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Bravo November » Blog Archive

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Holy crap, I just realized when overhearing a conversation while walking between classes…

It’s 2007 and people are still believing the bull about battery ‘memory effect’!

Therefore, I am writing about batteries. Details within.

“Memory Effect” is something you used to hear a lot, back when all portable devices, for the most part, used nickel-cadmium (NiCD) battery packs and cells. Basically, the myth of memory effect was that if you used your device then recharged the battery before it completely died, the battery would suffer a ‘memory’ and would not recharge beyond that point in capacity.

At the time, portable electronics were getting cheap and plentiful, but really good battery charging systems were not. Most devices simply charged via an unregulated voltage and current being passed through a ballast resistor to set the charge current to roughly C [battery capacity] / 10. Charge a fully discharged battery at C/10 for 14 hours, and it’s fully charged again. This works GREAT if you can ensure that the battery starts out fully discharged…

One charging system I built from a kit, the Vellemann Universal Battery Charger kit, did do this properly. Upon connecting the battery, setting the voltage and current via jumpers, and pressing the start button, the charger would fully discharge the battery before recharging. This worked great as long as you knew the capacity in milliamp-hours of the battery and set it correctly. Otherwise, you would get an undercharge or overcharge.

Overcharge must have happened a lot on the old C/10 chargers. Let’s say you took 10% off the capacity of your old brick phone’s battery, then hooked it back up to the charger for ten hours overnight. Your battery will be quite overcharged and warm to the touch in the morning, and will have suffered some permanent damage and capacity loss from overcharging.

My theory is that ‘memory effect’ did not exist. A symptom of permanent capacity loss would occur in systems that were recharged without discharging, but this was due to overcharging damage, and nothing more!

Eventually, we got smarter chargers.

Some of the first attempts to detect a fully charged battery and stop the charger seemed to work acceptably well. A NiCD battery begins to warm up once it’s fully charged (and actually going into a very slight overcharge). A thermistor was taped to the side of one of the cells in the pack, and would detect the temperature change and stop the charge cycle. This was used on a lot of 2-way radio products such as the Motorola HT220.

The best method of detecting end of charge on any nickel based battery is via the negative delta-V response. When the battery finishes charging, its voltage falls by a bit. Detecting this allows you to catch the end of charge before the cell’s warmed up much, if at all. The only downside to this method is that fast charging the cell leads to the negative delta-V taking place too early, and causing the battery to undercharge. This is harmless, though annoying. Better chargers fast-charge to the first negative delta-V, then either apply a small top-up current for a set amount of time, or top-off at a lower current until negative delta-V is observed again.

Chargers do tend to add in features to prevent or reverse “memory effect”. These include negative pulse charging, a discharge before charge option, and a variety of different pulse charge methods. I do not believe any of these are worth paying extra for on a charger, but if a charger does have pulse charging or negative pulse, it also features microprocessor driven negative delta-V detection, and will not overcharge your batteries. I suppose that makes them all worth it.

Common flaws in many NiCD/NiMH loose cell battery chargers are lack of end of charge detection, and not monitoring the state of charge of each cell individually. These can both be avoided; chargers with no control or just a safety timer should be avoided, as should chargers that put all the batteries on one circuit. Examples of the latter include chargers with a switch to set whether you’ve loaded 2 or 4 batteries (they’re all charged in series). These kinds of chargers will charge your batteries until the one that’s closest to full or has the lowest capacity charges and shows a negative delta-v, then stop, leaving the others undercharged. Worse yet, some will severely overcharge the first one to finish, getting it blazing hot. Ow.
Most portable devices have gone over to lithium ion batteries, but digital cameras and other gadgets still take AA cells for the most part. Nickel metal hydride AA cells (NiMH) are a good choice as a power source. They last longer than alkaline cells would in a high-drain device, and are, of course, reusable.

The Rayovac IC3 “15 Minute” charging system and batteries work really well. I’d certainly recommend them to anyone. A couple of notes on them, however: First, the charger’s indicator lights and fan will turn off once the 15 minute charge cycle is done, but at this point, your batteries are only at about 75-80% due to the false negative delta-V and heating of the cells. Leave them in the charger for a couple hours afterwards, if possible; a top-up charge will take place if you do, and that could mean the difference between getting that great last photo and getting a “Replace the batteries now. Ha ha.” message. The second concerns the fan. It’s a sleeve bearing fan, and tends to wear out quickly on some chargers. To fix this problem, take a pair of wire cutters and cut away the louver over the center of the fan. The charger would be a pain in the ass to disassemble to get to the fan in a less destructive manner, requiring lots of desoldering. Once you have the hub of the fan exposed, peel up its label and apply a bit of oil to the bearing, then put the sticker back down.

NiCD, lithium ion, and NiMH batteries all suffer self-discharge. It’s slowest on Li-ion, takes about a month on NiCD, and takes only a couple weeks on NiMH. This means you’ll need to recharge the batteries regularly if you want to have them ready to use.

Lithium ion batteries are kind of a sad case. They have some of the best power to weight/size ratios available for a rechargeable power cell, but they permanently lose capacity due to aging. On a high drain device like a notebook computer, this is noticeable within the first year, or the first month if you use a Dell. XD

CDMA cellular phones, PDAs, compact cameras, wireless headsets, and other devices with a low constant power drain don’t run into so much of a problem here, but laptop/notebook computers, digital SLR cameras, and hard disk based media players get hit hard once the battery wears out a bit.

The lithium ion battery, as it ages, will develop a higher internal resistance. Effectively, this means the power’s in there, but can’t get out fast enough. This makes the battery’s output current fall below usable limits sooner than it did when it was new. This will also cause charging problems after some time.

The aging process seems to be faster on devices that are stored or charged in a hot environment. A cellular phone regularly left in the open in a car is bound to have major loss within a year or less.

A word of warning on lithium ion: Most lithium ion cells have a rather reactive and potentially explosive chemistry within. This has caused some fires and explosions on damaged batteries and charging systems. A lithium ion battery that has reached 0 volts should never be recharged; an electroplating reaction takes place in a totally discharged cell that will cause an internal short! If you used the device until it shut off, you did not discharge the battery to 0 volts; any device using lithium ion cells will turn itself off early to prevent damage.

Lithium ion is really picky. Overcharge the cell by even a very slight amount, and it’ll short itself out and die (perhaps even BY FIRE). Therefore, all Li-Ion chargers have intelligent circuitry to regulate the voltage and current. I believe NO lithium ion charger should ever be made without temperature sensing, as a severely overheated cell will go into a self-heating thermal runaway and — this is the official term — “vent with flame”.

NiCD and NiMH batteries can leak if they get really unhappy. The electrolyte is acidic, and can be neutralized and removed with baking soda and water. Wear gloves, and do not get the electrolyte in your eyes. NiCD batteries must be recycled when they get old to recover the cadmium, which is toxic and can accumulate in the environment and food chain.

Lithium ion batteries should also be recycled when they get old and decrepit.

Lead-acid batteries don’t have any memory effect. They should always be kept as close as possible to fully charged for longest life. They’re still with us, in cars and UPS units.

PbSO4Lead-acid batteries produce electrical power from a reversible electrochemical reaction in which lead is converted to lead sulfate. If a lead-acid battery is discharged below a 0% state of charge (this is 10.8 volts on a 6-cell “12 volt” battery, as found in cars and most UPS’s) and left there, the lead sulfate (also known as galena in crystalline form) can crystallize, permanently blocking the electrochemical reaction from occurring on the surface of the battery plates! In extreme cases, the formation of galena crystals can mechanically force the plates apart, causing that bulgy nasty failure seen on really bad UPS and car batteries.
Overcharge a lead-acid battery, and excessive hydrogen gas will be produced. This is, of course, explosive. In addition, the iron conductive grids that hold the soft lead and lead oxide plates together will corrode, leading to internal damage and reduced current handling.

Flooded cell battery rack, with VISIBLE cell constructionFloat charger. This unit gets obscenely hot due to high operating current and the use of a lossy linear regulator.Fortunately, lead-acid batteries aren’t that difficult to properly charge. The simplest method is just to provide a regulated ‘float’ voltage, at which the battery will slowly come to a full charge, then stop drawing current. Float charging can be left on indefinitely. This is the method used in large “online” type UPS systems and telecom power installations.

Some NiCD cells can also be maintained on a float charge. Always check the manufacturer’s recommendations; some do not recommend it, and others will give a specified voltage.

Other battery types do exist; nickel-zinc (NiZn) is under development and limited use as a new high capacity battery for electric vehicle use, and the same charging methods as NiCD/NiMH apply for it.

Lithium ion polymer is in use on some devices already, and uses a dry chemical electrolyte. No word on whether or not this fixes the explosion problem. *g* They’re used in the iPod Nano, and can be made really small. Recent developments have allowed for the manufacture of curved and flexible cells!
Sodium-sulfur (NaS) batteries use a sodium metal electrode system and liquefied sulfur. They charge up fast, have a good storage capacity, and great discharge characteristics. You’re not going to have one in your cell phone, though, as they require the sulfur to be liquefied by heat (300-350 degrees Celsius). NaS cells have to be initially preheated before they can be used, however, a combination of grouping cells for heat conservation and good thermal insulation allow NaS battery packs to be kept in operating temperature by normal charging and discharging cycles, with no extra energy needed to heat them. Typical applications use very large NaS arrays for grid energy storage and ‘peak shaving’, storing power during periods of low usage, and reapplying it to a power distribution system during peak demand. In case you’re wondering about the whole sodium vs. water thing, the cells are well sealed, and from what I’ve heard, a breached cell will have all the sodium covered in sulfur. (If one of these ever gets breached in a thunderstorm, I fully expect you, Internet, to put video of the event on Youtube.)

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