How to keep your AIBO batteries working for years to come!

There are many myths surrounding the subject of battery life and battery care. For the scope of this page I have isolated just the minimal guidelines relevent to care and storage of your AIBO battery packs (this also applies to Li-Ion batteries in general, ie laptops etc).

Q. I do not use my AIBO often, how should I care for the battery when my AIBO is not in use?

A. Lithium-ion must be stored in a charged state, ideally 40 percent. This assures that the battery will not drop below 2.50V/cell with self-discharge and fall asleep. To assure the longest life in storage conditions, seal the battery (charged to 40%) in a moisture proof package and store in the refrigerator (0-10 degrees centigrade is ideal). Do not freeze the battery.
When you need to use the battery be sure to take the battery in it's still moisture prroof package and leave it to normalise to room temperature for a few hours before use. The last thing you want is moisture in the electronics!!!


Negatives of the 'smart' battery (such as AIBO battery)
A serious drawback is the requirement for periodic calibration or capacity re-learning. The Engineering Manager of Moli Energy, a manufacturer of lithium-ion cell commented, "With lithium-ion we have eliminated the memory effect; but is the SMBus battery introducing digital memory?" Why is calibration needed? The calibration corrects the tracking errors that occur between the battery and the digital sensing circuit while charging and discharging. The most ideal battery application, as far as fuel-gauge accuracy is concerned, would be a full charge followed by a full discharge at a constant current. In such a case, the tracking error would be less than 1% per cycle. In real life, however, a battery may be discharged for only a few minutes and the load pulses may be very short. Long storage also contributes to errors because the circuit cannot accurately compensate for self-discharge. Eventually, the true capacity of the battery no longer synchronizes with the fuel gauge and a full charge and discharge is needed to 're-learn' the battery. How often is calibration needed? The answer lies in the battery application. For practical purposes, a calibration is recommended once every three months or after every 40 short cycles. Many batteries undergo periodic full discharges as part of regular use. If the portable device allows a deep enough discharge to reset the battery and this is done regularly, no additional calibration is needed. However, if no discharge reset has occurred for a few months, a deliberate full discharge is needed. This can be done on a charger with discharge function or a battery analyzer. What happens if the battery is not calibrated regularly? Can such a battery be used in confidence? Most 'smart' battery chargers obey the dictates of the chemical cells rather than the electronic circuit. In this case, the battery will fully charge regardless of the fuel gauge setting and function normally, but the digital readout will become inaccurate. If not corrected, the fuel gauge simply becomes a nuisance.



Prolonged battery life through moderation
Batteries live longer if treated in a gentle manner. High charge voltages, excessive charge rate and extreme load conditions have a negative effect on battery life. The longevity is often a direct result of the environmental stresses applied. The following guidelines suggest ways to prolong battery life. -The time at which the battery stays at 4.20/cell should be as short as possible. Prolonged high voltage promotes corrosion, especially at elevated temperatures. Spinel is less sensitive to high voltage. -3.92V/cell is the best upper voltage threshold for cobalt-based lithium-ion. Charging batteries to this voltage level has been shown to double cycle life. Lithium-ion systems for defense applications make use of the lower voltage threshold. The negative is a much lower capacity. -The charge current of Li-ion should be moderate (0.5C for cobalt-based lithium-ion). The lower charge current reduces the time in which the cell resides at 4.20V. A 0.5C charge only adds marginally to the charge time over 1C because the topping charge will be shorter. A high current charge tends to push the voltage into voltage limit prematurely. -Do not discharge lithium-ion too deeply. Instead, charge it frequently. Lithium-ion does not have memory problems like nickel-cadmium batteries. No deep discharges are needed for conditioning. -Do not charge lithium-ion at or below freezing temperature. Although accepting charge, an irreversible plating of metallic lithium will occur that compromises the safety of the pack.
Not only does a lithium-ion battery live longer with a slower charge rate; moderate discharge rates also help. Figure 5 shows the cycle life as a function of charge and discharge rates. Observe the improved laboratory performance on a charge and discharge rate of 1C compared to 2 and 3C.  
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Longevity of lithium-ion as a function of charge and discharge rates.

Lithium-cobalt enjoys the highest energy density. Manganese and phosphate systems are terminally more stable and deliver higher load currents than cobalt.

Battery experts agree that the longevity of lithium-ion is shortened by other factors than charge and discharge rates. Even though incremental improvements can be achieved with careful use, our environment and the services required are not always conducive for optimal battery life. In this respect, the battery behaves much like us humans - we cannot always live a life that caters to achieve maximum life span.


Credits and Acknowledgements.

Information is reproduced with the permission of the author Isidor Buchmann, and has been taken from http://www.batteryuniversity.com. This is an excellent resource for anybody who wants to know more about batteries. As the author has been kind enough to give permission for me to use this information here I shall return the favour by putting in a quick plug to his book

The new 3rd edition of "Batteries in a Portable World - A Handbook on Rechargeable Batteries for Non-Engineers" is now available for pre-order through Amazon.com. The book will be available on April 11, 2011. Book format and details:


http://www.batteryuniversity.com/buy