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Topic: How to load a battery / wall-wart for a voltage test? (Read 1 time) previous topic - next topic

Nick Gammon

This is what my battery-tester multimeter (claims) to do:



In principle, what would a good load be? Assuming, say, that you have a 1000 mAH battery, draining 200 mA would be 1/5 of its capacity, or should take 5 hours, anyway.

be80be

There to kinds of test one to see how long the battery last and two to see if the battery is good.

To see if a battery is good doesn't need a big load to see how long a battery last and is still good the user load would be a better choice.

Testing a  Nicad with a 6 ohm resistor 1/2 watt would be like short not last not very long because the Nicad can dump out amps when shorted needs a 3 watt resistor.

zoomkat

My cheap HF multimeter battery check has 4ma for a good 1.5v cell and 25ma for a good 9v battery. As to dummy loads, I use various types of light bulbs. 12v auto bulbs have lower resistance than the 120vac types. An old car headlight comes in handy for a bigger load. One might also get a space heater/hair dryer type device with heating coils to use for a variable load. Connect to the coils at varying distances apart with alagator clips to have an adjustable resistance.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

tack


Testing a  Nicad with a 6 ohm resistor 1/2 watt would be like short not last not very long because the Nicad can dump out amps when shorted needs a 3 watt resistor.

A battery 'short' isn't really a short. The current limiting under conditions where a nominally 0R is placed across the terminals will be governed by the internal resistance of the cells.

A 6R means you are limiting the current to a known value. The true value will be slightly less than you calculate as there will be the cell internal resistance in series with your load.

The battery can't 'dump amps' just based on it's chemistry. It can only obey Ohms law in providing current into a fixed load resistance, based on it's terminal voltage, state of charge and internal resistance.

If you short the terminals then different battery types will be able to deliver a different 'short circuit' current, although it's not really a short as any voltage would drive an infinite current into a zero Ohm load.

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