Magazine article American Cinematographer

Talking Technically

Magazine article American Cinematographer

Talking Technically

Article excerpt

THE VANISHING BATTERY CHARGE

Why is it that all of a sudden a nicad battery which has given perfectly good service, is not old, has good connections, and has never given cause for complaint, will fail to deliver the goods?

It happens, we all know it happens, it happens to all of us, but why?

There are two factors that a cameraman or camera assistant responsible for keeping the batteries in good condition should bear in mind.

The first is that when nicad batteries are stored in a warm room they self discharge. Keep them cold enough and they will hold a charge, almost forever. The remedy to this one is easy: "keep it cool".

The other is regular exercise and this one is more difficult to persuade users to put into practice.

Nicad batteries are a bit like pet dogs-and many human beings we all know-if they're not exercised thoroughly and regularly, they become lazy and incapable of giving of their full potential when necessity demands.

If the charge of a nicad battery is frequently topped up without the battery being deeply discharged it will develop a taste for the lush life and the next guy who gets hold of it may have a problem. It is important, therefore, to use a battery regularly and to its full capacity if it is not to fail. Nicad cells, which have a nominal capacity of 1.2 volts each, need to be discharged regularly down to at least 1 volt per cell to keep them electrically fit. This means that a 12-volt battery, which is made up of 10 x 1.2 V cells, should regularly be run down to 10 volts to maintain efficiency and reliability.

The best thing to do with a suspect battery is to discharge and re-charge it several times in quick succession until it is right. It's just like punishing a naughty child.

To help avoid these problems we ask our customers not to recharge fully charged batteries too often. Give them some exercise first!

One of the problems with nicad cells of course is that you can't test them as you can the lead acid battery in your car. For a start, they don't have a liquid that you can measure the specific gravity of, (and in any case they are sealed). Neither does checking the voltage with a meter help because it will read the full nominal voltage per cell until they fail, at which time they go down with a whomp. …

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