Observed battery set discharge unbalance

[JBA]

I previously posted the below as a footnote on one my posts on the Mechanical Engineering Forum and it was suggested I start this thread on the subject here.

"PS On a general note about battery replacements. On my field GPS I use rechargeable batteries and always test them before recharging; and, through this process I have found that consistently the battery with its + pole connected to the device in any series battery set will lose its charge while the remaining battery(s) will still be good. I have since found this to be true for both standard and rechargeable ++A and +++A batteries. In fact, switching the position of my two GPS batteries at the end of each period of usage extends the field life of the GPS between charges by balancing the amount of charge loss in both batteries. I have no idea why this is the case."

In order to be a bit more specific regarding my testing and the effect:
1. I have not tested for this effect on standard alkaline batteries because I stopped using them years ago because of their relatively short life relative to the alternate lithium batteries and the fact that I have found that unlike alkaline batteries that can leak and damage the equipment after an extended period and/or exposed to extreme heat, I have never experienced any leakage or loss of charge from the lithium version even after being left for years in my vehicle where they experience many days of temperatures at and above 120° in the south Texas area where I live.
2. I have found the same effect in series battery sets regardless of the the number of batteries in the set or the type of service in which they are used.
3. When tested the remaining batteries in the sets show no observable voltage decline after the one noted is fully discharged.

As noted, I have no idea why this effect exists and any relevant feedback will be appreciated

 

[berkeman]

One suggestion from the other thread was that there might be intermediate taps in the middle of the string of batteries. Do you have a way of looking for this? Can you post a picture of the battery compartment?

I have seen a device (can't remember what it was) that taps between series connected batteries.

 

[NTL2009]

Curious. I'd like to see a picture as well. If this effect truly exists, it doesn't make any sense from a purely electrical view, if there is no center tap, or other leakage path (dirt in the battery case?). I don't know if there is any other explanation.

 

[JBA]

My GPS has 2 side by side series AA batteries with a tab connector between the batteries and I have checked to see if there is any intermediate tap to that connector and none exists. Amount the other battery sets acting similarly are straight nose to tail battery sets that have no possibility of a center tap ranging from LED flashlights to digital clocks to a digital inside/outside thermometer base unit that has a straight train of 4 AAA batteries.

 

[NTL2009]

A carefully controlled, documented study is needed! :)

In my sample size of one (the only thing I could find off hand with multiple cells that were accessible), a weather station with two AA batteries, about 9 months into their usual 12 month life, they both measured 1.29 on my voltmeter. No discernible difference. Batteries were from the same package, so very likely the same age/batch/characteristics.

 

[JBA]

NTL2009: It would be informative to know specifically what type of batteries you are using in your weather station.

 

[NTL2009]

NTL2009: It would be informative to know specifically what type of batteries you are using in your weather station.

Those were regular alkaline AA. Not rechargeable.

Wasn't sure if you were saying you only saw this effect on rechargeable cells, or just never looked for it on alkaline, since you don't use them anymore.

 

[JBA]

That is interesting to know. I see the effect on Energizer Ultimate Lithium non-rechargable batteries. Since I no longer use alkaline batteries I have not tested any of that type.

 

[JBA]

I have found 2 AA alkaline batteries that are fully charged (test at 1.5v) and placed my GPS unit on test with those batteries installed. So we will see what I find with those.

 

[berkeman]

I have found 2 AA alkaline batteries that are fully charged (test at 1.5v) and placed my GPS unit on test with those batteries installed. So we will see what I find with those.

What do you mean by "have found"? In a drawer, or in the same package that you bought from the store?

 

[JBA]

They are a pair that were in a new device I received that I immediately replaced before first operating the device. This may not be a perfect case but considering the extreme difference between NTL2009 measurements and mine they should give some indication if there is a difference. It has some 10 years or more since I stopped using alkaline batteries so the chances of my having a purchased packaged pair is nil.

 

[JBA]

Follow up: My test is complete and I have the same results as NTL2009 with both batteries showing the same voltage loss. I left the GPS display backlight on to increase the drain rate but the short drawdown time is not unusual for this type of battery in my GPS. I will run an equal test with a pair of lithium batteries from a new package I have as a comparison test.

 

[berkeman]

Interesting thread.

Curious. I'd like to see a picture as well. If this effect truly exists, it doesn't make any sense from a purely electrical view, if there is no center tap, or other leakage path (dirt in the battery case?). I don't know if there is any other explanation.

Yeah, other than a center-tap in the battery string, the only other thing I could think of was some unusual mechanical strain on one battery versus the other (from its position in the battery compartment), but that seems unlikely.

EDIT/ADD -- Plus I haven't heard of any small mechanical strain issues with battery chemistry anyway.

 

[anorlunda]

If this effect really happens in many devices with many types of batteries, then I would expect it to be widely known.

Exchanging the batteries would be included in lists of "household tips"

We would have had several threads about this in the PF archives.

All that makes me skeptical. I'll have to do my own tests.

 

[Averagesupernova]

Unless I have missed it, temperature differences have not been mentioned. Is it possible one battery is running warmer than the other due to external effects? Is the physical placement causing one battery to be closer to heat than the other? We generally like to think our battery operated devices generate as little waste heat as possible but I thought I would mention it.

 

[Tom.G]

They could be from different manufacturing lines that got packaged together.
There could also be a charger problem where there is a difference between charge stations. Perhaps you could measure the individual cell voltages (and hopefully currents) while they are still in the charger. Take measurements before charging, at start of charge, at middle of expected charge time, at end of charge time. Then a voltage measurement after a few minutes under load.

There could also be a random variation between the batteries such that one of them doesn't accept a charge as well as the other. This could show up as a cumulative difference in their run times, and you just happen to always return them to the same position in the GPS.

Mark those batteries you use in the GPS so you can tell them apart.
Make a list of the four possible position combinations of the batteries (in GPS and in charger).
Run at least three charge/discharge cycles for each position, writing down as much measurement data as you can get.

As others have stated, there is no known theory for your observation of dependence on electrical position. Individual cell variation is commonly known and addressed in high-end chargers that do an "equalization charge." This equalization charging is essentially a many hour (day-long?) float charge. For details see: http://batteryuniversity.com/learn/article/bu_803a_cell_mismatch_balancing

Cheers,
Tom