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Battery Power design


A Closer Look at Nickel-Cadmium

A rule of thumb states that, when using nickel cadmium multicell packs, set cutoff voltage to around 1V per cell to reduce the risk of polarity reversal. But a closer inspection of the battery pack allows a more scientific determination of cutoff. The method provides lower cutoffs and still protects the battery from cell reversal.

Although it is convenient to think of multicell battery voltage in terms of mean voltage per cell, the voltage rarely distributes itself in such a simple manner. In multicell packs, the voltage on each cell is related to where the particular cell is on its discharge curve. Because discharge qualities may vary from cell to cell over the life of the pack, the voltage on each cell may differ.

A mathematical model of a battery pack determines the minimum voltage where reversal takes place. It assumes the that the voltage on the cell with least capacity drops off to 0V or possibly to a small negative value. The other cells are at or just above their knee. The mnimum battery voltage at the onset of reversal is calculated as

Vpr1 = (Vmp-100)(n-1)
where, Vpr1 = minimum battery voltage that prevents reversal,
Vmp = midpoint voltage for the individual cell,
and
n = number of cells in the battery pack.

The factor of 100mV is subtracted form Vmp because the point just prior to the voltage knee is typically around 100mV below midpoint.
An example illustrates the minimum battery voltage equation. A 5 cell NiCd battery has a nominal voltage of 5(1.2) or 6V. Circuits powered by this battery pack should be designed with a minimum cutoff voltage of
(1,200 - 100)(5 - 1) = 4.4V


Forget The Memory Effect

The infamous label "memory efffect" has haunted nickel cadmium (NiCd) cells since the mid 1960s. It was originated to describe a phenomenon observed in the power system of a U.S. satellite. The memory effect implies that a NiCd cell can forget how to deliver all of its charge if continually partially discharged to the same level. In other words, if a rechargeable 2-h battery runs 5 minutes each day, it will turn into a 5 minute battery.

Memory effect seems to be brought on by certain conditions:

First, the battery must be designed with cell cutoffs above 1V.

Second, it must be repetitively discharged to the same level.

Third, it must receive no overcharge.

If the the 3 conditions are satisfied, then there is a remote chance of permanent loss of some capacity.

Since the mid 1960s, memory effect has been the "cause" of almost every conceivable rechargeable-product malfunction. To avoid it, fearful users have vigilantly enforced complete discharge of the battery on each cycle. But in reality, memory effect has been officially diagnosed in only that one instance in outer space. The overuse of the term may be a case of confusing memory with a similar phenomenon called "voltage depression".

Voltage depression was somewhat of a problem with early NiCd cell technology of the late 1960s. It was common with improperly applied batteries made with primitive electrode technologies of that time. Modern electrode technology, however, has nearly eliminated the problem.

The ability to prevent voltage depression is in the hands of designers. Keep the cutoffs below 1V, and the effect will not be noticed. Users could help by varying the depth of discharge or runtime occasionally. But voltage depression is practically a nonissue because it is completely reversed after the first duty cycle. If it does occur, it shortens only the first run.