Batteries for portable equipment

Choosing the right battery:

How long will it run for?

Alkaline batteries are advertised as "lasting up to ten times longer" - but don't buy them for your torch, as they are not well suited to high current drain applications. This graph shows the discharge characteristics of common AA size batteries when supplying a current of 2.0 Amps.  You can see that the Alkaline cell goes flat very quickly, (about 20 min) while the output voltage of the rechargeable NiMH cell stays above 0.8V for over 1 hour. The Lithium cell does even better - but costs more and is not rechargeable.

The capacity of a battery is given in Amp hours (Ah) - this is the length of time in hours that the cell will supply 1A.  Suppose our torch has a single AA cell with a capacity of 2.0Ah and a voltage of 1.2V.  That gives us total stored energy of 1.2 * 2.0 Wh or 2.4 Watt hours. We will only use about 80% of this i.e. 2Wh. So a 1W LED will run continuously for 2h on a single AA cell.

High power white LEDs require 4.3V and will stop working at about 3.2V.  They often use a driver IC that converts the 1.2V to 4.8V (X4) so when the battery voltage falls to 3.2 / 4 = 0.8V the torch goes out.  In any case you should never discharge rechargeable cells completely.

Batteries can be "primary" or "secondary"; primary cells have the energy built into them at manufacture, and cannot be recharged as secondary cells can.  Here is a brief comparison of some of the more common types.

Primary Cells: not rechargeable

There are three main types, the "heavy duty" Zinc Chloride batteries, the Alkaline cells such as Duracell Procell, and Lithium batteries.

Zinc Chloride : "Heavy duty" is a historic term based on comparison with earlier Zinc-Carbon cells, and not of any real significance now, as other modern types of AA cells store up to five times as much charge. They are OK in torches etc. for intermittent use, but don't last very long.  They can start to leak corrosive material to the detriment of your equipment!  Typical capacity said to be 400mAh.  AVOID
Alkaline or Manganese-Alkaline batteries give good performance in low-drain applications such as smoke alarms, but are not well suited to higher drain applications such as torches.  Despite manufacturers hype reviews claim  there is little difference (except in price) between different manufacturers AA alkaline cells. Duracell Ultra M3 nominal capacity is quoted as 2600mAh.
Lithium cells outperform alkaline AA batteries, do much better in demanding applications (e.g. digital cameras), and work over a greater range of temperature.  You need to check the voltage is correct as they come in a wide range of voltages, from 1.5V to 3.7V.  The Energiser Ultimate Lithium AA cell (pdf data sheet) has a voltage of 1.5V and stores 2700mAh ( about £1.30 each).   Lithium cells are used where long life, reliability and dependability are more important than cost.

Secondary cells:   These give much better service than primary cells in most high-drain applications

Some examples of modern batteries for torches

The new generation of "hybrid" NiMH cells are supplied precharged, have much longer "shelf life" than the older type, cost £1 - £2 each. Sold as GP ReCyCo, Uniross Hybrio, Varta "ready to use", Sanyo Eneloop, Vapex Instant.  review
This is a lithium battery, type 18650.   Its becoming very popular for torches since it has a voltage of 3.6 and capacity of 2400 mAH giving a total energy of  nearly 9Wh as compared with 3Wh for NiMH or Alkaline AA cells.  Don't worry about mixing it up with your AA's (14510) its bigger so it wont fit. Good ones are also also protected against over-charging and over-discharging which can be damaging to Li-Ion cells.  The naming of these cells is from their size 18mm X 65mm.  The last '0' confirms the shape is cylindrical.
As torches with higher output LEDs such as the SST-90 become available a higher output battery is needed. The 26650 cell (26mm dia * 65mm long) with a capacity of up to 5AH can deliver peak currents of about 10A safely. This cell is the newer LiMnNiCo or LiFePo which does not have the fire risk associated with some Li-Ion cells.
Another common size of cell, these CR123A size 3.6V Li batteries are available in both rechargeable (600mAh) and disposable (1300 mAh) forms (quite cheap in bulk on ebay!) and are also called 16340 i.e. 16mm * 34.0mm Some torches allow you to put two of these in place of one 18650; the higher voltage gives a brighter light, but note the reduced capacity means much shorter use time.

"C" and "D" cells

Most currently available "C" and "D" rechargeable cells are just carriers with the works of an AA cell inside.  I'm guessing (but it makes some sense) that the larger cells get hot during charging and in use, and that this heat cant be dissipated so effectively in a bigger cell.  Also of course the zinc cells they replace had no more capacity than current AA cells.
It is possible to get higher capacity cells , and these "D" from Ansmann cost £10 each at present (Jan 2009) while Ansmann 4500mAh "C" cells are £6 each.  GP are another good make that produce similar higher capacity cells. As an alternative, converters are now available cheaply (ebay) to let you fit two AA cells into a D cell carrier, giving up to nearly 6000mAh capacity.  Converters can also fit a single AA cell into a "C" carrier. Using these converters also means you don't need a special charger for the larger cells.  NOTE: If using these converters the batteries should be removed before charging - its not good practice to charge cells in parallel.

PP3 nine volt batteries

Bat detectors and lots of other portable equipment use PP3 9V type batteries.  I haven't been able to find rechargeable Lithium batteries in this category, but here is some information about currently available types.

PP3 / MN1604	NiMH rechargeable	Alkaline disposable	Lithium disposable
Voltage	8.4 or 9.6V	9.0	9.0
Capacity	170 - 250mAh	550mAh	1200mAh
Self-discharge	20-30% per month	1- 2% per month	negligible
Typical price	£3 (170mAh) - £8 (250mAh)	Duracell or Energizer from £2, other brands from £1	about £7
Notes	Hybrid NiMH technology is also coming in here making these a viable alternative to disposable batteries; however the lower (8.4V) voltage can be a problem giving lower volume and reduced operating times.  If its a problem try the 9.6V ones	Remember cheaper brands here (e.g. PIFCO)  can give almost as good performance for much less cost.	can last over 10 years in low-drain application - ideal for smoke detectors etc.  Fit and forget.

Do shop for batteries (and chargers) on the web as there are some very good deals around. 

Caring for your batteries

Forget the old rules about looking after NiCd batteries, they don't apply to NiMH batteries.  Most important things to remember for NiMH cells are:

DONT charge them in NiCD chargers - they cant tell when they are fully charged. However you can charge NiCd in most NiMH chargers.
DONT let them become completely flat - NiMH cells need reconditioning after a deep discharge, and may never fully recover. 
DONT store them when discharged and remember even if not used they will need recharging every 6 months or so.
DONT draw excessive current. NiMH batteries degrade rapidly if asked to supply current at more than half their capacity per hour (i.e. 1A for a 2Ah cell) 
 

Keep your batteries in sets (use an indelible marker to label them) and dont mix new with old or higher with lower capacities.
Store both Li-Ion rechargeables and NiMH batteries in a cool place (the fridge - not the freezer) to reduce self-discharge.

REFs: 1  2  FAQ1  FAQ2 

Chargers for NiMH cells

This "universal" charger (GP PB19) (£19): Can charge 2 or 4 AA/AAA/C/D and 1 or 2 9v PP3 size Ni-MH rechargeable batteries at up to 200mA and switches to trickle charging when they are fully charged (continuous trickle charging is not recommended for NiMH) NOTE: chargers that only let you charge in pairs cannot properly monitor each cell while charging, and will shorten their life..
This charger from GP provides 4 individual charging channels with LED icons to show status Termination methods: - Individual minus delta voltage (-dV) - Individual temperature sensor - Individual safety timer It also provides over-temperature protection and primary and damaged batteries detection Some chargers claim to charge batteries in 1/2 hr or less. Its better to charge NiMH cells at the 0.5C rate or below (i.e. 1.3A for a 2600mAh cell) so this will take a minimum of 2 hours to fully charge them.
The torch site sell a "1 Hour Super Quick LCD Fujicell AA & AAA Battery Charger SCH808F" for NiMH and NiCd cells that looks to meet all the requirements and has an LCD indicator to show the state of charge for each cell. It comes with mains, car and laptop adapters!

WARNING:

REF:  http://www.greenbatteries.com/bachfa.html   http://www.mpoweruk.com/chargers.htm

NEVER put a fully charged battery into a charger - good chargers look for changes in voltage or temperature to tell them the battery is fully charged, and putting batteries back to "top up" will almost invariably result in shortening their life. 

Charging a sealed lead-acid battery   (link opens in a new window)