I'm discharging a nickel metal hydride (NiMH) AA battery at 500 milliamps. After the first minute or so it shows 1.05 V. Between then and three hours later, the voltage steadily rises to 1.08 V. Is there something wrong with the battery analyzer, or can battery voltage increase very slightly as the battery discharges?
Or maybe I'm reading the analyzer voltage that produces the 500 milliamps through the battery? And that's the voltage required to produce the 500 milliamps? As expected, several other batteries show the voltage levels decreasing as they discharge, but this one is consistently unusual.
I'm using the Maha PowerEx MH-C9000 battery charger/analyzer.
Could be the analyzer. I've never had much luck with anything from Maha, especially their batteries. But it could depend too on what you might have been doing during testing. If you interrupted the discharge at any point, a battery will slightly recover some of its charge.
Think of any rechargeable battery (or any battery for that matter), as a bucket filled with some porous sediment. Now dig a hole out of the center to the bottom of the bucket. Fill the well that you dug in the center of that bucket with water. (The water in this analogy is the electricity you are putting into the battery.)
As you fill it with water it will top off (full voltage), but the water from the central well will leach into the surrounding porous sediment. Eventually you'll be able to add no more water as the sediment around the well becomes saturated. The "battery" is now fully charged.
In an analogous way, overfill it too fast or for too long and some of that porous sediment will be washed over the side, become too mushy to work properly and the well closes up or fills in with surrounding sediment, or it might even distort the bucket. I.e. overcharging/overheating for too long = bad, needed chemistry inside will boil off through the battery's vents, or destroy the laminations and/or chemistry inside.
Now comes the time to draw off some current from it. As you draw water out of that central well the main amount of water is still contained in all that porous sediment around it. Draw off the water too quickly and you can run the well dry, then the water slowly trickles in from the main reservoir around the well. In very low amperage batteries that well is very narrow and the sediment is very tightly packed so the water won't drain back-in to refresh that well as fast. You can only draw off a small amount at a time at a continuous rate (the available amperage).
If a battery was manufactured poorly and there are some contaminants in that sediment that cause it to be more clay-like, less porous, it will refresh that well even slower.
If you stop drawing off water from the well, the well will slowly refill to the water level in the surrounding sediment. This is why a battery will increase in voltage after you let it rest a bit after a discharge. If you ever get the "Change Batteries" warning and you have no more batteries with you but you MUST take a few more photos, wait a couple minutes. You'll be able to shoot off a few more pictures and then get the warning again. That central well in your AA buckets are slowly refilling from the porous sediment around it while you wait.
To confirm if its the charger/analyzer or the battery:
Try putting the "bad" battery into a different slot in the analyzer compared to a known good one in the same slot that the "bad" battery was in. If the known good one acts as the "bad" one then you know the analyzer is bad. Or it might be something as simple as finger-grease on a battery terminal or charger terminal. Get an ballpoint-ink eraser and clean all contacts with that. A ballpoint-ink eraser has a fine abrasive in it that acts as a very good electrical-contact cleaner. Just be sure to dust out all the bits of left-over eraser.
If the bad one still acts bad in a new slot then it was just a manufacturing error in chemistry or internal laminations in that battery alone. Shit happens.
If you had interrupted the testing at some point and it caused the peak in voltage reading, then see above for why this happens. If the contacts were dirty than just moving the charger/analyzer might have been enough to temporarily interrupt the current flow and cause the rise in voltage for a moment.
0.03v (3 hundredths of a volt) is not a lot to worry about too. For perspective, your own body will show anywhere from 0.1 to 0.5v of AC voltage in it at all times from surrounding radio transmissions and AC power-line sources in your home. Got a sensitive oscilloscope? Put it on a high sweep-rate and a 0.5v scale. Hold one probe in each hand and watch the way cool and complex Lissajoux patterns of AC voltage that are coursing through your body from all the radio-waves you are receiving. Move the probes to different areas of your body to change the patterns and voltage readings. The ear's lobes and folds seem to be a particularly changeable area for a wide variety of voltages and patterns. I suspect that the convoluted shapes of cartilage are acting as antenna-lengths for certain frequencies. (I also suspect that this is how acupuncture-fads seem to be able to pick up voltage fluctuations there so much. You can even do this with a simple digital multimeter set on 0.5 VAC scale. Hold one probe grounded in your hand and touch the other probe to parts of your ear. I've gotten some AC voltage readings as high as 0.9 VAC when testing this in the past. Much can depend on where I'm standing in the house.)
But I digress....
You might have dropped that battery at some point or it received a knock to its side and distorted the laminations inside a bit. It'll refill the well from its porous sediment a little differently now. There might be a slightly more porous section of its sediment that allows it to refill the well a little more rapidly when the water level gets that low. Might have been nothing more than a little unknown micron-thin contaminant on some layer when they were assembling the battery layers.
If the analyzer is applying only a light load, that is possible- the battery heats up a bit, and the light load voltage is temperature dependent. But 500 mA is not a light load, so that is surprising if it REALLY is discharging at that rate.
I've noticed that as well, but it's to be expected when the batteries are used too long between recharges. Cells in a matched set don't all have the same capacity. They're just placed in sets that have tighter tolerances, but one of them will *always* be have less capacity than the rest, and this 'weaker' cell will be the first to die.
Alkaline and NiMH batteries may have large differences in their measured voltage curves as they're used, but both really discharge fairly evenly. The big difference is that alkalines don't have voltages that decline precipitously when they're nearly exhausted. If you measure individual voltages of alkaline cells as they're used, there may be differences, but they're fairly close, and they remain usable at much lower voltages than rechargeables. If measured, you'll probably find voltages clustered around 1.1v, 0.9v,
0.7v and 0.5v. Using the last as an example, with 4 AA cells you might measure 0.53v, 0.50v, 0.48v and 0.47v under load. Cameras are different in that they tend to require slightly more than 1.0 or 1.1 volts per cell to keep operating. When cameras exhaust their AA alkaline cells, they're far from being really exhausted, will probably show more than 1.2 volts if measured without a load, and can continue to be used for hours, sometimes many dozens of hours in devices that don't require high loads, such as analog radios (at low or medium volume), LED lights, etc.
NiMH cells also discharge uniformly, but at the point were they're nearly exhausted (somewhere between 1.1v and 1.0v) the first to become completely exhausted will show a very rapid drop in voltage as it plunges from 1.0v to 0.0volts. The remaining 3 NiMH cells will still be pumping out more than 3.0 volts and you'd really want the device they're powering to shut down at this point, because if it doesn't, the depleted cell will start to become reverse charged, damaging or killing it. This is true even though the 3 remaining cells may have less than 1% of their capacity remaining. Most digital cameras require more than 3.0volts to operate, so they won't tend to kill NiMH cells, but if you don't remove them for recharging until the camera shuts down, one of the cells is pretty much guaranteed to appear near dead in a battery tester. The harm in doing this repeatedly depends on the individual camera, ie, at how low the voltage of the complete battery set has to go before it shuts down. At the point that digital cameras shut down when alkalines are used, they'll have a good deal of unused capacity, and can probably provide a little more than 1.1 or 1.2 volts under light loads to other devices for a long time.
It's possible that one or two cells may be loaded more heavily in certain slots, and if that's the case it might be to provide a lower voltage for clock or memory chips at very low currents. In this case the effect would be noticeable only if the devices are used very intermittently, where the batteries would last quite a while before needing to be replaced, depending on design anywhere from a couple of months to a year or more. This can be ruled out if you open the case and check the battery compartment to see if it has no more than two wires connecting the device to the battery pack.
I have an .mp3 player that uses a single AAA. I notice the charge indicator on the device starts of full, shows partial charge after usage and will return to show a full charge when next turned on. Never really gave the matter much thought. Dave Cohen
I was partly curious. After a charge/discharge/charge cycle, I switched the two batteries and started discharging them again at 500 milliamps, paying closer attention. The bad battery in the other slot rose from about .96 V slowly and steadily upwards. Later, after it reaches 1.15 V or whatever and is significantly discharged, it probably starts going back downwards (as I recall from the last time). Still, it charged to a higher capacity than the other same type battery. I just threw it away.
Batteries are not nearly consistent as I would have guessed.
Absolutely positively not. It's consistently discharging at 500 milliamps the entire time. After doing another "Break-In and Analysis" and again discharging but with the two batteries in opposite battery charger/analyzer slots, the weird battery starts at about .96 V and slowly rises, hours later to about 1.10 or 1.15 before it starts falling and finishes discharging.
I wonder what would happen if you put an alkaline in that slot, along with rechargables in other slots? The evil slot probably is powering something else as well.
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