Powerhouse - Battery Technology for Film and Video Cameras.

Structure of NiCad batteries
Most NiCad batteries are cylindrical and use spiral-wound cathode and anode materials with a separator in between, although button cells are also available for small applications such as memory backup. In many cases the positive is formed and the negative is pasted or formed. All batteries consist of a positive and a negative plate, separator, alkaline electrolyte, metal case and sealing plate with or without self-resealing safety vent.

The positive is a porous foamed nickel plate. The negative plate is foamed or a punched plate of thin steel coated with cadmium active material (NiCad).

The separator is made of polyamide fiber or, for high temperature applications, a non-woven polypropylene fiber or glass mate. The positive, separator and negative are sandwiched together, wound into a coil and inserted in a metal case.

The electrolyte is an alkaline solution that is totally absorbed into the plate and separator. The metal case is constructed of nickel-plate steel, welded internally to the negative plate. It becomes the negative pole.

The positive is welded internally to the sealing plate, so that it becomes the positive pole. A safety vent permits the discharge of gas in the event of an abnormal increase of internal pressure. This prevents the possible danger of rapture or other damage. The vent uses a special alkaline and oxidation resistant rubber to assure the retention of its operating pressure and safety characteristics over a long period of time.

A NiCad cell, regardless of capacity has a nominal voltage of 1.2V. When fully charged it will have slightly higher voltage and it is considered to be fully discharged when it is down to 1.1V.

The capacity of NiCads is measured in Amp/hours (AH), the average current drawn times the time in hours. A NiCad cell of 1 AH capacity could supply 1 Amp of current for one hour. It could also supply 2 Amps of current for a half hour or 0.5 Amp of current for 2 hours. NiCads can be found with capacities ranging from 50maH to 10 AH in different size and shape packages.

NiCad batteries discharge differently than do alkaline or other non-rechargable types. When the cell first comes off charge it will usually show a fairly high voltage (1.4V possibly). This will drop off quickly as the cell is discharged until close to the nominal voltage of 1.2V. The voltage will then drop off slowly throughout the bulk of the discharge. However, once the cell is nearly fully discharged, it will drop off very quickly again. Refer to the figure illustrating the cell voltage over the discharge time for a typical NiCad cell.

NiCad batteries have one flaw in that they can develop a memory. If a NiCad battery is repeatedly fully charged and then used an amount that is less than full capacity (let's say you charge and regularly have three or four flights in a session), after a period of time, it may not be able to deliver any more than the capacity frequently used. This is called NiCad memory.

To avoid NiCad memory, it is important to cycle the batteries. Cycling is where the battery is fully discharged under controlled conditions, as described below, and then recharged. By fully discharging the pack in this way every so often, the cells in the pack will not develop memory and will remain at maximum possible capacity.

It is also a good idea to measure the capacity of your battery packs every so often. This will tell you how long you can safely operate your camera in one session.

It is possible to check the capacity of the battery while cycling. If you discharge your pack at a constant, known rate, and measure the pack voltage at various time intervals during discharge, you can determine the capacity by multiplying the discharge current rate by the time it takes to fully discharge. Your pack is considered discharged when it reaches a value of 1.1 volts per cell. For example, if you discharge a 10-cell pack, it would be fully discharged at 11 volts (10 cells x 1.1 volts/cell). Do not discharge a pack below this level or cell reversal could result.

The normal charge rate for NiCads is C/10 or the capacity of the NiCad divided by 10. For example, a 4 AH pack should be charged at 6/10 or 400ma. This is known as the overnight rate. Although, ideally, a pack should be fully charged in 10 hours, due to inefficiency, it will probably take between 12 and 14 hours.

After being charged overnight, the battery should either be removed from the charger or the charge rate should be reduced to C/100 (the capacity divided by 100). This is known as the trickle rate. The pack in our example would have a trickle rate of approximately 40ma. The battery may remain on the trickle rate indefinitely. Keeping your system's batteries on trickle charge is a great idea as it will ensure that your batteries are fully charged when you go to the field. NiCads, just sitting around will probably loose 1% of their charge each day.

Most NiCads may also be charged at a higher rate such as a rapid charge of C (charge rate equal to capacity) or a quick charge of 10C or ten times the capacity. This is normal practice with the packs used in powering electric models. Some cells are better at accepting a fast charge than others and these are usually denoted by being an "R" type cell or "SCR". When charging NiCads, however, one has to be very careful to ensure that they do not get overcharged. Applying these high charge currents to a battery that is fully charged can ruin the battery and even make the battery explode. For this reason, fast chargers are equipped with charge termination devices such as peak voltage sensors and or thermostats for temperature cut-off.

A second type of fast charger is the peak detection charger (Delta V). This charger can automatically charge your battery packs while its circuitry monitors the voltage of your pack during charge.
As NiCad charges, the voltage will increase at a slow rate. However, once the battery is fully charged, the voltage will actually drop back slightly. The circuitry detects this drop and reduces the charge rate to trickle. You can safely charge your batteries with this type of charger and there is no need for initially discharging them.

NiCad memory and overcharging:
Memory, does it exist? Yes it does , but it is very unlikely to occur in the applications which we use. It first reared its head when satellites took to the sky, NASA found that the batteries on board were not delivering the power expected. This is caused by the timing of the charge/discharge cycle being absolutely fixed by the sun on the solar panels which charge the batteries and a uniform load when there was no sun.

What appears to be a memory effect is in fact the result of repeated overcharging reducing capacity. For instance, if you charge a battery which is partially charged for 15hrs at 10% rating, then at some point the battery goes into overcharge.

Overcharging causes the cadmium hydroxide electrode (like the acid in your car battery) to lose contact with the negative plate of the cell eventually the cadmium hydroxide will convert to cadmium metal, once that occurs only hydrogen gas and heat will be produced at the negative plate with oxygen at the positive plate, these gases will then vent (all the NiCads we use are vented specifically for this problem), hence reducing capacity.

Treatment:
Heat is the main damaging factor of Nicad cells. Heat occurs as a result of passing a high current through the internal ohmic resistance of the battery. It is recommended that when using high rate charge, a cooling system, such as a fan be used to cool the battery during charging/discharging . A point worth adding is that you should be aware that near freezing temperatures will affect your battery's ability to deliver its full potential power.

Discharging:
Discharge cells to 1.0 V per cell (normal charged state approx 1.4 V/cell): A 10 cell pack, nominal = 12V, discharged 10V. Do not go below this level, the reason is that not all cells are created equal, as the discharge proceeds then the weakest cell will use up its active material, the current through the dead cell becomes a charging current except it is reversed, this is the big danger, computer matching of the cells performance when assembling a battery is an attempt to reduce this risk as well as enhance performance.

Battery life:
With good quality cells your batteries should last for around 1000 cycles.