Battery and power

Introduction

The OLPC has put a lot of effort into building a device that uses very low power consumption because it will be deployed in areas with no power infrastructure, or poor power infrastructure, or just plain expensive electricity costs. The laptop is capable of taking a DC input ranging from 5 volts to 25 volts to charge the 5-cell NiMH battery inside. This is far more flexible than most portable devices.

In addition, the 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. The units will ship with some kind of human-powered charger that plugs into the DC socket.

Read more details in Hardware specification.

Supply current characteristics of OLPC

Switching regulators as employed in OLPC can be a very demanding load for a power supply. The input current typically exhibits an 1/Voltage characteristics (this f.e. 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.

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.

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 5 V.

car battery, class room situation

Sensitive to deep discharge. Depending on the internal resistance of the battery and length of cabling a bistable situation as with solar cells can occur. Algorithms to avoid deep discharge in a single OLPC situation could lead to several oscillation modes for voltage and current in the classroom.

car style electrical generator

Non modern mechanical ones probably need a torque roughly proportional to the current. High internal losses (heating), large torque and poor efficiency if the voltage is "stuck" at the voltage at which the switching regulators start.

wind mill

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

wall power (with a transformator 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.

Hand Crank Problems

I bought a small battery-powered radio with a hand-crank charger to experiment with human-powered devices. It cost me all of £15 at Maplins in the UK. After working with it for a week I have discovered two problems with hand-cranked power. --Memracom 16:34, 21 June 2006 (EDT)

Balancing Musculoskeletal System

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.

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.

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, the 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. The treadle mechanism is a larger version of what needs to be built inside a pedal charger.

Static

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 arial. Any power generator for the OLPC should be tested for static 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.

Ideas for Alternate Power Sources

Gang Charger

Have some kind of charger at the school that can charge several OLPC's at the same time. This could be some human guinea-pig-in-a-cage idea that people take turns on. Or water-powered. Or even a solar-powered reserve battery bank.

Pulley Power

Instead of cranking something small by hand, use a rope wrapped around a pulley (or several pulleys) in order to work like a gear to increase the speed of rotation. The operator (or an animal) then pulls the rope continuously around the pulley system. Has the advantage of being buildable with local materials (wood, fibers) unlike bicycle contraptions.

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 panals 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.

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. Therefore the shipping units will come with a pedal-powered recharger. If you want to prove this to yourself then try the Freecharge portable charger. You can buy both a hand crank unit for mobile phones and a foot pedal unit.

Bicycle Adaptor

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 costs less then any pedal units for all users that already have a bike, and doesn't require additional effort (or waste of time) from all the users that already are using bikes to go to work or to school. (If this generator is like the one I had for my bike light, it actually does require noticably more efffort.) --Tef

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.

Low voltage infrastructure

The power connector should be widely available, allowing connections to a variety of low voltage devices. I believe there are other projects promoting low-voltage, high efficiency lighting for the third world. These systems might be a source of laptop power.

There are two such connectors on the laptop. One is where the power generator plugs in to recharge the batteries. Obviously you could use the generator to power other things as well. The other such connection is called a USB port. One of the gadgets that you will find in computer stores is a light that plugs into the USB port.

Reuse of dead car batteries

Related to the above, it is my understanding that most 12 Volt vehicle battery failures involve a short circuit in a single cell, reducing the total voltage at full charge to a bit over 10 V. Such batteries should still be able to power 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.

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.

Make your own batteries

Is it feasible for the kids to make their own batteries to power the OLPC? This would be a fantastic science project if it is possible. If potato and lemon powered clocks are possible, this should be too.

Batt types and charges

If you are constantly recharging the battery all the time this will greatly decrease the life of the battery.

NiMH batteries do not suffer from "memory effect", in fact NiCd batteries don't normally either. But, both types do suffer from voltage depression; when they are fresh and new they give a voltage of 1.2v per cell so 8 cells will give 9.6v, a useful amount. But if the cell is over charged or just always on trickle charge the cell voltage soon drops to just about one volt per cell; just 8v total, the equlivent of removing an entire cell. The interesting thing is voltage depression does NOT effect the total capacity of the battery. I have NiMH and NiCd that are completely depressed (hmmm) but they still do the job as well as they did when new.

The problem with this is the discharge graph [1]. You will notice that it's mostly flat so some designers take the 1.2v as gospel and design thier electronics to shutdown at 1.1v. If the battery is depressed by 0.2v this line comes in at about 10%..15% of the way into the capacity so you only get 10%..15% of your two and a bit hours of charge ... 20 minutes... 62.252.0.11 15:42, 17 March 2006 (EST)

I heard that nicd and nimh batteries have some fake memory effect: it was said that they were afected by another redox transformation than the normal,when they keep staying pluged on after having fully recharged the batteries: this redox transformation will divide the battery in 2 and have a part with the normal voltage and another part with an inferior voltage usualy that caan be easely detected because there is a voltage jump and so they needed to be full discharged with a resistor in order to get back their capacity but i don't remember the formula for calculating the value of the resistor 213.189.165.28 19:48, 4 April 2006 (EDT)

There is an article explaining NiCD and NiMH battery management here: http://www.camlight.com/techinfo/techtips.html

It would be a good idea to get some people to work on battery management issues. It should be possible to write an ebook that explains how to repair batteries. For instance, a slow charge at one-tenth the normal rate will solve some problems. Obviously, this is something that is fairly straightforward to do with a human-powered generator. Similarly, zapping the battery to melt crystals could be done using a partly charged battery from another laptop, or by the hand generator.

Quiet Laptops

I think that for people interested in buying these computers for personal use, one of the major appeals of this design is that it should be absolutely silent (well, except for typing and occasional cranking) - no hard drive, and with the low power consumption there shouldn't be any need for a fan. This, combined with its portability, makes it perfect for writing or other uses in remote, natural settings. I hope the design makes this happen, and that it is used as a selling point in developed countries.