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Q: When
is it OK to use rechargeables instead of ordinary disposable
cells?
A: The
main issue when using rechargeable cells is that the termal voltage
may be somewhat lower than that of the common zinc - manganese
dioxide disposable cell. NiCd and NiMH cells, in particular,
give about 1.2V during the major part of the discharge cycle,
whereas Zn-MnO2 cells give 1.5V. The question
therefore is, how well will the device to be powered perform
with a voltage reduction of 20%.
The old Sea & Sea Motormarine cameras, for example, ran on
3V (2x AA alkaline Cells), but 2.4V just wasn't enough. Most
flash units work fine, but the maximum guide number will be affected
if the circuit has no voltage regulation. You can tell if your
flash has voltage regulation by listening to it. In regulated
strobes, you can hear the whine of the inverter stop when the
target voltage is reached, then it will come on from time to
time to keep the capacitor topped up. In this case, the capacitor
voltage is independent of battery voltage, at least until the
battery is too flat for the voltage regulation to work. Unregulated
strobes whine constantly (like small children), and the light
output depends on the battery voltage. If you use a typical unregulated
strobe designed, to run on 6V, and run it on 5V, you should multiply
the stated guide number by about 0.8. This means effectively,
that you lose just under 1/2 stop; and while a TTL exposure control
system will correct for this, it means that you can't stop your
lens down quite so far.
Apart from the reduced terminal voltage, the
capacity of a NiCd cell is considerably smaller than that of
the same sized alkaline cell; and NiCds are easily damaged, especially
if you pay too much attention to popular myth. Many sources advise
that NiCds batteries should be completely discharged before recharging,
in order to avoid the memory effect. This advice is incorrect.
If you discharge a NiCd battery much below 1V per cell, and leave
it in that condition for any length of time, it may be ruined.
This mode of death is far more pernicious than the loss of capacity
caused by not discharging it far enough. If you store NiCds for
any length of time, you must take them out periodically (8 -
12 weeks) and recharge them, otherwise they will self discharge
below 1V, and when you come to use them you will find that they
have committed apoptosis (sorry, I just couldn't resist using
the word in this context). Fortunately, the fussy and environmentally
nasty NiCd cell is obsolescent.
NiMH (1.2V) cells are the preferred alternative
to NiCds. They have the advantages of capacity comparable to
that of a Zn-Manganese cell and RoHS compliance, and the memory effect is
reduced to the point that it can usually be ignored. The have
their own problems however, the main one being that they are
intolerant of over-charging, i.e., an intelligent electronic
charger is required. They also should not be discharged to less
than 1V per cell when series connected in battery packs, and
they will self-discharge when left unused for long periods. |
