Battery and power

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Removing the battery from a B4 model OLPC laptop.

OLPC has put a lot of effort into building a device with very low power consumption as it will be used in areas with poor or non-existent power infrastructure, with electricity often very expensive for the user.

Early concept devices were shown with a hand crank on the side to demonstrate that they would work in areas where the only electricity available comes from devices like the Freecharge portable charger. This was removed due to concerns about stresses on the casing, and ease of use.

Read more details in Hardware specification.

XO power input specs

Also see Battery replacements for why opening up your XO's batteries is a bad idea.

Electrical

take a DC input ranging from 11 volts to 25 volts to charge the battery inside. This is far more flexible than most portable devices. Polarity is positive (+) center pin and negative (-) outer. Reversing the connection will not harm the XO, but it won't charge either.

Switching regulators as employed in OLPC can be a very demanding load for a power supply. The input current typically exhibits a 1/Voltage characteristic (this e.g. means that an OLPC needs 3 times as much current at 5 Volts than at 15 Volt).

The impact differs depending on the type of power source and can range from small over poor efficiency to malfunction.

Following contributed by Sreeram Dhurjaty:

The switching power supply of the XO will tend to be more efficient at the higher voltage. A higher voltage, for a given power rating, will draw less current. Switching FETS (used in the XO) have an on-resistance and therefore the losses will be lower at lower currents for a given FET. Furthermore, diodes that are used in rectification will be less lossy for lower currents as the forward voltage of the diode is multiplied by the average current in order to determine the loss.

Be aware when wall warts are used to generate the requisite voltage, there will be a greater loss as many of the Wall warts, in developing countries are linear power supplies and have efficiencies ranging from 30-50%. For example, consumption of 2 Watts by the XO (assuming 90% efficiency) will necessitate a power consumption of 6-7 watts from the mains. Wall Warts also tend to be more efficient at higher voltages due to lower copper losses that are entailed by lower current-draw.

Mechanical

The XO DC jack specs:

• 1.67mm Center pin diameter
• 5.5mm Outer barrel diameter
• 11mm contact length

Mechanical Drawing File:DCJack.pdf

Note that a mating plug is shown on that drawing.

This connector is very non-standard. The "glass is half-full" people say that a completely weird connector is a good idea to avoid kids being mugged to have their charger bricks stolen. The half-empty guys say it's a clever conspiracy by some unmentionable dark force to keep control over OLPC by throttling the delivery of plugs to hardware hackers. I need plugs to deliver XO Solar kits. Yamaplos 17:18, 19 April 2008 (EDT)

With luck, this connector will be revised for Gen 2. See this ticket.

Some off-the-shelf parts that work

• There is a good existing Australian adapter from Farnell for Australia, New Zealand and the Pacific Islands.
• I finally found a 5.5x1.75mm connector as part of a laptop plug set made by Velleman. A distributor in New Jersey, Tequipment, sells the plug set (Model# PLUGSPSET3 adapters for USD $3.80. You'll have five extra plugs in other sizes to add to your parts box for other projects. Ordering from Tequipment was easy and delivery was prompt. I have photos posted on Flickr showing the plug and the slight modification necessary to use it with the Radio Shack 2-pin adaptaplug socket or you can solder directly to the pins on the back of the plug and seal appropriately. --Mike Lee 21:37, 4 June 2008 (EDT)
  • Velleman also sells a PLUGC5 "lighter plug" for USD $3.00. One would expect that they would mate, but the PLUGC5 end is round and the PLUGSPSET3 adapters are rectangular so they don't mate out of the bag (if you look at the pictures closely, you can see the round/rectangular shapes). I used an X-Acto to cut the rectangular shield off the adapter which allowed them to mate. A little heat-shrink tubing and it will look almost professional... --gvb 29 June 2008.

• Digikey part #CP-2199-ND (right angle) Current recommended part, tested at the OLPC office in Boston
• Digikey part #CP-2195-ND.
  The barrel for these parts is 4.75 mm and the length is 9.5mm but they still fit nicely. The right angle part won't swivel around 360 degrees because it interferes with the high side of the plastic casing. 9.5 mm looks like its about the smallest you can go with the length. These parts have a 6ft wire and a molded plug housing, and terminate in 2 bare wires. The outer cutout in the plastic casing of the XO is 10mm so make sure that the plastic molding of the plug is < 10mm. These plugs conform to the inner dimensions of the barrel better, which is optimal because the spring contact that touches the outer barrel has more flexibility to size variations.
  (courtesy Ed Leslie)
  The Digikey CP-2199 and CP-2195 look like standard EIAJ RC-5320A-03 jacks which are really meant for 6.3...10.5 V. "In theory", these should be easy to get.

• Kensington tip N2 and N2B work with User:AaronPeterson's XO. the package of his universal Ultra Portable Notebook Power Adapter K33197US says that plug is for certain IBM/Lenovo laptops. It's not a perfect fit.

• Someone at a San Francisco OLPC meetup had actually cut his original XO power cord and put RCA connectors where he cut it. This way he was able to use the original connector with various power sources (solar, car battery, etc) -- perezda

• A small number of free raw connector samples are available from an OLPC experimenter here.

Parts that DO NOT work

• Digikey CP-014-ND is not the right thing. I ordered a few on 24 April 2008. $18 bucks down the drain. Yamaplos 16:42, 27 April 2008 (EDT). Richard Smith adds: The problem is that it's too short, by about a .5 to 1mm. The interesting note is that the datasheet specifications are exactly the same as the plugs that _do_ work. So I dug a bit deeper. The end result is that a barrel length of 9.5mm is on the hairy edge of too short. The reason one set of plugs work and the others do not is that the the CP-2199 and 2195 plugs have better holding force inside the connector. They grip the inner pin much better, so they still work. They feel like they have some sort of internal taper that holds the center pin tight. So if you are trying to find connectors:
  • Even if the specifications on the datasheet are the same they need to be tested.
  • I think the minimum length needs to be >=10mm and perhaps 10.5 mm.

• Radio Shack part 274-1569 is a pair of 5.5 mm OD, 2.1 mm ID plugs, one of which can be soldered to a source of power (in my case, Radio Shack part 270-1559 -- 12VDC cigarette lighter plug with a 2.5 m cord with an on/off rocker switch and green LED power indicator) to make an inexpensive 12VDC power cord. However, when trying to use these Radio Shack parts, the connection was too loose to provide a solid connection. I wouldn't recommend that people purchase them.

Ideas for Alternate Power Sources

Solar panel, directly connected

They deliver an output current roughly proportional to light. This gives a bistable situation. Lets assume 100 mA here, the OLPC can then get either 100 mA at 5 V (0.5 W) or 100 mA at 15 V (1.5 W). These numbers might be typical for a 2 W solar panel with 16.5 V peak voltage. See [1] - since there's no regulation with a direct connection, efficiency can vary greatly depending on a lot of factors.

dunno about calculating that - if I did, I would have another degree (really, long story). Anyway, solar has been shown to work, see XO Solar

Car battery, single OLPC

Sensitive to deep discharge (due to thin plates compared to lead acid batteries specialized for solar cells). Premature end of life if discharged down to under 10 V.

This battery should be the lightest/cheapest, easy to find in the launch countries, and should be of good quality.

Car battery, class room situation

With very long cables, a similar bistable condition can occur.

Car style electrical generator

Older ones need a torque roughly proportional to the current. High internal losses, large torque and poor efficiency if the voltage is "stuck" at the voltage at which the switching regulators start.

Wind mill (turbine)

Poor efficiency if the voltage is "stuck" at the voltage at which the switching regulators start.

Wall power (with a transformer based device)

Little to no problems.

Wall power (with a switching regulator based device)

Most of these devices use a current limit or current foldback technique. Problematic on startup. A 60 Watt notebook (19V, 3.16A) supply could handle 6 OLPC but it will have difficulties providing their startup current.

Diagrams like "Current versus Voltage, charging/not charging" and "Current versus Time, @6V, 12V, 24V" would be needed to give good advice on dimensioning supplies. A means to limit or control the input current of the OLPC by software is needed to make the most of some of the above given energy supplies.

For most of the above cases, a simple regulator, that only connects input to the OLPC when the voltage rises above a programmed threshold (determined by the power source - 11V for a lead-acid battery to avoid deep discharge) would avoid the problems at switchon.

Balancing Musculoskeletal System

Human body

The human body, at its best, is a nicely balanced symetrical machine. My experience of hand-cranking is that it is too one-sided. You always crank with the right hand which means that it tires while the left hand does little work. Over time this means that the muscles of the right hand and arm will become a lot stronger than those of the left. This musculoskeletal imbalance can lead to various problems including back pain caused by unbalanced muscular tension. This is likely to be a greater problem with children than adults, because a child's musculoskeletal system is still developing.

Two-sided cranking

I tried to crank with the left hand but there were two problems. First, I couldn't see the charging LED which was disconcerting. I don't know if it is necessary to see it, but since it is there, using the crank system backwards and left-handed seems unnatural. But the more fundamental issue is that I have to crank in the opposite rotational direction when I use my left hand.

There are two possible ways to solve this for the OLPC. One way is to have a removeable crank that attaches to either the left or right end of an axle that runs through the generator. This way, I don't have to flip the device around so any indicator is still visible. And, when I crank left handed, it rotates in the same direction.

Pedaling

The other possibility is to get rid of hand cranking entirely and use a pedal on the charger. The pedal would work like those pedal pumps for inflating airbeds except it would run a charger. Interestingly the Freecharge portable charger seems to be moving in this direction. In addition to being easy to pump right-footed or left footed with no modifications, it uses the much stronger foot and leg muscles. If the generator still maintained an exposed axle, then it would be easy for users to rig up animal powered devices to turn the generator instead of the foot pedal. For an example of the pumping mechanism, see if you can find a working model of a Singer treadle sewing machine, a treadle potter's wheel or a pump organ. The treadle mechanism is a larger version of what needs to be built inside a pedal charger.

The chief advantages of a treadle pump system, or the reasons why such a system was successful historically are:

1. Legs are stronger than hands or arms
2. Legs have essentially infinite continuous stamina at a rate in the magnitude of 2 to 20 Watts for two legs.
3. Leg power generation doesn't interrupt work (this is why treadle sewing machines and pump organs are successful)

This is the better option because the kids can use the XO and generate the electricity at the same time.

RFI

While cranking my radio at the same time that it was playing, I noticed that there was static. But I didn't think much of it because the whirring sound of the generator was even louder. But then my wife complained that the cranking was disrupting our television reception when I was too close to the wall-mounted TV aerial. Any power generator for the OLPC should be tested for RF interference generation because it will not go down well if the child's cranking interupts vital economic information such as weather broadcasts or market price reports. In remote regions radio is a vital communications link and a device which disrupts radio reception will be frowned upon. Perhaps proper shielding will prevent this or attention to the generator design.

Wood Gas Generator

Electricity can be easily produced by generator powered by an internal combustion engine converted to run on wood gas. Gasifiers create combustible gas through the burning of wood or charcoal, that can then be used to power a gasoline generator with a slightly modified carburettor. Gasifier can be made from common materials such as 50 gallon oil drums and metal cooking pots. I am enclosing a link to a research document written by FEMA on the use of gasifiers as a fuel source during a petroleum crisis or fuel shortage. http://www.webpal.org/webpal/b_recovery/3_alternate_energy/woodgas/fema_wood_gas_generator.pdf

Direct Methanol Fuel Cell

MTI Micro, a subsidiary of Mechanical Technology, Inc. has developed direct methanol fuel cell (DMFC) technology that promises 2-10 times more up-time in normal portable devices, as compared to lithium and nickel-based batteries, and can re-charge without wires. I don't know specifics (costs, voltage, how recharge works exactly), but there could be a very interesting partnership with OLPC here... MTI Micro has been supplying power solutions for U.S. government laptops and packs, but are now moving into more consumer-base markets, and have just entered a deal for Samsung cell phones, to be shipped in 2009.

(Added comment: Methanol is quite poisonous. -- Nick Bodley)

(Added comment: So is nickel, cadmium, lead, sulfuric acid etc. Let's not lose focus here -- Matt Burnham (whatwouldmattdo at gmail dot com)

Gang Charger

OLPC is designing a charger for schools that can charge many OLPC batteries at the same time, and can also power running laptops so that their batteries can be removed and charged. This charger can be powered by a high power AC or DC source.

Child Powered Play Charger

Battery charging playground equipment could be connected to a Gang Charger. Unlike other existing human powered options, this solution does not require users to think that their energy is being used to charge the laptop. This is similar to the Playpump concept where water is pumped by children at the school's merry-go-round.

Pulley Power

Instead of cranking something small by hand, use a rope wrapped around a flywheel to increase momentum and output. The operators then pull the rope occasionally keeping the flywheel at a high rate of speed. Has the advantage of circular engineering instead of linear solutions that all lead towards a dead end.

Trevor Baylis, the British inventor whose work led to the original Freeplay radio, has also discussed using a bucket full of water or sand which would raised via a pulley and human power, and would drive a dynamo as gravity pulled it back to the ground.

Server with Solar Power

This server version could be built with a project laptop with the lid sealed shut. This machine should be a low power device -- a server should operate by solar panel in the lid or wind. It can be left unattended at high points in the area. This could be mounted on a pole or large tree at a hill/mountain top.

Furthermore, solar panels might be built into the laptop lid to charge the battery. I don't know if this is possible, but I am sure that the sun is the most readily available source of energy in the third world. Chemists have found a way to make cheap plastic solar cells flexible enough to paint onto any surface or mold into the case and able to provide electricity for wearable electronics or other low-power devices. Several available technologies such as Quantom dots have recently, made 10 x leaps in efficiency and are very cheap to produce.

We should separate the server idea (which belongs on some other page) from the solar power idea. Since the laptops will accept any DC power source, external solar panels could very well be used to provide that power. They don't need to be built into the lid or anything.

Hand cranked power

First of all, remember that the hand crank shown on the side of the original prototypes will not be used in the shipping units. The mechanical forces of cranking turned out to be incompatible with the laptop case. In addition, the hand crank relies on some of the smallest and most delicate muscles in the human body, namely hand and wrist muscles. As any anatomist will tell you, the strongest human muscles are in the leg. If you want to prove this to yourself then try the "Freecharge portable charger" (see below). You can buy both a hand crank unit for mobile phones and a foot pedal unit.

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The Freeplay Energy company, noted for their wind-up radios, also makes portable power chargers.

The FreeCharge Weza ($269 by ebay) is a foot-pump device similar to the one that will be used with the OLPC. The FreeCharge Mobile Phone Charger ($59 by ebay) is a hand-crank device that could easily be used in conjunction with a Sharp Zaurus or PepperPad to emulate the charging environment of an OLPC.

Freeplay Hand-Cranked Generator in development

Freeplay , the 'clockwork radio' people, are developing a hand-cranked generator specifically for OLPC.--195.137.93.171 15:37, 25 November 2007 (EST)

Potenco’s Pull-Cord Generator

Potenco’s Pull-Cord Generator (PCG) looks like a large yo-yo : you pull a cord out and a dynamo generates electricity. Claims 20W peak average power.

Quads are among the body's strongest muscles. Why not put leg/ankle straps on the Pull-Cord Generator? That way, the child could strap the generator to one leg for stability and essentially do leg lifts with the other. It would be easy to switch legs, and this set-up is more portable than a pedal-powered one.

Hand held Wind Power

A wind power source on a individual level could be more efficient when it comes to wind power instead of the conventional 20 foot tall 6 feet wide wind mill trying to power 20+ XO laptops. Individual wind chargers will have the children find the best spot for wind gusts or become the wind creator themselves through kinetic energy.

The portable, power storing HYmini comes to mind. It is 5.4 inches high, 3.4 inches wide, and 1,3 inches thick. Its 1200mA/h lithium ion polymer rechargeable battery stores power from three sources.

• Wind power from a small palm sized fan where a 9 mph gust will offer 5 volts
• Solar power from their cardboard solar panels
• Wall socket power from a AC/DC power adapter.

The stored charge can power most devices from the supplied USB cable or one of the five adapters. their website gives the example of 20 minutes of uninterrupted 19 mph wind will provide an iPod with 30 minutes of power. While the price is high at $50, a bulk order or a chat with the company could bring the price to reasonable limits.

It even comes in a white and green color scheme.

Bicycle Adapter

Practical trials

I've been trying to get the XO to charge from a bicycle alternator (bottle dynamo) this week, but no results so far. I am using the standard "Miller 12V 6W" alternator here in India that puts out between 0 and ~24 Volts AC. The set up is currently: AC voltage in goes through diode rectifier with three caps on the output (4700 uf, 0.1 uf, 0.01 uf) and an 8712 to limit the voltage going into the laptop. The four diodes produce a fully rectified voltage, as shown here. The capacitors help smooth out the bumps.

The XO charging light and icon come on, but there is insufficient current draw available to actually charge. To provide more power, I hooked up a second, identical alternator to the input of the electronics above. Again, the charge light came on. I wasn't able to measure current draw, but the 8712 was heating up quite a bit. My thought is that the AC was not in phase, and so there might have been destructive interference. I did not pedal for more than 20 seconds for fear of letting smoke out somewhere along the line.

These bottle alternators can only provide 0.5 Amps, and so I'm not even sure that two would charge the laptop. The stock wall adapter is rated for 1.42 Amps, so we still may be short.

From here there are two options:

1. Try and see if there is a simple circuit to get the two (three?) AC signals in phase.
2. Try and track down a larger (1 Amp?) dynamo that seems to exist for bicycles.

(Two days later)

Can't get the second option (larger alternator), no bicycle shops seem to know about this.

I hope someone else can try the first option. Here is a possible schematic. The "x3" by the capacitors mean to use what is mentioned above (4700 uf, 0.1 uf, 0.01 uf)

Hbonwit 10:27, 1 May 2008 (EDT)

Debate on effectiveness as a method of recharging

Bikes are very good pedal units and are often used by many students and workers. A simple add-on could allow to charge the computer while, for example, going to school or to work by bicycle as usual, this kind of add on could be really welcomed in many countries.

The simple add on it's just 2 rims (one to be added to the user's bike, one to to the hand-crank on the charger ),and a chain. Something like 2 U-bolts applied to the bike rack used to transport the laptop could avoid any stress to the computer case when charging This tool is not a replacement for the hand crank but a complement to it. It would cost less then any pedal units for users that already have a bike.

Another approach to using a bicycle would be to build or adapt something like an indoor bicycle trainer. a trainer is a kind of simple stand, the back wheel goes on a roller which is attached to some kind of resistance device (in this case a generator). sometimes a simple flywheel can be included to smooth the output. this is good if for instance there is only one bicycle (or only one generator/addon) in town: you can set up the "charging station" at a central place like a school. i think you will see from the picture that it should be possible to make these very inexpensively: just some tubing and a roller.

http://stonek.com/montevideo/afilador/afilador4707bx.jpg

Bicycles have been adapted to many things... if the thirld world is good at something is to grab something and reuse it for their own needs. To wit: here's a typical picture of an 'afilador' (somebody that sharpens your knives, scissors, etc.) whose bicycle has a stand that swings under the traction wheel so that when you pedal the sharpening stones rotate—much 'simpler' and portable (it's the bike itself). --Xavi 08:48, 2 May 2007 (EDT)

The roller that presses onto the tyre in the trainer is not efficient - neither is a 'bottle cycle dynamo' - they are hard work and wear out the precious tyres. Much better is a hub dynamo even one costing UK £15. Building a wheel or adding it to a bike will also be an educational experience and the 'flywheel effect' makes it much easier than 'foot-pump' systems. Maybe 'One bike per family/village' would make a good sister project ! --195.137.93.171 15:16, 25 November 2007 (EST)

Maybe this should go into a discussion page, but I disagree with the above post. I think bottle dynamo is the way to go, based on what I've seen here in rural India. Bottle dynamos are widely available and understood. As well, you can use the kick stand and be pedaling in place to charge the laptop (with 0% of your pedaling going towards forward motion) I'm working on getting our XOs powered this way. (Challenge is that some are AC and some are DC.) More info to tie in here: a Wiki.laptop page and a wikipedia page

--Hbonwit 09:35, 26 April 2008 (EDT)

Salvage Old Motors

People could salvage old electric motors and turn them into a generator which could be run by bullock power or running water.

Car and Motorcycle Batteries

People who have the use of vehicles can charge up extra batteries and rent them out to OLPC users.

Reuse of dead car batteries

Most car batteries fail due to decreasing ability to supply starting current. Such batteries can still be able to supply the much smaller amount of power to the lap top and could be charged by solar cells or a manual generator. They could even be placed in a car or truck and charged from a cigar lighter adaptor during work trips. A discarded vehicle battery should be able to run an OLPC machine for a few days after a full charge.

Now that is a prime topic for an e-book to be distributed with the OLPC. Some guy will take his kid's generator, adapt it to water buffalo power and use the beast to charge up car batteries for a fee.

Failure due to not supplying starting current is not that common. Most lead acid storage batteries fail due to a cell sulfating up. At that point the battery is useless. In regard to toxic materials; lead-acid storage batteries contain metallic lead, lead salts, sulfuric acid. Benefits may outweigh hazards, and proper handling minimize them, but they are there. Lead is particularly toxic to young children.

Fuel Cells

When you look at military PDAs,they are powered by fuel cells that aren't so available in these countries and are not in the price range being considered. We have to do something with the options available.

Buying 5 to 10 million fuel cells, and taking into account that the rest of the world would need or produce something between 100 and 1000 millions fuel cells, they could be in the price range. BUT fuel cells are going to be disposable or refillable, with all the distribution problems it may have in many of the areas where OLPC are supposed to be used.--S112 02:07 16 August 2006 (EST)

The US military does not use fuel cells in PDAs or in any field equipment that is general issue. The main source of power is batteries. The problem of powering all of the electronics soldiers must carry is very similar to getting an XO charged in a remote location. The military's research into powering those devices can cross over to powering XO laptops. --Brandonj 00:21, 16 November 2007 (EST)

Biological Fuel Cells (aka. Microbial Fuel Cells.[2]) are fuel cells that generate their electricity not from hydrogen, methane or methanole as known from conventional fuel cells but they actually digest organic matter (leafs, fruit, meat) and generate electricity. No kidding! There are already prototype robots driving around the countryside feeding themselves on what they find on the way! The structure is rather simple. You have a bioreactor where you put the organic material. Special bacteria living on electrode surfaces digest the "food" and deliver electriciy. Therefore such a power supply lives from the land as people and animals do. [3] [4]

Kinetic energy

As laptops, those computers are going to move around. A Kinetic energy system to power OLPCs might not be little enough to be integrated into the computer. But their users will probably run, dance and jump around even more than they will walk. As the project is already suggesting external power systems, what about recycling or reusing human energy that's already wasted?

It might need a second OLPC battery, instead of another energy storage device.

Just for keeping the kids untied from their laptops, while recharging... I mean, while playing.--S112 02:31 16 ago 2006 (EST)

Darpa has done lots of research on this, and several companies are producing small cheap energy harvesting devices that could be employed and would be superior to the hand crank or pedal. The charging device used in Faraday flashlight that produces power by shaking is very simple and cheap, and could be tuned to charge by almost any movement especially walking.

Another possibility in this category is a backpack that captures wasted vertical motion when walking or running. There have been various news stories about such devices. The top of a backpack is also a decent place to put small solar panels.

IBM has published an exhaustive paper with viability calculations of solutions related to human-powered computing a decade ago, it's available here. They conclude that legs are the best. -- bkil 21:20, 14 April 2007 (EDT)

In addition to human carried energy harvesters, it is possible to install gear in a floor to harvest energy from the people (or livestock) walking above. ex a busy town square or playgound or buillding or animal pen. See an interesting Wired Magazine Article. See also a company with some very cool ideas about energy harvesting: The Facility.

Solar Power

Some solar cells in the case can be useful, they will be charging batteries slowly every time of the day, while walking or just using the OLPC outdoors. And they can provide some extra time when the computer is off, in mesh mode.

Even the cheapest Chinese calculators (worth of USD 1) are equipped with SOLAR BATTERY - so it must be very cheap. And it is very light. And it is very wear-resistant. So why isn´t it mentioned in OLPC design? Not as the only source. Not as the main source. But why not allow it to contribute in energy balance of OLPC? Katerina Tlusta, hajtl@volny.cz

Most cheap 'Chinese' calculator's solar cells need only supply power in the order of a few microamps, while no less than a miliamper (or more, depending on temperature) would be needed in order to counter the self-discharge of a typical NiMH (this is not a capacitor as in BEAM robotics ;->). IMO, at least half a Watt of power should be considered worthwhile for most uses, providing roughly an hour of uptime after a day of sunbathing. Don't forget, that the user already has the option of connecting a proper small panel to the DC connector of the current design! -- bkil 20:08, 14 April 2007 (EDT)

The military frequently uses a folding solar panel to charge laptops and devices in remote places. Though the panels can be a bit expensive they are very rugged and a 10 watt or larger rollable panel which is cheaper if not as compact could easily charge a few XO laptops and possibly even operate them while charging. It might be feasible to apply one of the flexible solar modules to the back of an XO screen. --Brandonj 00:41, 16 November 2007 (EST)

Solar or Fire heated Stirling Engine

The Stirling Engine [5] is an old but none-the-less smart thermomechanical device that transforms heat energy into mechanical power (rotating shaft) that could drive a small bicycle generator. The stirling engine has two heat exchangers. One that is heated up by e.g. a fire or solar heat (maybe concentrated by reflectors). The second one has cooling ribs to stay as cool as possible. The larger the temperature difference of the two heat exchangers the more power it can produce given a fixed engine size. The automotive industry has improved the stirling engine for years now and has achieved an astonishing efficiency. As a small, simple and rugged battery charger produced in large quantities it could be low price (a few dollars) and it could be heated by a whole range of heat sources. Solar heating should be preferred since kids and fire is often a hazardous combination.

Solar or Fire heated Thermo-electric Generator

'No moving parts' makes these attractive and reliable. They are direct heat-to-electricity, instead of heat-to-motion-to-electricity. Direct fire might take them to solder-melting temperatures - maybe use a sealed heat-pipe, boiling water in the fire and condensing steam on the TEG. Solar concentrator (lens and/or reflector) can be designed to limit power input. Cold water from a stream or water evaporation could provide a 'heat-sink'. Tellurex make devices to generate 1.5 to 5.7 Watts of electricity from a 150 degC heat source and 50 degC heat sink, costing $22 to $37. Might need 2 in series to increase the voltage, or if the temperature difference is lower. Quantity discount applies to 25+. Sounds worth investigating ! Bismuth Telluride isn't the nicest material, but not generally high risk unless you grind it up and breathe or eat it.

Generation vs. Storage

Solar is an excellent hybrid option - but it cannot be a sole solution. There are plenty of places on the planet that coud use the olpc system but have very short days in the winter. It may be much better to separate the power generation issues from the power storage issues - and find a series of collaborative generation options including both solar and turbine with water/wind/human extensions for charging batteries.

I'd like to expand on this idea. What about some sort of waterwheel hydro generation to put on the downspout of a school's roof during the monsoon season? Of course it's seasonal, and erratic, but it seems like a decent amount of energy to harvest. Hbonwit 11:21, 1 May 2008 (EDT)

I came to this website trying to find a solution for a school in the western Himalayas - where there is very little sun in the winter - and not much water or wind either - and of course there is no mains power! I guess we will look at finding a longlife laptop battery with good human-powered generators available. Maybe some flywheel-based pedal generator that can be built with local materials is best. If we can find a reliable solution for pedal generation, then it may be possible for batteries to be made redundant - which would be great, as they are expensive, consumable, and don't work well in low temperatures. (86.142.27.202 14:54, 7 December 2006 (EST))

Above poster, please look in this article at the section on [Bicycle Dynamo practical trials]! Hbonwit 11:21, 1 May 2008 (EDT)

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For instance: Just as a fairly constant supply of electricity can be produced by a water wheel from the gravitational energy of water descending from a higher to a lower level, so it should be possible, in dry conditions, to use the potential energy of -- for instance -- a large boulder which descends from a height of (say) a meter or so to "ground level".

The way this might be done is to arrange a rack-and-pinion drive with quite a lot of gearing-up; the boulder's platform sits on top of the rack, whose descent is slowed by the pinion and its associated gear train, at the far end of which a small generator is turned by the now speedily-rotating shaft and produces a very small, but fairly constant, current.

The torque-resistance of the small generator would then act, as a flywheel or an air propellor acts, as a "brake" on the pinion's shaft via its gear-train; and the end effect would be that the (very slow) descent of the boulder on its platform produces a tiny but constant electrical current at the terminals of the generator, which is turning at a similarly constant speed.

All that we now need to arrange is that, at intervals and when it has almost reached ground level, the boulder is removed from the platform on top of the pinion, which is allowed to rise (or is pulled upward again) to its 'meter-high' position. The boulder is then lifted to that height -- perhaps via a pulley-system if it is sufficiently heavy -- and replaced upon the platform, whereupon the cycle of descent-and-power-production starts again.

The advantage of this suggested system is that, like a water-wheel, it's simple and robust. Its output isn't quite as constant as that of the water-wheel, since it's interrupted by the need to lift the boulder at intervals. This disadvantage can be smoothed out in various ways, though, from the use of several such boulder-wheels in tandem to the storage of the electrical watt-hours produced in fairly large (perhaps car-type) batteries, these being then 'tapped' by whatever actually uses the avaailable current -- usually a small computer (or several computers) of the type already discussed here.

--Later comment: This isn't going to be very effective. A 10-kilogram weight descending one meter releases 100 joules (100 watt-seconds) of energy. That's probably less than a minute of operation. The human body is not well designed for lifting weights, and children certainly shouldn't be doing it. Anyway, this is not the One Laptop per Sisyphus project. Generally, people, take the time to run the numbers for your proposals; if the numbers don't work out, maybe you should think a while longer before offering the proposal.

Mechanical Shaking

This weekend I bought an ingenious LED torch (flashlight).

It operates by using a 15mm magnet that slides inside inside a 9 mm tube that in turn is surrounded by a 25 mm bobbin at the centre of the tube. There is a rubber buffer at each end of the tube to absorb excess impacts and to encourage the magnet to re-enter the coil in the reverse direction. The user just gives the torch a shake, and this shaking charges up the capacitor and hence the battery. The minimalistic pc board contains an on-off switch, full wave rectifier connected to the coil, a current limiting resistor in series with the LED and switch, and is connected to what looks like two disc batteries in series, connected in parallel across a capacitor, and in parallel with the direct-current side of the full wave rectifier.

Similar devices are shown e.g. at http://www.vidcam.com.au/sales/faraday.htm

and at http://www.hc-gifts.com/3_Eternity_Flashlight_Hand_shaking_Forever_Flashlightsmall_623.htm

I was wondering if such an idea could be incorporated horizontally into the base of the OLPC so that when the child walks to school carrying the OLPC, the natural movement charges the laptop.

I have been running this torch most of a weekend, without any deliberate shaking of it, and it has not yet extinguished itself!

Certainly, the OLPC load is more that a single bright LED, but I think this approach will have promise, especially in developing countries where there is no power at home or the school, without the problems incurred by the mechanics of an external crank. --Olpcme 08:49, 31 May 2007 (EDT)

I think the power and efficiency are quite low. I am sceptical about 'most of a weekend' : this site says 5 minutes light for 30 seconds shaking, and the light only needs ~0.01 Watts of power, rather than 2 or 3 we want for OLPC! The cranked models are slightly better - 30 mins light for 1 minute cranking.--195.137.93.171 16:43, 25 November 2007 (EST)

Some of those shake-lights are sham. The disc batteries aren't really rechargeable, they're just ordinary long life lithiums that will run out eventually, although they will accept some charge. Try removing the batteries and running the light on the capacitor alone.

A few notes about shake flashlights/torches -- They use a supercapacitor, probably one farad, which can be damaged by overcharging; the few that I've seen don't have overcharge protection. The magnet is extremely powerful, as well. I've owned a couple of these, and imho they are not practical (they work, but the supercap discharges fairly quickly). In mine the magnet sticks to the bumpers and does not slide freely at all, but vigorous shaking does work. Far better is a well-designed crank flashlight. -- Nick Bodley

See also

• Power Peripherals

• World Bank report on return on investment for rural electrification, March 8 2007