Keene Retail Hints & Tips Library

A quick guide to Nickel Metal Hydride (NiMH) batteries

NiMH batteries batteries are now available as alternatives to nicads for video power. Both types have their advantages and disadvantages.
NiMH batteries exhibit a relatively high rate of self-discharge. A battery fully charged on Friday but not used until Monday could have lost 10-15% of its charge. A nicad would normally take a month or more to self-discharge to the same degree. The self-discharge rate of the NiMH battery increases rapidly with temperature. At 40 degrees centigrade, loss of charge could be as much as 70% in a week.
NiMH batteries are more sensitive than nicads to the rate of discharge during use: the current drawn from a NiMH battery should not exceed 3C (3 times the battery's capacity in amp hours) amps to avoid damage. For example, using a 6V NiMH battery to power a 30W video light would result in a current drain on the battery of 5 amps (Watts divided by Volts = Amps). This wouldn't be good for a 1500mAh (1.5Ah) battery where 3C = 3 x 1.5 = 4.5 amps, but would be OK in the case of a 2000mAh battery where 3C = 6 amps.
NiMH batteries are less robust than nicads when it comes to charging, and caution should be exercised. A replacement charger or car charger which is OK for nicads may not be sensitive enough to detect the exact full-charge point of a NiMH battery, and damage due to overcharging could occur.
The big advantage of the NiMH battery is its high capacity/volume ratio. A NiMH battery packs about 1.5 times the capacity of a nicad into the same size casing. This probably isn't so important if you're only carrying one battery, but if you have to lug around several spares the saving on space (and weight) could be considerable.

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My nicad battery only lasts for 10 minutes...

It is a common temptation to recharge your batteries after each use, to keep them topped up. However, when this practice is followed only the first part of the battery's total capacity is ever used, and after a time the battery will 'remember' the point to which it is being part discharged.
Then, any attempt to use all the battery's capacity will result in the 'battery' indicator coming on when 'remembered' rather than full discharge is reached. A good part of the battery's capacity can no longer be used.

This 'memory' can be erased, and the full battery capacity made available once again by giving it a full deep discharge/recharge cycle (nb 'memory' cannot be erased by using the battery in the camcorder; it will not be discharged far enough).
The battery should NEVER be discharged to below the absolute minimum critical voltage of 0.9V per cell. Keene Electronics dischargers automatically shut off at 1V per cell, the optimum safe but effective level.

For the technically minded, 'memory effect' is caused when the nickel and cadmium undergo a chemical change, either because of prolonged overcharge at high current or through charged portions of the battery not being used. In both cases the nickel and cadmium change to crystallised forms, which clog up the cell plates rather like fur on a kettle element.
The total capacity of the cell is still available, but now at a much increased impedance, with the result that the terminal voltage falls more quickly when current is drawn and the change in impedance falsely trips the camcorder's 'end of battery' circuit.

Repeated discharging and slow charging removes the crystals and will in most cases return the battery to its original condition, but in the worst case the crystals can short out one or more cells in the pack. This is not easily reversed.

Keeping nicads ready for use over long periods

If you're a twitcher, storm spotter or similar, you are faced with the problem of keeping a camcorder nicad battery ready for immediate use even though it may not actually be used anything from days to months ahead.
If not to be used for some time, a nicad should ideally be stored in a fully discharged state, but then having to wait for 12-24 hours while it charges is hardly immediate!
The best solution is to fully charge the battery then give it a full discharge/slow charge cycle every 2-3 weeks, even if not used in between.
It is important to discharge the battery and then recharge it, rather than 'topping-up' which could introduce problems. A discharge/charge cycle does not put any undue strain on the battery, which will be kept in good condition and 'ready-to-go'.