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, and cheap. Sorry, it can't be done! However new torches using LEDs are rapidly improving. Look for torches using genuine LUXEON LEDs at about 5 Watts, be careful there are fakes around. For finding your way in the woods in the dark any of the multi-led torches now on the market are excellent.
After searching for some time for a suitable rechargeable torch I finally decided to make my own. All my design choices were based on sound engineering principles - i.e. what could I get for nothing? (;-)
I wired a small car spotlamp through a switch to a lead-acid car battery. The result was ideal (except that I couldn't carry it). The lamp had a 55W halogen bulb. It took 4 Amps at 12 Volts. I wanted at least one hour and preferably two, so I needed a battery with a capacity of 6 Amp hours or thereabouts.
I bought a 12V 6Ah sealed lead-acid battery designed for a burglar alarm control panel. It weighs 2.4kg and cost about £12. Although it is a bit heavy the battery is quite small, and fits easily into a shoulder bag. The spotlamp is a separate unit, connected to the battery through a length of 6A mains flexible cable. A standard jack plug and socket allows me to change lamps and eliminates the need for a switch. The lamp is light enough to be swung to follow bat movements, and is very satisfactory in use, having a very bright narrow beam. Total cost about £15 as I already had the spotlamp. However 'probe' spotlamps are currently available from some car accessory shops for £3. I couldn't resist buying one and it is in some ways better than the better quality lamp as it gives a useful all-around illumination as well as a bright spot beam.
The battery needs charging and a conventional battery charger won't do. Maplin sell suitable chargers, but I built my own. Following enquiries I have attached the circuit diagram here.
Battery manufacturers recommend charging at a charge voltage of 13.5 - 13.8V for standby, and 14.4 v - 15v for cyclic (charging and discharging) - but if using the latter, which gives a faster charge, you must turn it off when fully charged. I use the 13.8V option, and leave my battery charging until I need it - although it's always fully charged after about 12H.
The charger uses a LM317 adjustable regulator. A 6 - 0 - 6V (or 12V) 6VA transformer feeds a bridge rectifier, and the signal is smoothed with a capacitor of 4700uF at any voltage above 16V. This gives about 17.5V to the regulator. The output voltage is set to 13.75V by the two resistors. Only 6 components, it can be built as a birds nest on the transformer, or as I have on a little bit of veroboard. The regulator doesn't need a heat sink.
All parts available from Maplin's.
I am now searching for a lower power halogen or xenon bulb (about 20W) that will fit the spotlight. This would give enough light, with up to four hours continuous use, or allow a smaller, lighter and cheaper battery to be used. Another idea is to use a motorcycle headlamp, which includes a sidelight bulb. This would give a very useful broad beam, switching to the main beam only when needed to pick out a bat.
I have bought a halogen desk lamp from IKEA. For £5 I got a 20W halogen bulb in a reflector, and a 12V 20W transformer - ideal for a charger! The lamp isn't very directional but gives a good even spread ideal for walking around a wood at night!
By the way, if you can afford them and want a good reliable light, you can buy one from a wide range produced by
Cluson Engineering, Unit 6 Bedford Road, Petersfield, Hampshire GU32 3LJ
or from Alana Ecology (bat links)