I guess most of us use spotlights so I thought I'd expand on
John and Llew's remarks about batteries.
The sealed lead-acid batteries John mentioned buying do seem a good choice
for spotlighting. The amp-hour rating refers to the capacity of the
battery. Notionally it means a full charged 6.5 amp-hour battery can
supply 1 amp of current for 6.5 hours or 6.5 amps for 1 hour and so on.
John's 30 watt spottlight will consume roughly 30/12 = 2.5 amps.
Notionally this means John spotlight will run for roughly 6.5/2.5 =
2.6 hours. In practice, the capacity of a battery depends heavily
on the discharge rate, the temperature and the battery age.
The bottom line being John will probably get signicantly less than 2.6
hours running time, particularly if its cold and he discharges the battery
quickly (i.e he uses his spotlight continuously). I'd guess at Canberra
temperatures his battery would last maybe 1.5 hours with the spotlight
run continuously.
Use of battery chargers designed for car batteries is not recommended with
sealed lead acid batteries for 2 reasons. These chargers are likely to
exceed the maximum recommend charging rate for the battery and they are
likely to charge it to too high a voltage. Both can damage the battery
and in more extreme cases produce an explosion. The charger John has
bought will avoid these problems. You can also buy suitable chargers
which will run off car cigarette light sockets (Jaycar sell one for
about $30 I think).
I must confess to using a small car battery charger to recharge the sealed
lead acid battery I use for spotlighting. I've measured the current
and it doesn't exceed the manufacturers specs for the battery by much.
I try to remember to take the battery off the charger before its over
charged.
Completely discharging a lead-acid battery will reduce its life. Its
better not to discharge the battery below about 11.5V. I'd guess this is
about when you notice the spotlight becoming dimmer.
Lead-acid batteries have a longer life if kept charged. It also helps
if they are stored in a coolish place (< 25 degrees). They self-discharge
over period of several months (depending on temperature and battery age).
If you are not using your spotlight I presume its better to every few
months to recharge the battery.
The above comes from semi-authorative sources. I hope its accurate
and of interest.
Andrew Taylor
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