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Old Nov 13, 2004, 10:22 AM   #1
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Hello, I'm new to this forum but have read many posts and have learned a lot from the experts here. Great forum!

Now my question: What does everyone use for testing theirbatteries? I have an old Radio Shack tester that simply shows a needle pointing to a range from Replace to Good. It only seems to work on primary batteries. Any rechargeables always show as Replace, even fully-charged ones.

Can anyone recommenda good battery tester that will show the actual voltage or charge left ina battery? Ideally, it will work on any primary or rechargeable battery (NiCd and Nimh).

Thanks much!




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Old Nov 13, 2004, 11:48 AM   #2
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AA NiMH rechargeable batteries have an output voltage of 1.2 volts. AA Alkaline batteries have an output voltage of 1.5 volts.

So, a charger looking strictly at voltage would probably think that your fully charged rechargeables need charging.

I'm not sure that any of the battery testers would be very accurate (it's difficult to determine how much charge is left in a NiMH cell, which is one reason that the battery life remaining indicators don't work well on most models).

Thomas Distributing does carry some (but I don't know how well they work):

http://www.thomas-distributing.com/battery_testers.htm
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Old Nov 15, 2004, 7:18 AM   #3
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Due to their voltage characteristics it is difficult to test state of charge of Ni batteries without discharging them. There are some testers that draw some current for a short time and try to estimate the remaining capacity but they are not very reliable. With NiMH cells I top them off whenever I'm not sure if they're 100% charged.
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Old Nov 15, 2004, 9:01 AM   #4
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I checked that link to Thomas and those testers claim to test Nimh and primary, so in theory they should be smart enough to tell the difference between 1.2v and 1.5v? They look exactly like my old RadioShack one, the one with the inexact needle display, but maybe that's the best I can hope for without spending a lot of $$$. I'll drop by RadioShack again and see if they have special Nimh testers along withregular testers.
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Old Nov 16, 2004, 11:20 AM   #5
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ed777 wrote:
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I checked that link to Thomas and those testers claim to test Nimh and primary, so in theory they should be smart enough to tell the difference between 1.2v and 1.5v? They look exactly like my old RadioShack one, the one with the inexact needle display, but maybe that's the best I can hope for without spending a lot of $$$. I'll drop by RadioShack again and see if they have special Nimh testers along withregular testers.
If it is a simple voltmeter, the only thing it can tell you is if the battery is totally flat. If terminal voltage is below 1.2 V/cell then it's most probably 100% discharged. Everything else would be guessing.
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Old Nov 16, 2004, 1:47 PM   #6
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Well, Radio Shack was no help. They had a tester that seemed to test both alkalines and NiCads, but the salesgirl didn't seem to think it worked very well (how's that for asales technique).

blr, when you say "it's difficult to test without discharging them", isn't that true of all testers? Don't they all drain some power in order to determine the charge?

Maybe I'm just electrically-ignorant, but if rechargeables use 1.2v instead of 1.5v, couldn't they just make virtually the same tester that tested for 1.2 instead of 1.5?




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Old Nov 17, 2004, 11:44 AM   #7
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ed777 wrote:
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Maybe I'm just electrically-ignorant, but if rechargeables use 1.2v instead of 1.5v, couldn't they just make virtually the same tester that tested for 1.2 instead of 1.5?

The answer to your question is no. Primary cells (alkaline etc.) have nearly linear voltage versus state of charge curves. This means that the cell has close to 1.5V when fresh and its voltage gradually decreases during discharge. Thus the state of charge can be determined by simply measuring the terminal voltage. Rechargeable NiCd and NiMH cells are different. When freshly charged they mreasure 1.30-1.35V per cell. When you start discharging them the voltage initially drops quickly to 1.20-1.25V per cell and then remains almost constant until approx. 95% of the capacity is discharged. After that it drops quickly again. So, if you measure say 1.20V per cell the cell may be 90% charged, but it may be 10% charged as well. Even fully discharged cells can measure close to 1.2 V but as soon as you put a load on them the voltage drops. Hope this helps
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Old Nov 17, 2004, 1:19 PM   #8
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Thanks blr, that was very informative. I guess I'll forget about the tester. Unfortunately, my camera, a Canon A75 only has a low battery indicator. I would have preferred a constant indicator showing the strength of the battery in "bars". I know some cameras have them so there must be a way to test NimH's. Perhaps those indicators aren't very accurate?

While I was waiting for my Nimh's to charge, I put some old but charged-up NiCads in the A75 and they lasted about 15 shots. But by the time the low battery indicator finally came on, I couldn't even take one more shot. But that may just be the old NiCads.

Anywasy, the question still remains. Does anyone know of a good Nimh tester out there? That is, you've tried it and it works?


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Old Nov 18, 2004, 5:27 AM   #9
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To give an accurate voltage reading re state of charge you must load the battery with a suitable resistor & measure the voltage across that resistor with a digital mutimeter.This is a system that works well with a little experimentation.i.e.Take measurements of varying battery states:-Fresh off charger, discharged out of camera etc &you will quickly realize the relevancy of various states of charge(with accuracy).I have found general battery testers to be useless as their simple "good/fail" result is too lacking in finesse to give useful information.The following is copied from a post re multimeters that I wrote in another battery forum.

"If using a Digital Multimeter or Voltmeter you will need to load the batterys
with a suitable load(resistor) to give anything like an accurate reading.Simple experimentation will verify this,even an exhausted cell will show something in the order of 1.3 volts if measuring an open circuit value(ie straight off the terminals)Loaded it will show the true 1.1/1.0 etc value.You can do this by buying a 1.2 Ohm resistor,alligator clips,banana plugs and digital multimeter.Connect the resistor up to the battery and measure the voltage across the resistor with the meter.A reasonable meter will give values down to 1000th of a volt (.001Volts) You will see ranges from 1.330 Volts(fresh off the charger) to 1.100(no useful charge left)In fact you can use the setup to fully discharge a cell & visually monitor its progress to get an idea of voltage trends in relation to time if you wish.(It will confirm a typical NImH discharge curve, Stable from 1.250-1.100V approx,drops off the scale rapidly after 1.100V).
Radio Controlled model plane flyers,car racers etc have traditionally used Loaded Expanded Scale Voltmeters,a voltmeter scaled to show only the useful range(4.0-5.0 V in small increments{They use a 4 cell pack} )
All battery monitors of averge cost work the same way,they load a given battery with a given resistance and read the resultant voltage.A Digital Multimeter is superior in the degree of sensitivety of the reading.
By the way, at 1.2 Ohms its sucking approx 1000mA out of the cell at any given time.This is OK for rechargeables,because you can throw them back in the charger, but will chew up Alkalines pretty quickly so I'd advise against putting the Alk AA's in the test rig.If you think 1000mA is too much substitute a 2.2 or 3.3 Ohm resistor.
In case it's not clear, this rig is only to test one cell at a time(which is obviously what you want to do IMO) and only for rechargeables.I have made up a more permanent solution which consists of a Jiffy box,banana plug inputs,switch & attached AA battery carrier hard soldered together.
You plug the DMM in, put the Batt in the carrier, throw the switch & you're right to go.If you go with a 1.2 Ohm resistor it needs to be a 5Watt from memory(As distinct from a 1 Watt [Power=V'2/R] )"
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Old Nov 18, 2004, 7:25 AM   #10
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This is exactly how some testers on advanced chargers analysers work. I agree it's much more accurate than a simple voltmeter.
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