Rechargable battery technology overview

I’d like to get into specifics, but for now I’ll go with an empirical study of the current popular rechargeable battery types. As I get more detailed information about the actual chemistry, I’ll include that.

Characteristics

NiMH batteries are viewed by many people as a replacement for NiCd. They have the same (exactly?) cell voltage as NiCds, are lighter (lower mass density), and can deliver more energy in the same volume (higher energy density). They also don’t exhibit the infamous NiCd memory effect. Seems pretty good all-around. They are also less toxic to dispose of in the environment (you recycle your batteries in accordance with the law, right?), but they cost more than the old familiar NiCds we had as a kid.

Modern Living claims that NiMH batteries last at least 30% longer than NiCds, and only take about 20% longer to charge.

There seems to be a new type of long-life NiCd cell that has close to twice the capacity of classic NiCds. I believe the comparison is 1100 mAh versus 650 mAh for a AA cell.

Cellular Advantage claims that

Motorola uses a foam type NiCd for its Portable and Personal cellular telephones because this chemistry does not exhibit memory effect and provides ample talk time at a very good value.

Interesting.

NiMH have one down side: They self-discharge quickly. That is, if you leave a NiMH pack sitting on a desk unused, it’ll lose its charge much more quickly than NiCds will. The rumored rate is 3-5% per day, compared to the NiCd loss rate of less than 1% per day. I’ve noticed that if I let a fully-charged NiMH battery sit unused for a week, it becomes useless to my GSM phone (until recharging, of course).

LiIon batteries do not seem to suffer this self-discharge problem. They are also lighter and more energy-dense than either NiCd or NiMH cells. They are also more expensive.

Lead-acid batteries, and in particular sealed lead-acid "Gel Cells" seem to be used only for vary large-capacity situations, such as in UPSes. (Lead-acid batteries are also used to start your car, of course.) I imagine this is largely because they are very heavy (think: lead), so no one wants to carry them in portable communications devices, and because NiMH batteries in such large-capacity configurations would be very expensive. And if you keep a trickle charge going on a sealed lead-acid battery, it just puts out a little heat, and it’s ready to perform as soon as you need it.

Tomas Distributing lists some useful NiMH facts that I have yet to distill here. Interestingly, they quote the same MAHA literature I quoted above. :)

Random notes about charging

I have seen combination NiCd/NiMH chargers. Presumably, this is because the cell chemistry is so similar. In fact, I have a MAHA brand NiMH replacement battery pack for my ham radio, which only knows about NiCds. MAHA claims that their battery pack "is compatible with your existing slow charging system." The pack does have an internal thermistor that cuts off charge current at 55 C. This implies to me that a NiCd charger may attempt to charge the pack too quickly and cause heat damage.

I had thought, though, that higher charging temperatures were fine for NiMH cells, such that they could be rapid-charged with impunity. I know my Nokia 2190’s NiMH battery became very hot during its 45-minute or so charge cycle. (Battery: 6V nominal, reportedly 550 mAh. Charger: 12V, 760 mA [one document claims 800mA as a typical charge current])

MAHA defines a trickle charger as one whose charge current is less than 1/10 of the battery capacity (they interchange mA and mAh for this calculation, ignoring the hour part). For rapid charging, MAHA requires "negative delta V at a sensitivity of 10-15 mV/cell", thermostatically-controlled cutoff, and peak voltage cutoff. They don’t specify what values should be used for the cutoffs.

MAHA warns against recharging the NiMH battery when fully charged, noting that rapid chargers need 3 to 7 minutes to initialize, during which time the charge will not be properly terminated, even if the battery is already fully charged. They also warn about charging the battery for longer than 24 hours. (Trickle charge time for this pack is just over 15 hours.)

Every source I’ve come across so far claims that trickle-charging a battery will provide greater energy storage than rapid-charging it. In the grand scheme of things, this fits.

I’ve also heard about the advantages of not filtering the AC too much in the rectification stage of your charger, since the ripple can actually improve the charging characteristics. According to Advanced Battery Systems of Massachusetts:

NiCd

Nickel-Cadmium rechargeable batteries are very durable and reliable. You may slow or fast charge most NiCd’s, but some manufacturers make different types of cells specifically for rapid charging or specifically for slow charging. NiCd battery performance is improved dramatically by interspersing discharge pulses between charge pulses. This is known as "burp" or "reverse load" charging. This method of charging allows the battery to more efficiently degass while charging.

NiCd batteries should not be left in a charger for more than 30 hours. Also, NiCd batteries should not be subjected to shallow discharge (i.e. using the battery for a short period of time, then recharging). This type of use may result in crystalline formation inside the battery which will diminish performance. This is known as the "memory effect".

NiMH

Nickel-Metal Hydride rechargeable batteries are the next level up from NiCd. They offer up to 40% more run time per volume than NiCd. They are also more environmentally friendly. The biggest advantage of NiMH over NiCd is their ability to accept a charge at any time without suffering from the "memory effect". The best way to charge NiMH batteries is either with the "burp" charging described above, or with a Delta V terminating charger. Before charging your NiMH battery, check with the charger manufacturer to make sure their charger can handle NIMH.

Li+

Lithium Ion is the latest technology in rechargeable batteries for portable equipment. They have the highest energy density among commercial batteries; twice that of NiCd. They also have a very low self-discharge rate.

Li-Ion batteries are the most expensive batteries available commercially. Disposal of lithium based batteries may cause some concern since any moisture which may creep into the cell after corrosion could present danger of explosion.

Advanced Battery Systems also gives a formula for calculating how long to charge a NiCd or NiMH battery. Divide the mAh rating of the battery by the charge current (in mA), and multiply by 1.4 (for trickle chargers) or 1.5 (for rapid chargers). This gives charge time in hours. The numbers this gives are in accordance with my equipment manufacturers’ claims. This formula seems to imply that around 30% of the energy that goes into a battery during charging is lost to heat. Not too bad, I suppose.

Cell voltages

Type Standard NiCd "Hi-Power" NiCd NiMH LiIon Lead-acid Lithium (not rechargeable) Alkaline (not rechargeable)
Cell voltage (V) 1.2? 1.2? 1.2? ? 2? ? 1.5
Typical capacity (mAh) 600-650 850-1100 1,250-1,500 2,100 2,800

More research is clearly indicated...

Questions left

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