Torches, batteries and chargers

Torches

Watching bats can be a problem as they are dark coloured, very small, fly and change direction very quickly, and are seen against a background of dark trees water or dark sky. You need a light that is: directional, powerful, reliable, robust, lightweight, cheap, and runs for ages on AA batteries.  New torches using Light Emitting Diodes (LED's) are improving very rapidly but we still need to make some compromises.

Seoul Semiconductor have recently (Feb 08) brought to market the Z-Power LED P7 series a single LED package providing the worlds highest brightness of 900 lumens (90 lumens per watt) and lower power (1W) leds in development are now achieving up to 160 lumens per watt. (Jan 2009) (By comparison our "low energy" lightbulbs deliver 60 lumens per watt, and quartz-halogen headlight bulbs a puny 24 lumens per watt.  A lumen is the standard unit for measuring total light output, and a candle produces about 13 lumens)

"Led lenser" torches use a focussing lens that is matched to the shape of the led to give a beam that can be focussed from spot to flood!  Also look for cheaper torches using genuine LUXEON, CREE or other big-name LEDs at about 1 - 5 Watts, be careful there are fakes around. For finding your way in the woods in the dark any of the  cheaper single or multi-led torches now on the market are excellent.  Beware of torches that use AAA size batteries unless size is really important, as the capacity of these is small compared to AA's (see below)

Choosing the right battery: Some facts about batteries for portable equipment

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 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 10 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 - 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
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 by a factor of two or more, 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

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, Rayovac Hybrid, Sanyo Eneloop, Vapex Instant.

"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 too hot during charging and that this heat cant be dissipated 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 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) 
DONT charge them in NiCD chargers - they cant tell when they are fully charged.

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 or 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.

"Smart" chargers are to be preferred and here are some indications of specifications to look for. Microprocessor control -dV control (8 channels) Over-temperature sensing on each channel Reverse polarity protection and short circuit protection. Individual channel status display Dont be tempted to buy a "fast" charger, its better to charge NiMH cells at the 0.5C rate or below (i.e. 1.3A for a 2600mAh cell)

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)