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I wish to understand how a battery charger may best be designed in order to maximise the life of Lithium Ion batteries. I have specified Lithium Ion batteries as they are what is used in most laptops and in many other pieces of modern equipment.

There is much advice and opinion available but as some is contradictory it is clear that any one opinion or offered advice may be partially or largely incorrect.

I wish to understand which parameters are genuinely important, how varying them will affect battery lifetime and which of the "commonly given" advice is actually incorrect.

In orer to make the question more specific initially and to provide a finite target for answers people could consider addressing the following charging scenarios. These may not be the only options.

  • Force charging-Charging even after reaching 100% of Lithium Ion battery capacity.
  • Charging only after the battery is empty.
  • Charging the battery in parallel while the powered equipment is working.

How do these affect battery life?

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  • \$\begingroup\$ I heard from some of my friends that maintaining a battery charge level above 90% will increase its performance and life.. I am not a electrical student. I am here to ask about how it is affected So that it will help many of our members. Can anyone explain \$\endgroup\$ – Dinesh Kumar Apr 30 '12 at 8:09
  • \$\begingroup\$ @noah1989: thanks a lot for your explanation.. \$\endgroup\$ – Dinesh Kumar Apr 30 '12 at 8:09
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    \$\begingroup\$ @DineshKumar there are serveral diffent kinds of batteries, each having their own chemistry. What is good practice for one kind of battery may be rather harmful to another one. If you want to learn more about this go to batteryuniversity.com/learn they explain everything in detail. \$\endgroup\$ – Stefan Paul Noack Apr 30 '12 at 8:22
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Preamble:

I'll assume a standard Lithium Ion (LiIon ) battery tyoe as used in the very large majority of laptops and similar equipment. Lithium Polymer (LiPo) is for practical purposes the same.
Lithium Ferro Phosphate (LiFePO4) is of the same family as LiIon but has some fundamental differences which mean these nswers apply only partially. I may comment on LiFePO4 separately.

I'll assume "normal" ambient conditions - say about 10C to 35C unless otherwise noted. I may comment on results out of that range.

I'll assume the battery (or cell) has not been deep discharged below the normally recommended minimum discharge level. All well designed equipment will not allow deep discharge below minimum recommended level. This can cause battery damage or destruction and special care is needed to recover a battery from that condition, if it is possible. I may comment on that at the end.

I will tend to use the term "battery" to mean cell or battery (= N cells) when the text applies to either. If I mean 1 cell specifically I will use "cell".


Answers:

Force charging-Charging even after reaching 100% in battery backup.

  • You cannot "force charge" a LiIon cell as long as you do not exceed design parameters of maximum charge rate and maximum charging terminal voltage. The battery is normally charged at design current until maximum terminal voltage is reached and then allowed to accept whatever current the chemistry involved desires until a cutoff point is reached.

Two parameters that affect life and charge capacit are the maximum terminal voltage used and the % of maximum current that you allow the current to fall to before you terminate charging. Reducing maximum terminal voltage and/or limiting that the current falls by before charging is terminated will increase cell life, at the expense of storage capacity in both cases.

Here is an immensely informative chart, care of Battery University, that tabulates the affect on capacity of various endpoint voltages and charging cutoff point. eg the traditional maximum charge voltage is 4.2V. When this cell voltage is reached the cell has 85% or maximum capacity. If charging is at 1C hen this occurs in about 85% x 1 hour =~ 50 minutes. If charging is allowed to continue for another 180-50 = 230 minutes the capacity will be 100% and charge current will be close to zero - say maybe 1% of max. Leaving the charger connected will have minimal additional effect. Disconnecting the charger when 4.2V is first reached will reduce available capacity by 15% BUT will increase cell lifetime by much more than 15%.

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Charging only after the battery empty.

More soon.
Worst case.

Better to charge little and often with battery more full.
Recharging back to point of 1st reaching 4.2V is best.


Charging parallel while working.

Good. Charger may charge battery plus operate computer if of enough capacity - this is almost always the case with chargers supplied with the computer. If not, it will slow down the discharge rate.

BUT, Best

Charge to cutoff voltage as per table above.
Disconnect battery and operate computer from mains supply.
This is the best point to maintain the battery at.
Battery will achieve maximum calendar life.

More on the above plus other comments later probably ...


Added comments:

Manufacturers tend to produce chargers which achieve close to maximum capacity. This gives longer operating life which assists the "consumer experience" [tm].
It also much shortens the available battery cycle life which is not so noticeable to users. This increases the number of batteries needing to be bought at accessory level prices during the equipment lifetime. This enhances ghe manufacturer and reseller experience :-).

While it would be possible for manufacturers to stop charging at less than complete capacity, and while some may truncate charging somewhat early in the final "current taper" part of the charging cycle, the majority dio not reduce capacity as much as is desirable for long life.

The OLPC laptops use either LiFePO4 or NimH cells. By limiting NimH charge and discharge to not include the top 10% and bottom 10% of capacity they get 2000 cycles from a NimH cell !!! LiIon can be extended by such methods.


Best cycle life a a given end-point voltage is achieved by stopping charging when the voltage "pedestal" is reached. As per the above table this gives 85% capacity at Vpedestal = 4.2V, and 75% at Vpedestal = 4.0V.

Batteries when unused last longest when stored at the end of constant current / start of constant voltage point.


Charging while working was covered above. Stopping the battery discharging further is a major gain. Extremely high temperature is not tolerated wll by LiIn cells but temperatures to about 40C are tolerable with no great problem.

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  • \$\begingroup\$ How come you can answer a closed question?? \$\endgroup\$ – Federico Russo Apr 30 '12 at 8:54
  • \$\begingroup\$ @Federico - Yes, I noticed that too: closed 1 hour ago, answered 4 mins ago. Strange. \$\endgroup\$ – stevenvh Apr 30 '12 at 8:57
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    \$\begingroup\$ witchcraft! burn him! \$\endgroup\$ – Stefan Paul Noack Apr 30 '12 at 9:01
  • \$\begingroup\$ @Russell McMahon: Thanks a lot. You made me to understand the concept. \$\endgroup\$ – Dinesh Kumar Apr 30 '12 at 10:21
  • \$\begingroup\$ @FedericoRusso - we try :-) \$\endgroup\$ – Russell McMahon Apr 30 '12 at 10:52
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Force charging-Charging even after reaching 100% in battery backup.

I guess you mean tickle-charging: Keeping the the charger attached, so the battery keeps being held at 100%. This is indeed not the best you can do to a Li-Ion Battery, but the manufacturer actually took care of this: 100% on the battery meter corresponds to ca. 80% of what's theoretically possible with Li-Ion batteries. There is a trade-off between capacity and longevity.

Charging only after the battery empty.

It's common that people believe that full-cycling (charging only when empty and then completely) batteries extends their lifetime. While this is is true for Nickel-based batteries, all modern laptop batteries as Li-Ion. Full-cycling is the worst you can do to such a battery, because they hate both being empty, and being full. They last longest when kept half-charged. However, it's recommended to full-cycle the battery once in 6 months to re-calibrate the battery meter since the capacity decreases over time and readings may become incorrect.

Charging parallel while working.

Charging while working is no different to the battery from just being charged electrically. But the heat caused by the Laptop adds to the heat from charging. Heat is what Li-Ion batteries hate most. It will reduce their lifetime. However, if the Battery is thermally well seperated, this should not be a problem.

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