Usually the biggest damage with deep discharging mismatched cells is that the
reverse charged (
low capacity ) cell voltage soon reaches a potential at which electrolysis
takes place. It
actually depends on the chemistry what this voltage is, so for lead acid
batteries for example it
is much higher than niCd say, although in electronic devices, deep discharge of
Pb does not
normally cause cell reversal because battery impedance becomes too high as
cell aproaches zero
volts. In reverse charge (in niCd,NiMh), unlike in forward charge mode, any
evolved gasses do not
recombine, so pressure rises and the cell vents through the normal pressure
relief valve (under
the cell +ve end) causing loss of electolyte. This causes further loss of
capacity every cycle as
the weak cell quickly gets weaker and hence more and more reverse charged each
cycle.
Deep discharges thrpough 0V also encourage dendritic shorts.
Interesringly in lead acid batteries it is actually possible to completely
reform the battery so
positive plate becomes negative etc, if you reverse charge with an external
source, but gas
recombination does now not work properly at end of the (reverse) charge. In
normal use with
electronic loads like flashes ,lead acid batteries cells are never reversed by
deep discharge,but
the weak cell is often not fully recharged each cycle and eventually the
battery cells become more
and more unbalanced reducing capacity to that of the weakest cell. This is
termed "cycle down". It
actually is related to two effects : the reduced charge efficiency of the more
deeply discharged
cell and the voltage of the higher capacity cells which reduces the voltage and
current to the
lower capacity cell, so the deeply discharged cell is never fully recharged
(battery rebalanced)
in a reasonable time.
The vendor's markings on the cell often bear little reality to actual capacity,
so even in cells
marked the same capacity, there is a good chance they will not be well matched,
unless from same
manufacturing lot. Reputable Manufacturers of battery packs, actually match
cell capacities and
use cells from same manufacturing lots. In the case of Pb batteries they form
the plates
simultaneously and go to a lot trouble to control manufacturing to produce
matched cells.
If your charger has a test mode you can match cells yourself, otherwise try not
to completely
discharge them!
Tim Hughes
>>
Most but not all battery types can be reverse charged, and
at the swing stage they can get very hot. Normally when this
happens the discharge capacity of the battery will be greatly
diminished unless they are correcty charged again but still
with a very high likelyhood that there life expectancy will
be greatly diminished.
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