The effects of cell deterioration on the available capacity are significant. If
the battery is charged for long periods, gassing takes place. The gases are
Hydrogen and Oxygen, derived from the water in the battery acid. Loss of
this water needs to be made up by topping up the cells if possible, or by
avoiding lengthy overcharges in sealed cells.
Other irretrievable effects include sulphation (encouraged by leaving the
battery flat for long periods), and deterioration of the cells' plates. If the
battery voltage falls below 10.7 Volts (for a nominally 12 Volt battery), and
charging is not started, sulphation of the plates can begin. The BM1/BM2
has an alarm which flashes the bell symbol when the voltage falls below
10.7 Volts. If the alarm is triggered, it is important to reduce the current
being drawn immediately, and if possible place the battery on charge, to
avoid permanent damage to the cells. If the alarm is ignored, the total
number of charge/discharge cycles which the battery will survive before it
loses a substantial fraction of its nominal capacity may be substantially
reduced.
All of these (and other effects) reduce the available charge after fully
charging the battery. If the effects are ignored, the BM1/BM2 will
incorrectly estimate that more capacity is available at any state of discharge
than is actually the case. If so, it is wise to alter the nominal capacity
stored in the unit to match the reality of the battery's condition.
NORMAL OPERATION
The NASA BM1/BM2 offers four normal modes of operation:
Volts and Amps;
Ampere-hours;
Time to go.
Starter Battery Voltage
In all these normal modes, the battery charge state is also displayed on the
scale on the right of the display. Also, if the battery voltage falls below the
preset alarm level representing dangerous discharge, the alarm bell symbol
is flashed.
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