Rechargeable Batteries and Charging FAQ

Rechargeable battery technology has changed drastically over the past 10 years. Chemistries evolved from the  Nickel Cadmium (NiCad), followed by  Nickel Metal Hydride (NiMH) technology,  and now Lithium Ion (LiIon).  LiIon an lithium variants are  the new wave – the current and rapidly maturing state of the art.    In miniature devices, NiMH cells have been largely but not completely supplanted by LiIon technology.  The battery chemistries have different tradeoffs in characteristics such as capacity, voltage, discharge, self-discharge, charging, stability, durability, and life expectancy.

Ideally, a rechargeable battery is like a gas tank.  Fill up, run until the gas is gone, then fill up.  Recharging should be like filling up a car and take little time.  So how are rechargeable battery technologies doing in approaching this ideal?  The NiCad cells were good enough to make portable electrical devices possible but as everyone who ever used them knows, left much to be desired.  Run time was too short and characterized by a brief period of great performance, then a gradual drop-off in voltage performance.  Engineers profile this graphically as the so-called discharge curve, where given a fixed power drain, the voltage declined with time until the battery “dumped”, voltage went to near zero.  Internally, this behavior was determined by the internal resistance of the cell.  A low internal resistance resulted in  a longer period of good performance and a fast dump at the end (like running out of gas in a car).  A high internal resistance produced lower voltage performance overall, a slowly declining voltage dump.  As cell chemistries advanced, internal resistance dropped dramatically resulting in flatter discharge curves and gas tank like performance.  NiMh cells overcame the limits seen in NiCad, delivering much lower internal resistance, higher cell voltages, and increased cell capacities by a factor of five or six in a few years of development.  Now,  Lithium Ion cells have the flattest discharge curves to date, can sustain voltage at the highest levels of discharge ever.  This contributes to cell capacities that equal and exceed NiMh capcacities. Perhaps the most significant of the LiIon advantages is the absence of memory effect, meaning that the battery can be recharged at any time without losing capacity.

In many areas, the LiIon seems to be superior to the previous generation of nickel based cells but with technology, as in life, getting something for nothing just doesn’t happen easily.  LiIon has an Achilles heel, thermal runaway, a tendency to self-destruct if discharged to too low a voltage or charged to too high a voltage.  By self–destruct, I mean that the cell shorts internally and then reaches temperatures high enough to cause the cell to vent, explode, melt or otherwise burn and destroy the equipment it is meant to power, such as that $2000 Dell laptop.  This tendency might give one pause given that the device is often in your pants pocket, the kids backpack, or the luggage compartment of your airplane.   Despite this feature, LiIon is now a mature technology  due to a dogged determination by engineers to overcome thermal-runaway. Ways have been found to prevent the cells from operating outside their voltage range.  The problem has not gone away but by building in defensive measures into how LiIon cells are charged, discharged, and physically packaged, the incidence of thermal runaway has been progressively limited to the point that the primary barrier to technic logical feasibility, the risk of a multi-million dollar lawsuit, is close to zero.

What about the charging process?  We already talked about thermal runaway and how LiIon does not suffer over-charging mildly.  How long must you charge? Can LiIon be fast charged like NiMh and NiCad could?

Here are some frequently asked questions about batteries and how to charge them.  There are three sections:  Lithium Ion, NiCd/NiMH, AA and AAA cells.   The information here applies primarily to consumer electronics and not necessarily to hobby applications like radio controlled cars or hi-performance flashlights that have   extreme charge and discharge requirements .

CELL PHONE, LAPTOPS, iPODS and other devices using Lithium Ion batteries.  The original equipment is shipped equipped with their own chargers from the manufacturer.  Charging is done using only the manufacturer’s charger.

  1. How long must you charge? Can LiIon be fast charged like NiMh and NiCad could?  Answer: Regardless of method, the charge voltage must stay well within the operating voltage limits of the cells, which eliminates the possibility of fast charging LiIon cells.
  2. Can I top off the charge or should I wait until the battery is low?   Answer: Charge the device at any time. The lithium battery is designed to work equally well within the limits of maximum and minimum charge capacity.  The equipment is designed for convenience and a fully charged phone is the most convenient, a dead phone most in-convenient.
  3. Can a device with an LiIon battery be overcharged?  Short Answer: No. Consumer devices with a lithium battery have battery protection built in to specifically prevent an overcharge.   There is a long answer (yes), but that is why consumer devices have battery protection circuits built in.  Protection circuits can be in the battery pack itself, in the device itself (special chip), and/or in the charger which plugs into the wall.  It is best to limit charging activity to getting a full charge and then disconnecting the equipment.  Why 1) trust the manufacturer more than you have to and 2) use more energy (i.e. money spent of electricity) more than necessary?
  4. Well, is it OK to leave equipment on the charger overnight?  Answer.  No.  But: Everyone does it, and manufactures know this so they build in overcharge protection (see the previous question).    Better to charge while you’re awake and unplug when you sleep.  LiIon can be charged anytime and a partial charge is better than nothing.
  5. What is meant the “dangers” of LiIon batteries?  Answer:   This applies to most kinds of batteries, including LiIon.  If the battery pack is short circuited, for example by contact with a metal object like a pin, tinfoil, coin, bare wire, or other metalic junk, the metal object and the battery pack can become hot.  Hot can be anything from red hot, posing a true hazard of serious burns, or, not as serious, just smoking, spitting hot with smoke, steam or jets of caustic  chemicals venting from the battery.  The heat can cause a pressure build up and a hazard of explosion, big enough to hurt someone within a couple of feet of the device for small devices.  The physical hazard increases with size.
  6. Ok, I didn’t follow the manufacturers guidelines and now gasses and smoke are shooting out of my battery pack.  What do I do?   Answer: Think people first and your safety, then property.   If there is fire, deal with the fire first.   If the problem is only with the battery pack or device, while you think about what you will actually do, unplug from wall power if that can be done safely,  put on work gloves and eye protection. Don’t breath the fumes.   A covered fireproof container would be handy to have about now.  Metal is fine, it is too late to worry about shorting the battery.  By any means possible, get the smoking fuming mess out the door and out of the house.

Nickel Metal Hydride (NiMh) or Nickel Cadmium (NiCd) Cordless Phones, Power Tools, Dust Busters and hand Vacs.

  1. Should I top off the battery charge or should I wait until the battery is low?  Answer:   With NiMh cells, there is  a trade-off between battery life (months or years before you need a new battery),  and convenience (how long you can use the device between charges).   Where convenience or need is more important, charge now.  Otherwise, the service life of the battery will be best served by fewer charge cycles. Do the same with NiCd but as soon as possible, let the battery discharge completely as soon as possible, then charge fully.
  2. Is it OK for the battery to sit on the charger?  For NiMh batteries – No.  After a full  charge,  remove from the charger, unless you have a charger that automatically stops charging.
  3. Is it SAFE for the NiMh or Nicd battery to sit on a charger?   Answer:  It depends on the design of the charger.  Safety depends on how hot the battery and charger get which depends on the charge current.  A trickle charge may be safe … e.g. cordless phones often use a very low amp charge and the phone can sit there indefinitely without getting hot.
  4. How do you know if the NiMh battery is charged?  Answer: A good peak detection battery charger stops the charge and indicates full charge.  Otherwise, with simple brick charges and no charging circuit in the device, there are at least two methods, one requiring measuring the voltage of the charge, and the other is temperature.  Because we’re talking about practical household applications, temperature of the battery is simpler indicator.   As a NiMH battery approaches full charge it gets warm.  When it overcharges it will get hot.   Unless you know what you are doing (there are situations when overcharge is acceptable and you will know what those are), don’t let a NiMh battery charge to a temperature over 125 deg which will be uncomfortably warm in your hand.  If you do, well that’s how it goes and you got away with it.  After you take a NiMh battery off the charger, it may increase in temperature for a few minutes.  The main enemy of a battery is heat, but then, the main cause of heat  is just using a battery , so you really can’t win.
  5. How do you know when a battery needs to be replaced?  Answer: If you are not happy with the device performance because the battery will no longer hold a charge for a useful period of time, that’s it.

Charging AA, AAA NiMH Batteries

You can get high quality rechargeable AA and AAA batteries at most hardware stores and even Costco.  Duracell, EverReady for example carry decent rechargeable cells, often packaged with a plug in charger.  I’ve found these  work well.  Some pointers: look at the mAH rating.  If you could trust the numbers, the higher numbers, e.g. 2200 mAH for AA for example,  mean a higher capacity and a better battery.  Low numbers, for example less than 850 mAH generally mean an inferior product these days.  Therefore most manufactures tend to exaggerate the numbers. At the current time, numbers greater than 2700 mAH are suspect.  Some brands that tend to be better are in decreasing order,  Sanyo, GP, Panasonic,  SAFT – these three are original manufacturers.    Duracell and EverReady are resellers of products from these, and perhaps other manufacturers.  When it comes to the best manufacturers of NiMH technology, some excellent Chinese manufactures license American technology, and produce excellent cells.  Sanyo and Panasonic develop their own technologies and are usually among the best.  Even within a brand, there can be a wide range of quality.

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3 Responses to “Rechargeable Batteries and Charging FAQ”

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  2. Thank you for the valuable information, I have been searching google for the past 2 days looking for this. Bookmarking your site for future reference. Thanks again.

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  3. AA/AAA NiMH Chargers to look at:

    Advanced battery charger/analyzers –
    Maha C9000
    http://www.amazon.com/Maha-Powerex-MH-C9000-WizardOne-Charger-Analyzer/dp/B000NLUSLM

    General consumer charger, good quality
    Duracell Mobile Charger CEF23

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