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Application Note # C47

CoverMaster CM52&9 Batteries

The CoverMasters use Nickel-Cadmium rechargeable cells: they are of full-specification professional grade, not the reduced-performance domestic/consumer grade.

The Topics covered in this Application Note are:

Self-discharge

All NiCd cells exhibit self-discharge, typically at the rate of about 1% per day, and slightly temperature-dependent.
CM52 As a result, a CM52 battery may be expected to lose a third of its stored charge, even when not in use, over a period of a month.
CM9 The CM9 actually never switches completely off: when the [O] button is pressed, it goes into a "standby" mode with extremely low current consumption. The current taken when "off" will be temperature-dependent, but is unlikely to exceed the equivalent discharge rate of 2% per day.

As the combined result of these two effects, a CM9 battery may be expected to lose half of its stored charge, even when not in use, over a period of two to three weeks.

Therefore we recommend that if an instrument has lain unused for more than a couple of weeks, it should be given a full overnight charge immediately before being taken on site for a full day's use.

Low temperatures

Any type of battery is an electrochemical device, and all chemical reactions are temperature-dependent: as a result, the performance of the CoverMaster cells deteriorates (temporarily and recoverably) at low temperatures.
This usually shows itself as an apparent loss of available capacity and operating hours.

To avoid that effect, and any other undesirable electronic effects, we recommend that the instrument should not be stored at low temperatures prior to use. If low overnight temperatures are unavoidable, we recommend that the instrument be allowed to warm up to nearer room temperature before use; and on cold days, it is worth turning the instrument on half an hour before use (on the way to the site, for example) as its own internal dissipation may help to maintain the electronics at a more reasonable temperature.

Operating hours

This can be calculated as (battery capacity) divided by (current drawn).

The cells used have a generic specification of 1400mAh capacity. That figure could be expected to reduce to about 1200mAh towards the end of the battery's life; but we actually fit "above-spec" cells of at least 1600mAh up to 1800mAh capacity.
CM52 With sound off, the CM52 electronics draws about 85mA, which would give at least 14 hours operation in the worst case, with 17 to 20 hours being more typical.

When sounding, the CM52's current consumption increases by a significant amount, to between 100 and 170mA depending on its intensity; obviously the instrument will not be giving a maximum-intensity signal continuously (and even if it did, you would still get between 7 and 10 hours of screaming) -- in more practical circumstances, you would expect the figure to be 15 to 18 hours.

CM9 With both sound and backlight off, the CM9 electronics draws about 130mA, which would give at least 9 hours operation in the worst case, with 12 to 13 hours being more typical.
With the backlight on continuously, the current consumption increases to about 175mA, and the operating hours become about 7 worst-case and 9 to 10 hours typical.

When sounding, the current consumption increases by a larger amount, to between 170 and 300mA depending on its intensity; obviously the instrument will not be giving a maximum-intensity signal continuously (and even if it did, you would still get between 4 and 6 hours of screaming) -- in more usual circumstances, you would expect the figure to be 7 to 9 hours.

However you look at those figures, it boils down to "a full day's use (possibly two for the CM52)".

Recharging

The chargers supplied by Protovale are designed to recharge at the C/10 or "ten hour" rate; in fact, NiCd cells should be recharged by putting in to them 150% of their nominal capacity. We therefore recommend that, for a full charge, they should be charged for 16 hours, which can be conveniently done overnight.
If the battery was not completely discharged, then 16 hours would be more than necessary; but the cells used can be safely overcharged at the C/10 rate with no deleterious effects.

Memory effect

This is a phenomenon which manufacturers of rechargeable batteries insist is insignificant, but which rival manufacturers of dry batteries claim is serious.
If for example you draw out just one tenth of a cell's charge (discharge to 90%), then "top up" the cell with a short recharge, and repeat this process many times in succession, the cell can "forget" what its original capacity was, and so appear to have lost much of its capacity if/when you subsequently attempt a deeper discharge.

In our experience, this is not a serious problem; nevertheless, we recommend that users do not attempt to "top up" the batteries after only an hour or two of use, but wait until either at least four hours of use have accumulated or it is known that the following day a full day's use will be required, and then in either case give the instrument a full overnight charge.
If any form of "memory (amnesia?) effect" does occur, it can usually be cured by putting the batteries through a couple of cycles of deep discharge and full recharge.

Overall life

Under normal conditions of use, NiCd batteries can be expected to last for about 500 full charge/discharge cycles.
If they are mistreated, for example by exposure to extremes of temperature, or by repeated shallow charge/discharge cycles, or more seriously by allowing a battery of cells to discharge beyond their endpoint by forgetting to turn an instrument off after use, the battery may need to be replaced before the full 500-cycle lifetime has been reached; but conversely, well-maintained batteries may last for up to 1000 charge/discharge cycles.

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