $0.40 more than you proabably wanted, but good luck.

Justin

Holden Bonwit wrote:    Updateswill 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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