Q: Asymmetric battery drain?

[rea5245]

Hi,

I have a battery powered circuit - it's basically a PIC and an XBee transmitter. It's powered by three AA batteries in series. I'm using Energizer Ultimate Lithium batteries.

The mystery is, the batteries don't drain evenly. I'll put in 3 new ones, all reading 1.6V. A couple of weeks later, two of them are still at 1.6V and the third is seriously dead - like 0.5V. So I throw it out and put in a new one at 1.6V. And a couple of weeks later, a different battery is dead. Again, the other two are at 1.6V.

How is this even possible? They're in series; how can one die so thoroughly while the others are at full power?

This circuit has been operating well for three years. Until now, batteries have lasted a year or so, and now every couple weeks, one dies. Obviously, something has changed - maybe the circuit isn't sleeping anymore. But regardless of that, how can just one of the three batteries drain at a time?

Thanks,
   Bob
 

[bji900]

Odd only thing that I can think of is a possible bad batch of cells that have some form of internal short? What happens if you set a set of cell aside and measure them after approximately the same amount of time.

[rea5245]

The fresh battery that I replaced the dead one with is the same age as the dead one; they're all from the same package.

- Bob

[Audioguru]

1.5V Lithium batteries are still "new" and not made by many battery manufacturers so maybe they are not all the same.

[janekm]

Any chance you got a fake batch of those batteries?

[BrianHG]

Hi,

I have a battery powered circuit - it's basically a PIC and an XBee transmitter. It's powered by three AA batteries in series. I'm using Energizer Ultimate Lithium batteries.

The mystery is, the batteries don't drain evenly. I'll put in 3 new ones, all reading 1.6V. A couple of weeks later, two of them are still at 1.6V and the third is seriously dead - like 0.5V. So I throw it out and put in a new one at 1.6V. And a couple of weeks later, a different battery is dead. Again, the other two are at 1.6V.

How is this even possible? They're in series; how can one die so thoroughly while the others are at full power?

This circuit has been operating well for three years. Until now, batteries have lasted a year or so, and now every couple weeks, one dies. Obviously, something has changed - maybe the circuit isn't sleeping anymore. But regardless of that, how can just one of the three batteries drain at a time?

Thanks,
   Bob

What is your average current consumption.  If you are drawing less than 25ma, don't even bother with lithium, you are throwing out your money.  Normal Energizer Alkalines will give you the same life span at low power consumption.  In fact, Alkaline actually lasts longer than lithium below 10ma loads with AAA sized batteries.

[rea5245]

The reason I'm using lithium is that the board is outside in the cold. I was lead to believe that lithium does better than alkaline in sub-freezing temperatures.

- Bob

[BrianHG]

Yes that's true.  Well now, if you are drawing 1ma average, well....

Also, sub-freezing temperatures may be why one of you lithium batteries are dying out of balance.  All it takes it a slight amount of additional humidity in the manufacturing of one battery from another, and the freezing may cause an internal ice crystal to form and expand damaging one of the batteries while the others would be immune.

Energizer Alkaline's are rated from -18c to +55c.
Energizer Lithium's are rated from -40c to 60c.

I do not believe that Energizer does a -40 degree C stress test on all the lithium batteries they sell, it was probably only done once in development.

[denelec]

Lithium cells will stay at around 1.6V until they're almost depleted and then the voltage drop precipitously.
So you can't easily verify their state of discharge with a voltmeter.
A slight difference in battery capacity can therefore result in huge voltage difference.

See datasheet:
http://data.energizer.com/pdfs/l91.pdf

Lithium cells are useful when the discharge current is high or the temperature is very low.
Their capacity in mAH is not hugely higher than alkaline at low current.
So at low drain, alkaline are more economical.