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I bought a Stanley SL5W09L spotlight about four years ago and it has performed wonderfully ever since. Recently however it has stopped working.

I can plug it in, and the charge light will come on - and after a little while (not sure how long) the "Completed" light will come on. At that point I can turn on the light for a brief period while it is plugged in. It will not turn on while it is not plugged in.

Thinking it was a battery problem (they are a bit old at this point) I took it apart (probably not the best idea) to see what sort of batteries it contained and it has 6 NIMH Grepow AAA 800 mAh batteries. However, I tested it with my multimeter and it read out fine, specifically 7.8 volts.

Is it possible that this is a false reading of some sort, i.e. it can "burst" sufficient voltage, but can't sustain it, or are the batteries fine and I should look elsewhere for the problem?

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The batteries have increased in internal resistance due to aging. Since the multimeter does not draw significant current from the batteries, this internal resistance does not affect the reading significantly. The easiest way to validate this hypothesis this is to read the battery voltage while applying a load across the battery terminals, such as a resistor, so that a few tens of mA flow through the resistor. The multimeter reading should drop.

As batteries age, they tend to suffer a variety of deterioration modes, including pitting, oxide formation and scaling of the electrodes, as well as depletion of electrolyte. As the battery chemistry remains valid, the voltage across the cells will show more or less as expected. However, due to the deterioration mentioned, the ability to supply current reduces. This manifests as increased internal resistance, opposing the flow of current.

In brief: Those batteries cannot fulfill the current demand any more, though the voltage under no-load condition remains as expected.

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  • \$\begingroup\$ That was it. I replaced the battery pack and now it's working perfectly. \$\endgroup\$ Sep 3, 2013 at 13:10
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NiMH batteries do seem to have the classic memory effect problems like the old Nicads. If you don't discharge them far enough, the internal chemistry inside the battery will adjust to basically assume that its minimum charge is above what it should be. Thus, cumulatively, the charge capacity of the battery will decrease as time progresses. So voltage-wise, your batteries may have not gone bad, but they will not hold the maximum charge that they were originally rated for.

Also NiMH's have that rather annoying self-discharging problem (according to Wikipedia, 20% at full charge if left unplugged from anything for a day, then 4% for every subsequent day in that state). This effect could, and likely does, increase over time as the batteries age, meaning after 2 years of high stress applications, you could lose 60% over 24 idle hours, and maybe even 100% (my first set of camera batteries would barely last just sitting in the camera after a fresh charge. I'd charge them up, put them in the camera, turn it on the next morning, and get only about 5 seconds of use).

As for your voltage reading fine, most batteries are not linear with voltage vs. charge measurement. NiMH's could read their full 1.2V for 80% of the discharge cycle, then at 80% will suddenly discharge to nearly 0. And basically, it looks like your discharge cycle has been significantly reduced in length.

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  • \$\begingroup\$ Why was this downvoted? Just curious \$\endgroup\$ Aug 28, 2013 at 2:07

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