(resisting) Resistance is futile!
... or, how rechargable batteries fail silently.
I'm brushing my teeth and my toothbrush stops. No worries, pop the end off, shake out the AA batteries, pop a pair off the charger, and I'm back in business... or not: *click* *buzz buzz buzz buzz* -- the signs of a dead set of batteries.
WTF mate? The charger says the batteries are full at 1.43v (NiMH flavored batteries). My battery tester says the batteries are good. Previous deep-cycle runs with the charger show a capacity of 2000 mAh. These should work!!
I spit out the toothpaste and rinse out my mouth and the toothbrush. This is going to take awhile. Out come the batteries. I clean the immaculate contacts on the batteries and the toothbrush. Still no go. Swap out each cell, one at a time, to test for a bad cell... still no go.
Aha! But wait! The intermittent buzzing got stronger when one of the cells was swapped out. Useful information! Maybe the toothbrush draws a lot of current, and these batteries can't supply it, like old lead-acid car batteries when they fail. Some quick math suggests that the cells can withstand being shorted, one at a time, for a short time. Perfect! I can measure their maximum current using a DMM, almost like doing a load test on a car battery.
Let's find out....
Cell 1: 2.0A
Cell 2: 1.0A
Cell 3: 2.2A
Well, that's poopy. These things will provide less than 3 watts of power; probably closer to 2 when factoring in the voltage drop from loading. That sounds a little weak. Their internal resistance is a bit high.
Rummaging through my box of charged batteries, I grab a couple of different brands and repeat the test.
Cell 4: 8.7A
Cell 5: 6.3A
Oh yeah, that's more like it.
Cells 1,2,3 go in to the box for remote controllers and other low-drain applications. Their capacity is still good, but their internal resistance makes them useless for applications with high, or spiky load.
Cells 4 and 5 go into the toothbrush. Yay, clean teeth!
The rest of the batteries in my collection will get load tested as they come off the charger.
I'm brushing my teeth and my toothbrush stops. No worries, pop the end off, shake out the AA batteries, pop a pair off the charger, and I'm back in business... or not: *click* *buzz buzz buzz buzz* -- the signs of a dead set of batteries.
WTF mate? The charger says the batteries are full at 1.43v (NiMH flavored batteries). My battery tester says the batteries are good. Previous deep-cycle runs with the charger show a capacity of 2000 mAh. These should work!!
I spit out the toothpaste and rinse out my mouth and the toothbrush. This is going to take awhile. Out come the batteries. I clean the immaculate contacts on the batteries and the toothbrush. Still no go. Swap out each cell, one at a time, to test for a bad cell... still no go.
Aha! But wait! The intermittent buzzing got stronger when one of the cells was swapped out. Useful information! Maybe the toothbrush draws a lot of current, and these batteries can't supply it, like old lead-acid car batteries when they fail. Some quick math suggests that the cells can withstand being shorted, one at a time, for a short time. Perfect! I can measure their maximum current using a DMM, almost like doing a load test on a car battery.
Let's find out....
Cell 1: 2.0A
Cell 2: 1.0A
Cell 3: 2.2A
Well, that's poopy. These things will provide less than 3 watts of power; probably closer to 2 when factoring in the voltage drop from loading. That sounds a little weak. Their internal resistance is a bit high.
Rummaging through my box of charged batteries, I grab a couple of different brands and repeat the test.
Cell 4: 8.7A
Cell 5: 6.3A
Oh yeah, that's more like it.
Cells 1,2,3 go in to the box for remote controllers and other low-drain applications. Their capacity is still good, but their internal resistance makes them useless for applications with high, or spiky load.
Cells 4 and 5 go into the toothbrush. Yay, clean teeth!
The rest of the batteries in my collection will get load tested as they come off the charger.