Battery Brieff

In response to power-hungry portable devices, battery suppliers are developing new systems with ever-increasing power ratings.  The choice of batteries is critical on any wireless device (such as smoke detectors, presence sensors and remote controls). Batteries are also used as back-up power for systems such as the security, fire alarms, external phone lines, some lighting and possibly, the main control unit. There are two types of batteries: disposables (also referred as "primary" or "non-rechargeable") and rechargeable (also referred as "secondary"). The main type of disposable battery is the one referred as "Alkaline" although this may change soon. Good alkaline batteries have an equivalent power rating around 800mAh(1) for an AA form. More powerful newly introduced Nickel Zinc (also referred as Oxy Nickel) batteries give more than double that (approx 1800mAh). And Lithium batteries will more than double that again (say, around the 4000mAh for same AA form). The main type of rechargeable batteries are the "Nickel Metal Hydride" or "NiMH" ones. These can give power ratings of up to 2300mAH for an AA form and can be recharged about 1000 times. The other common rechargeable battery (apart from the lead acid ones) is the Lithium Ion. This gives similar power ratings with the advantage of being 30% lighter.

Battery terms

Discharge/charge rates. These are current rates and are often defined not in Amperes but in terms of the number of hours to fully dicharge the battery in question.  So a discharge rate of "1C" equates to the current required to discharge the battery in 1 hour.  An example will make this clear, say you have a battery with a capacity rated at 1700mAh (milliampere-hour).  A rate of "1C" will mean a constant current of 1700mA (or 1.7 A) and will fully charge/discharge the battery in 1 hour; a rate of 0.2C will be a current of 340mA and will charge/discharge the battery in 5 hrs and;  a rate of 2C will be equal to a current of 3.4A and will discharge the battery in 30 minutes.

Battery power ratings are typically specified as power supplied from fully charged at 0.2C continuously until voltage drops below roughly 90% of nominal. Because this is the number that suppliers attempt to optimise, you should try to select batteries that discharge at 0.2C or less. For instance, if a device is rated at 150mA (per battery), you should install batteries rated at least 750mAh (150 x 5).

Discharge characteristics. The voltage supplied by the battery will vary over time with the charge held. You want a battery which shows a "flat discharge" - i.e. keeps the voltage for as long as possible.

Self discharge.  All rechargeable batteries will self-discharge over time (NiCd batteries are particularly bad).  Generally, it is recommended that you maintain your batteries connected to the charger when not in use.  This is referred sometimes as "trickle charge" (which by the way, is defined as 0.05C to 0.005C).

Operating temperature.  Temperature at which the battery can deliver power.  It depends on the chemicals used.  For instance, NiMH batteries operating temperature for instance is -20C to 60C. Military-grade batteries have greater operating temperatures.

Service life.  For rechargeable batteries, the time or number of cycles it takes a battery to drop its capacity to 80% of its original one (about 60% for lead-acid batteries).

Disposable batteries

Non-rechargeable batteries come in four main types:  alkaline, nickel-zinc, Lithium-Iron and Lithium cathode-type.  There also (older) zinc-carbon (also referred as "dry battery") and zinc-chloride batteries - which have been around since 1860s, but have been superseded in almost every way by alkaline batteries.

Alkaline. This is today the most widespread used battery, providing the longest life (about 6 times more than zinc-carbon and about 4 times more than zinc-chloride - for same size).  Other designs that fit the alkaline model including mercury oxide (which is being phased out), silver oxide and zinc air cells (mostly in development).

Silver Oxide.  Used in 'button-type' batteries uses silver oxide as positive, zinc for negative and potassium hydroxide as electrolyte.  It provides a very stable 1.55V voltage - at a price.

Nickel Zinc.  Just beginning shipping in 2003, it appear to have about double the capacity of the now classic alkaline.

Lithium (or Lithium-Iron.  it appears to provide 2-3 times the alkaline capacity but only under high current discharge capacity (digital cameras and motor-driven toys, for instance).  At constant low discharge rates they appear to offer no advantage.

Lithium Cathode. represent good performance but are relatively expensive and thus limited to military applications to date.  There are actually three different chemistries Lithium-Thionyl chloride, Lithium-Sulfur dioxide and Lithium-Manganese dioxide. They have very low self-discharge rates (1-3% pa) thus suitable to long term storage.

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Rechargeable Batteries

Rechargeable batteries come in three main varieties (so far, things are changing quickly here) Nickel metal-hydride (NiMH), Lithium Ion (Li Ion) and Nickel-Cadmiun (Ni Cd).  There are also Rechargeable Alkaline, and of course, the lead-acid batteries.

The most powerful to-date is NiMH. A top-end NiMH AA form battery can now deliver 2.3Ah at 440mA discharge (0.9Ah for AAA).  And can be recharged about 1000 times.  No memory effect.

Li-Ion provides a higher voltage per cell (3.7V vs 1.2V) and it is lighter.  It provides for 500+ charge-discharge cycles.

NiCd are falling into disuse due to the "memory effect" (although the use of new manufacturing techniques claims to have removed this) and low power (half of NiMH for same size).

Rechargeable Alkaline.  Relatively new development. Provides 1.5V (rather than 1.2V of NiMH or NiCd) and can be stored for longer (5 years versus 1 year).  But can only recharged 25-50 times (using a special charger).

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Battery Care

Some (obvious) precautions using batteries:

• Always store batteries in cool, dry place.  For rechargeable batteries, after a long storage (more than a year) it is recommended to cycle (charge/discharge) the batteries three times - leaving the battery charged.
• Do not short-circuit the batteries - or store them with metal pieces which may short-circuit them.
• If batteries are in the assembly, connect to charger during long periods (a month or more) of inactivity.
• Do not use different type of batteries in the same assembly.

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Uninterruptible Power Supply (UPS) considerations

Typically, you will need UPS (local or central) for main control unit, security system, file server (if installed) and the telephone system. You may also want to connect some lighting to the UPS system to act as emergency lighting. The specific requirements vary depending on your situation.

Battery UPS usually protects you for 6 hrs or 24 hrs. If you are subject to frequent black outs, you may need to install a power generator. This is the only way to get power on a medium to long term basis.

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Helpful Battery Sites

Yuasa Batteries

All Batteries

Powerstream Batteries

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