I am in the UK. I have a battery tester from years ago which is still available. It may also be sold in the US.
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My multimeter shows that this battery tester puts a load of 500 mA on the 1.5v battery under test.
I have alkaline, NiCad, and NiMH batteries. I have AA and AAA.
(1) Almost all gets a steady reading of 'GOOD' in green. (2) No battery goes to 'REPLACE RECHARGE' in red. (3) One battery starts in GREEN, then slides into RED over 10 seconds.
Is this tester measuring: (a) the general "health" of the battery (b) the battery's current state of charge?
Presumably (a) could be done crudely by displaying current and (b) by displaying voltage? Is this correct?
Yes/no/maybe, you'd have to determine your thresholds for the particular cell technology and size per any rate and charge state, and health is a bad term to use at all.
It will be measuring the voltage even if indirectly but the load is too high, many cells have an internal impedance that will drop the voltage rapidly when trying to deliver 500mA, particularly alkalines far moreso than NiCad or better quality NiMH.
"Health" is not a very applicable term to a battery if only checking the voltage, rather whether it's discharged past the threshold or not. You might hook up a voltmeter in parallel with the tester to see at what threshold the lights turn from green to red.
Perhaps I should now ask, what is your goal, exactly? Analyze this tester? Understand batteries? Test what batteries you have and be done already?
Different battery chemistries can have different voltage curves during discharge. Unless your tester was expressly designed to test NiCad or NiMH, odds are it was mean for alkaline and older battery technologies which tend to drop further in voltage more rapidly, a steeper voltage:discharge curve.
On Thu, 6 Jul 2006 16:02:19 +1000, "Rod Speed" Gave us:
They place a fixed load, and test for voltage. The problem is that with the batteries the world has been using, the charge/discharge curve is a long straight line right up until it avalanches with a pretty quick slew to near zero volts.
I'll bet that repeat tests will have the cycle occur more quickly each time. That battery is likely near discharge.
The meter is reading the voltage through a fixed load, so I'd say it's the voltage that is dropping. When removed from the load, certain batteries "bounce back" a bit. This is only by voltage, and it will again fall under loaded tests, eventually yielding no further rebounds of significance.
To be certain.
Loaded testing has always puzzled me with batteries, as they are limited fill storage devices. Upon a loaded test, I find myself wanting to "top off" the battery again. Also, as you stated, performing such tests doesn't really reveal charge level. At the rates modern batteries are slow charged, the best solution to one's unknown battery condition is to "top 'em off". In other words, put them back in the charger and let its built in detection routine decide the battery's fill level. Those "watchdog" chips are pretty cool stuff.
Yep.
For charge rate, and or fill level, one would have to know the physical characteristics of the battery under test.
It's fully charged internal resistance, and it's internal resistance right at the discharge avalanche point, and its very nearly fully discharged internal resistance. With this knowledge, one can test a battery while it is being charged at a known fixed current limited rate. It can be determined by knowing the at rest fully charged cell voltage, and comparing it to the voltage required to get the battery to take charge at any given point during a charge cycle (not including fully charged of course).
With these chargers being current limited, what takes place is that the voltage is just above that required to pump electrons into the battery. Any more and the charge rate current would be exceeded.
So, a dead battery impresses a very small voltage, or emf when near dead. Hook up a smart charger, and it will raise its voltage until it just starts to pump current into the battery, it will bring it up to the current limit rate and slowly raise the voltage as the battery charges up keeping said current rate steady as she goes. At some point near the end of the charge cycle the chip is programmed to provide, the voltage of the battery will no longer continue to rise and the current will begin to fall off. The chip will sense this and change the charge indicator to green and discontinue charging operations. Usually, in chargers, there is a chip for each battery. I have even seen batteries themselves with them built in. Particularly when there is an array, or true "battery" of cells arranged, and designers want to insure that all cells get charged evenly and fully. Memory effect got shot in da head.
Er. No. They go pretty steadily from their nominal voltage down to 0.8V, when they are considered depleted. The voltages are even less under any sort of heavy load. NiMH and NiCd are almost entirely flat until just before depletion. Alkalines do have a flatter section in the middle of the graph, but it really isn't very flat at all and measuring voltage under a small load is a reliable method of determining remaining capacity.
There are some timevoltage graphs for alkaline here:
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And for NiMH here:
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They clearly show that NiMH are essentially flat after a brief initial period with a much more pronounced knee at depletion. This is particularly the case if you take into account the Alkaline graph going down from 1.5V to
0.8V, whilst the NiMH graph goes from 1.4V to 0.9V. The graph even goes vertical at about 1.1V.
On Thu, 6 Jul 2006 18:55:32 +1000, "Rod Speed" Gave us:
The activity doesn't puzzle me, the logic does.
Even a loaded test doesn't tell one a battery is full. It can only tell one something if one witnesses the battery in its death throws.
So to me, it is pointless at any point during a battery's life as the thing is LIMITED, why take more away from a fixed amount?
A simple voltage test, and knowledge of the battery tells a lot. If it is a 1.2 or a 1.5 volt battery, and you are reading that or even a bit more, you know the dang thing is charged.
I have seen NiCads in particular discharge with an incremental drop in voltage through the cycle. I can tell when it is charged too. I was using an HP lab supply with a 4.5 digit meter on it and very precise constant current or constant voltage modes.
Really all one needs is a good meter and a lab supply. Read the battery voltage. Set the supply to just over that, say a half a tenth of a volt to start. Set the current limit on the supply to the battery maker's spec. Place the battery on the supply, and read voltage with the handheld, reading current on the supply meter. If the power supply is set to the battery spec charged voltage, and there is no current flow, it is charged. Turn up the voltage to see the current rise up to the limit point, and read the supply voltage. It should not be much more than the correctly charged battery voltage is.
Remove battery from charger, and let it sit for a few minutes to let its internal thermals re-homogenize. Read battery voltage with meter. Make a report that has each battery's serial number and fully charged and settled battery voltage (we should really be saying "cell" Replace the word "battery" with "cell" throughout this post). Do that with all of your CELLS. Any time you want to know a CELL'S condition, read its voltage, and refer to the chart. The closer it is to that value, the closer to fully charged it is. The increment is very small.
One could also find out the rate the battery is, and discharge what would be half that, and take readings on each battery, and log the value. Then one could extrapolate charge level from voltage reading fairly well. They avalanche late in their duty cycle, but the line from the beginning to the avalanche point is a smooth, slow, small decline over a couple tenths of a volt. Meter needs to read hundredths at least.
Please excuse the intrusion, but I need to interject a related question.
Recently purchased a new radio scanner which uses AA batteries. Currently, I'm using the last of my 'Renewal' alkaline rechargeables. I was under the impression AA, AAA, C, and D are all 1.5 volts (when new), but only differ in the amount of availabe milliampere-hours. I just measured a freshly recharged set (4) of the AA's and infact they measure 6.02 volts on my DVM.
Since I will have to replace my alkalines eventually, I started checking the various rechargeable systems available. In my search, I noted that AA NiMH cells were labeled as 1.2 volts and had varied milliamp-hour ratings. As my scanner reports low-battery condition (based on voltage), I want to maximize my listening time between charges.
Are there any AA rechargeables that are rated at 1.5 volts?
No. It is down to cell chemistry and the nominal cell voltage is a slave to this.
However, do not fear, almost anything that runs well on 1.5V alkalines will run fine on 1.2V rechargeables. This is because anything using alkalines will already have to deal with the actual voltage of the alkalines, which goes down well below 1.0V before expiry of the battery. As a NiMH battery will maintain 1.2V to the bitter end, the equipment should handle this. If it can't, then it can't use alkalines, either!
However, any voltage based battery capacity monitoring will fail. Just ignore the battery life indicator.
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