Your batteries
are probably the most critical element in your entire digital camera equipment
kit. Choose the wrong batteries, and you can be left with a camera that's
no more than an expensive paperweight when that once-in-a-lifetime shot
appears. Changing batteries underwater is not possible. Your dive depends
on batteries that don’t quit when you need them the most.
Although there's no issue of which brand and type of batteries to buy,
just be sure to get an extra battery pack or extra set of back-up batteries
and keep them charged as a spare set. Most digital cameras use conventional
AA-size batteries, which opens Pandora's box of potential battery issues.
Some have proprietary lithium batteries, but you have to make sure you
have fresh extras with you. Your batteries have to handle the constant-power
loads that most digital cameras require. When batteries are used in a
camera (and their terminal voltage drops), the camera draws proportionately
more current from them. This is a bit harder on the AA-equipped batteries
for cameras. It's well established that standard alkaline batteries are
almost completely worthless and cost more to use than good rechargeable
batteries.
A big problem with batteries that are under a constant load is HEAT. Hot
batteries lose capacity and give off heat. This heat build-up in a closed
underwater housing can effect the camera’s operation. Digital cameras
do not like heat. Your instruction manual will tell you that optimum operating
temperature is 50F to 75F degrees. Operating your camera at temperatures
beyond these values can adversely affect your picture quality. Also, hot
batteries expand in size and may be difficult to remove from the camera
while they are warm. Batteries with larger capacities tend to expand more.
How are batteries rated?
Most of us are accustomed to seeing batteries rated in
milliampere-hours (mAh), (other than Alkaline that are rated in Volts)
as measure of how much current they can provide over time. A rating of
1600 mAh means that the battery should theoretically be able to supply
1600 milliamps (mA) for one hour, or 160 milliamps for ten hours, at an
ambient temperature of 65F., etc. The best NiMH AA cells today carry ratings
of 1400 to 2200 mAh and mAh is really only part of the story.
What you should be concerned about is how much total energy a battery
can deliver. Energy is measured in Watt-hours, the product of voltage
and current over time, or volts times amperes, measured over hours. (A
milliamp is 1/1000 of an ampere.)
Your digital camera requires constant voltage when it is turned on. The
internal flash will require additional voltage when it recharges after
the flash is used.
To measure total energy, it is necessary to measure the voltage and current
moment by moment throughout the battery's discharge, multiply the two
values together, and total up all the individual readings.
The total run time will be an approximate indicator of energy capacity.
Some batteries will run shorter periods of time but deliver more energy
than ones with longer run times. It is possible that the measured mAh
capacities of the batteries won’t correlate perfectly with total
energy capacity.
Watt-Hours are what really count.
There's a lot of advertising about mAh ratings, but even
the standard way of measuring mAh will give optimistic values when compared
to what the batteries actually deliver in typical camera usage. Just because
the battery has the big NiMH numbers on its side doesn’t mean the
battery performs at that rating. The problem is that digital cameras use
a lot of power, which translates into a lot of batteries. Batteries are
much less efficient when driving consistent heavy loads rather than light
ones. Manufacturer’s test and report their batteries' capacities
according to the industry standard and the results may have little to
do with how well the batteries perform in a real-world usage using digital
cameras and accessories.
The Importance of the Charger (!)
One of the most interesting things is that a charger can
make a huge difference in the charging capacity of your battery! The worst
chargers (in terms of completeness-of-charge) will produce "charged"
battery with only half the stored energy of ones charged with the best
chargers. No matter how good your battery is your charger is the most
important companion.
Trickle-charger versus rapid charger:
For topping-off and charge maintenance, a charger must be able to do both.
This combination is also the gentlest on the batteries. Rapid charging
heats up the battery and the charge is never completely full. Trickle
(slow, 8 hrs or more) charging avoids the heat problem and the battery
has a much better chance of getting a full charge.
Battery performance is very dependent on the charger used. Having the
best batteries in the world won't do you any good if you've got a lousy
charger.
Battery cost isn't terribly relevant for digital camera usage:
Spending $4-5 more for a set of batteries for your $800 camera makes sense
if it'll net you an extra 10% run time, time after time. One missed picture
would easily erase any benefit the cheaper batteries offer.
Some chargers charge batteries in series. This means if one battery has
a problem with charging due to high internal resistance all the rest of
the batteries will be charged to the rate of the lowest capacity battery.
It is good to avoid this kind of a charger. Get a parallel style charger
that independently charges each battery according to its needs and will
have an individual indicator light for each battery to let you know when
the charging is complete. Then you will know that each battery is correctly
charged and you will get the maximum use from them.