When Your NiMH Battery Goes Bad

[+Renegade Knight]

My batteries are down to about 3 hours of life. This tells me one or more are bad in my group of 4 I use in my GPS.

 

Using a multimeter they all have the same voltate output so that's not how I'm going to be able to tell. When I put them in my Maha battery charger and tell it to discharge the batteries before recharging them one set dischages for a long time. That's got to be the good set.

 

So now I've got it narrowed down to two. But I still need to be able to figure it out. I'd rather toss the one bad one and use the rest than junk the pair. These things are not cheap yet.

[+EScout]

I would take the voltage after using them in your GPSr ( I am assuming your GPSr uses 4 AAs.) However, if they are the same age and one is bad, the others may not last much longer. They are fairly cheap now. Frys just had 10 AA 2000 MAh for $10 on sale recently. I have this brand and they work fine.

[+embra]

This reminds me of brain teasers I used to wrestle with back in the days when I liked to think...

 

First off, is there some injunction that one is supposed to treat batts as pairs? i don't know that (if it is so) one should throw out both of the pair if one goes bad, but I sorta remember something about not mixing up the batteries one uses on an ongoing basis arbitrarily. I dunno.

 

That being said, if you know (or are reasonably confident) that you've identified two good batteries and have narrowed the bad one to one of the other two, I would take one of the suspects and one of the known goods and discharge them as a pair. If they take a long time, then probably the other suspect is the culprit--which you could try to confirm by discharging it with the other known good. If the second pair discharge more quickly, that would boost my confidence in my detective work. If not...uh, back to the drawing board.

 

Disclaimer: that's an off-the-cuff idea and *not* an informed electrical engineer's opinion.

[+GeckoGeek]

I'd either measure the battery after a deep discharge or use embra's method.

[+JayFredMuggs]

A voltmeter isn't a good tool to use to determine if a battery is dead. A voltmeter will measure 1.2 volts even on a totally useless NiMH battery. The battery must be under load to measure the voltage drop across the battery. The internal resistance of the meter is so small that it produces almost no voltage drop across the internal resistance, leaving the full voltage drop across the battery terminals, therefore giving you a good reading. It would be better to put the battery in an external battery holder, connect the voltmeter as an amp meter, (current running through the meter) and turn on your GPSR. As soon as you turn on the GPSR, the meter should measure zero amps telling you the battery is dead. It will still measure voltage.

Ever stick your tongue on a 9 volt battery that you thought was dead? Try it sometime.

[+CacheCreatures]

A voltmeter isn't a good tool to use to determine if a battery is dead. A voltmeter will measure 1.2 volts even on a totally useless NiMH battery. The battery must be under load to measure the voltage drop across the battery. The internal resistance of the meter is so small that it produces almost no voltage drop across the internal resistance, leaving the full voltage drop across the battery terminals, therefore giving you a good reading. It would be better to put the battery in an external battery holder, connect the voltmeter as an amp meter, (current running through the meter) and turn on your GPSR. As soon as you turn on the GPSR, the meter should measure zero amps telling you the battery is dead. It will still measure voltage.

Ever stick your tongue on a 9 volt battery that you thought was dead? Try it sometime.

Ah hah! I was just wrestling with this yesterday. The things they don't teach you in school...

 

Thanks for the info.

[+geckoee]

"When Good Batteries go Bad" Home movie clips of batteries attacking their owners! Next on Fox!

 

... The internal resistance of the meter is so small that it produces almost no voltage drop across the internal resistance, leaving the full voltage drop across the battery terminals, therefore giving you a good reading. 

Just a little correction. The internal resistance of a volt meter is relatively large compared to that of the battery. This makes the internal resistance of the battery negligible. With an amp meter the internal resistance is small, making the internal resistance of the battery noticeable. The internal resistance of a battery increase as it gets old, so it able to supply less current as it ages.

 

But JayFredMuggs' method works.

 

You could also measure the voltage under load. You don't need any external stuff for that, and it is easier to hold together. Charge up all your batteries. Put one known good, and one suspect in the unit. Power up and record the voltage of the two (should over 2 volts, not 1.something) Switch to the same good battery, and the other suspect. And again with both goods, (just for a second base line) One of the readings should be significantly lower. That's your bad guy.

 

This internal resistance model of a dying battery can also be used to explain why you should not mix batteries of different types, or life. Basically the battery with lower internal resistance ends up charging the battery with a high internal resistance instead of powering what ever it is supposed to… but that’s another thread.

Edited March 20, 2004 by geckoee

[CoyoteRed]

Well, if you're able to tell you've got a bad battery in a pair by the method you've mentioned...

 

Mark each sequentially 1 through 4. Do your (dis)charge cycle. Note which batteries are part of the bad set. Take a battery from the good set and replace one in the bad set. Run them through a cycle. If it indicates one of the batteries is bad, then the battery that's in the charger and that was part of the orginal bad batch is the bad. If not, the other battery from the original batch is bad.

 

Make sense?

[+geckoee]

I was about the method embra, and CoyoteRed mentioned, but am afraid that the batteries may not be in parallel when they are discharging. And it may just stop discharging when both are done. Each discharging in its own time.

 

But it is worth a try, it's certainly simpler than trying to get the volt meter probes to stay in the right spot. Bad connections when measuring voltage can cause voltage discrepancies of 10-20 percent, possibly making the good batteries look bad. Poor connections during current measuring can cause voltage and current fluctuations to the GPS, which can cause the GPS to shut down, or at worse, reset it. Similar to when you start your car with the power cable attached, but more severe.

 

Deleted last paragraph.

Edited March 21, 2004 by geckoee

[+Corp Of Discovery]

Couldn't you just try them one by one in something that only takes one battery? The one that doesn't last as long as the others should be the bad one.

[+Stunod]

Throw them both in a fire. The one that explodes first is the good one.

[robertlipe]

While I could answer the EE part of this test, let me offer that if you place any value on your time, you can get 8 2100 mAH NIMH batteries for the cost of dinner and optimize this question away

[CoyoteRed]

I might be worth going to WalMart and picking up a Rayovac PS4 1 hour charger. That's what I use. It charges the batteries individually, 4 at a time. When one is fully charged only that one will shut down--the rest continue to charge. Mine came with a car adapter, too.

[+mochalatte]

Using a multimeter they all have the same voltate output...

Check your multimeter to see if it has a battery tester mode. If it is like mine, you will have to move the positive test wire to the ammeter connection.

 

A battery tester measures voltage that a battery can supply while delivering current. I got a cheap handheld multimeter at Home Depot a few years ago and 95% of the time I pull it out it is for checking batteries.