4x AA NiMH in series discharge @ different rates. Why?

I've got a motorolla walkie talkie that's powered by 4xAA's in series. They go in a magazine (holder). I am having a problem destroying my expensive new eneloops from over discharge, I test each battery when it comes out (the walkie has a low battery indicator but sometimes I'll be out and will have to use it till it dies) 3 of the batteries will be around 1.2v but the 4th will be .8. Why is this happening? I know I shouldn't let them drop below 1.0 so it can't be good, I make sure all the batteries are matched, not only by charge voltage but by individual pack they come in (they're all numbered when I get them)

how can I prevent this?

edit: I just want to point out this isn't a little GMR/FRS walkie talkie, I'm licensed, its a professional level radio.

With all 4 batteries in series, in theory they all should discharge pretty evenly. I don't think 0.8v is going to damage the battery if it just happens once in a while. On a couple of occasions, I've had a battery so low that the charger wouldn't even start charging it and I had to manually put some charge into it with a parallel battery until it would be recognized by the charger and yet it still works fine. Is it possible that this one cell wasn't quite charged up all the way compared to the other 3? Note the number on this battery and see if it's the same one that is always undercharged each time. Perhaps its a little weaker than the others.

I have heard that you only damage eneloops by reverse charging them as there have been people that have purposely discharged them to 0v alone. It is possible that you could have cells vary a lot but discharging them to below 1.2v will have most cells depleted and from there even a 50ma variance in capacity of cells will have cells caving in on voltage harder than others

Get could yourself a Maha C9000 charger and do a break-in test, you will then see which battery is the weakest, then just replace it.

John.

It probably doesn't have the same capacity as the other 3 cells so it ends up at a lower voltage when discharged by the same amount. You need a charger that can measure / record the amount of energy / capacity put in the cell or a battery discharge meter. Once you can characterize your batteries you need to find 4 cells with the same capacity if you want them all to end with the same voltage.

Just echoing some of the other comments really.

Possibilities.......

You say they are "expensive new eneloops - are they all from the same batch? All with the same model number?
(There are several types of eneloops still around)

What charger are you using? It is possible that one charging slot is not functioning correctly. A MAHA C9000 will tell you exactly what is going on, but it is an expensive longer term investment. Stay away from the chargers that require two or four cells to function and get one with individual channels. (That rules out some of the standard eneloop chargers.) These 2/4 chargers will likely make the problem worse by charging a weaker cell in series with a stronger one.

Mark/number your eneloops to see if it is the same cell each time. Or the cell from one particular slot on the charger.

A really "clutching at straws" chance here but depending on the radio, is it possible that one cell is getting warmer than the others when in use due to proximity to other electronics in the unit?

The eneloops are pretty robust and I'm not sure you are actually "destroying" them unless you are forcing the cell into reversal. If you really do get caught out and have to use the radio until "it dies" how about you consider carrying a spare set of cells so you can change them as soon as you get the low battery indicator. 4*AA cells in a small plastic box is not that hard to put in a pocket or pack somewhere.

Cereal_Killer said:

I test each battery when it comes out (the walkie has a low battery indicator but sometimes I'll be out and will have to use it till it dies) 3 of the batteries will be around 1.2v but the 4th will be .8. Why is this happening?

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In fact, it must happen this way. It would be a miracle if all four batteries were at 1.2 V when you tested them.

The explanation is in the discharge curve of an NiMH battery. See the illustration below showing the discharge curve of an AA eneloop. On this chart two batteries are represented with slightly different actual capacities (this is to be expected, since all batteries have slight variations between samples).

You can see that when one battery is discharged right down to 0.9 V, the other battery is still up around 1.2 V, even though the difference in actual capacity is not that great.

When you measure the batteries after removing them the apparent difference will appear even greater, since discharged batteries tend to recover to 1.2 V by themselves once you remove the load. Only the battery that got right down below 0.8 V will retain a low voltage.

Wow some great info here, you guys are so far above my head.

My charger is an original nitecore i4 (as in NOT the v2, I'm getting a BC-900 for Father's Day, I choose it OVER the maha c9000 for the individual displays and ease of use (ie programming all 4 cells at once vs having to program it for each cell))

As of now I can't really know for sure what each battery is doing. Like I mentioned each battery is labeled by batch and individually and I have noticed some batches don't have a problem and the radio can die yet all battery voltages will be equal (radio shuts down when all battery's read ~1.21v) however I notice some specific batteries that'll be down under 1v sometimes but not others. I am using a spreadsheat to record the voltage of each battery as well as its position in the magazine. All batches seem to last right about 24hours of use however sometimes 24h will be straight where as other times it'll take me 3 days to have it on 24 hours.

As I record more data and get my new charger (wife said I can wait till Father's Day, gotta wait for my preon penlight too) ill continue to update this tread and see if we can't solve this. Sounds like first thing I'll be doing is refreshing all my batteries.

Thanks for the help /advice / answers so far.

I have another question-the radio has a 12v in and is capable of charging the batteries (in series) when plugged in. Is that safe to charge 4x NiMh AA's in series like that? I have no idea what sort of charging technique/ algorithm it uses, is that just overall a bad idea?

Cereal_Killer said:

Wow some great info here, you guys are so far above my head.

My charger is an original nitecore i4 (as in NOT the v2, I'm getting a BC-900 for Father's Day, I choose it OVER the maha c9000 for the individual displays and ease of use (ie programming all 4 cells at once vs having to program it for each cell)).........

I have another question-the radio has a 12v in and is capable of charging the batteries (in series) when plugged in. Is that safe to charge 4x NiMh AA's in series like that? I have no idea what sort of charging technique/ algorithm it uses, is that just overall a bad idea?

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Depends on your definition of "safe".

With NiMH it is certainly "safe" to charge in series - as in - they won't blow up (I think the folks here prefer the term "vent with flame" )

However, it is generally not best practice for your cells because it can gradually exacerbate the problem of cell imbalances. I think if you have a better option, like your charger with individual channels, then you should use that instead. Consider the inbuilt charging as a convenience for someone with no better alternative.

i would suggest to mark that battery and change its place in the pack, this way you will see if it is simply defective cell, or one cell in the pack, in certain position (first , last, second...) discharges below the rest.

Cereal_Killer said:

I'm getting a BC-900 for Father's Day, I choose it OVER the maha c9000 for the individual displays and ease of use (ie programming all 4 cells at once vs having to program it for each cell)

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Oh no, bad move (IMHO). The C9000 is able to operate successfully over the whole range of settings and charge currents up to 2000 mA charge rate x 4 batteries. I do not believe the BC-900 is as well engineered nor as predictable or reliable. I believe if you try to charge four batteries at 1000 mA on the BC-900 the batteries and charger will overheat and may suspend charging until it cools down again. Not only that, but LaCrosse has very little interest in supporting the charger if you have any problems with it. I know, I sound like a C9000 fanboy, but it is a far more solid and reliable charger and I actually find it much easier to operate the controls than the IQ-328 I purchased (similar to the BC900).

I was going to start a new topic on this, but the topic is very much along the lines of the question I was interested in.

I have a repaired Casio 9750GII calculator which is powered currently by 4 x AAA eneloop (generation 3). These came from the same packet and are fairly close in value (797,795,804,796).

However I have tested this calculator on 3 discharged AAA eneloop (last battery holder shorted across the terminals to simulate 3 batteries) and it will still power on. Casio should be complemented on their efficient power regulation, but after learning about reverse charging due to imbalanced cells, this seems to be likely a problem especially with such a low power drain on the batteries. It seems inevitable that in this case the 804mAh will reverse charge one perhaps even two of the others before the calculator shuts down due to low power.

Due to alkaline batteries leaking because of reverse charging, do these devices have protection circuits that detect discharged impedance values of the series batteries and shut off the circuit when they become too high (hence why certain devices are NiMH compatible and others are only Alkaline(lower internal resistances for NiMH are not detected correctly for Alkaline only circuits))?

Should I be inserting a standard power/zener diode after the series batteries to create a voltage drop, to force an earlier cut-off to protect my poor little batteries in this alkaline only recommended calculator ?

And I am sorry for semi-hijacking the thread with my numerous questions. I do realize that in real world terms the damage is likely to be minute (especially as I will not be charging these batteries often in this case), I am just pedantic :thinking:

Baqar79 said:

I was going to start a new topic on this, but the topic is very much along the lines of the question I was interested in.

I have a repaired Casio 9750GII calculator which is powered currently by 4 x AAA eneloop (generation 3). These came from the same packet and are fairly close in value (797,795,804,796).

However I have tested this calculator on 3 discharged AAA eneloop (last battery holder shorted across the terminals to simulate 3 batteries) and it will still power on. Casio should be complemented on their efficient power regulation, but after learning about reverse charging due to imbalanced cells, this seems to be likely a problem especially with such a low power drain on the batteries. It seems inevitable that in this case the 804mAh will reverse charge one perhaps even two of the others before the calculator shuts down due to low power.

Due to alkaline batteries leaking because of reverse charging, do these devices have protection circuits that detect discharged impedance values of the series batteries and shut off the circuit when they become too high (hence why certain devices are NiMH compatible and others are only Alkaline(lower internal resistances for NiMH are not detected correctly for Alkaline only circuits))?

Should I be inserting a standard power/zener diode after the series batteries to create a voltage drop, to force an earlier cut-off to protect my poor little batteries in this alkaline only recommended calculator ?

And I am sorry for semi-hijacking the thread with my numerous questions. I do realize that in real world terms the damage is likely to be minute (especially as I will not be charging these batteries often in this case), I am just pedantic :thinking:

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My advice is to determine how long the calculator can run with the eneloops including parasitic drain and estimate about 80% of that and schedule charges based upon that. Put a sticker on the back of it when you last charged it.

batteries in series..usualy the 'header' battery takes the full voltage so releases its energy more readily than the others with less combined voltage accross them.

e.g. first battery 1.2v second battery (also a 1.2v) is at 2.4v, third battery (also a 1.2v) has 3.6v running through it..as a simple exsplanasion.

as the voltage goes up current (amps) flows more readily, or 'easier', also the voltage drop could be seen as the more amps in store in the battery the easier it maintains its voltage, as it wares down theres less 'potential' so the voltage sorta sags a bit lower, the more amps coming out the more the voltage sags.(the voltage sag depends largly on the make of the battery, some are better than others...anything ultrafirey..well, they're ok for shooting at to see if they exsploid lol).

and back to your pack, i'd say at a gues the header battery is papped...it exsperiencing all the current flow at the highest voltage so it burns out sooner than the other two, as time goes on, it just gives up a lot easier than the other two untill 'you'd throw the battery pack away' with two good cells out of the three to it.

Baqar79 said:

I was going to start a new topic on this, but the topic is very much along the lines of the question I was interested in.

I have a repaired Casio 9750GII calculator which is powered currently by 4 x AAA eneloop (generation 3). These came from the same packet and are fairly close in value (797,795,804,796).

However I have tested this calculator on 3 discharged AAA eneloop (last battery holder shorted across the terminals to simulate 3 batteries) and it will still power on. Casio should be complemented on their efficient power regulation, but after learning about reverse charging due to imbalanced cells, this seems to be likely a problem especially with such a low power drain on the batteries. It seems inevitable that in this case the 804mAh will reverse charge one perhaps even two of the others before the calculator shuts down due to low power.

Due to alkaline batteries leaking because of reverse charging, do these devices have protection circuits that detect discharged impedance values of the series batteries and shut off the circuit when they become too high (hence why certain devices are NiMH compatible and others are only Alkaline(lower internal resistances for NiMH are not detected correctly for Alkaline only circuits))?

Should I be inserting a standard power/zener diode after the series batteries to create a voltage drop, to force an earlier cut-off to protect my poor little batteries in this alkaline only recommended calculator ?

And I am sorry for semi-hijacking the thread with my numerous questions. I do realize that in real world terms the damage is likely to be minute (especially as I will not be charging these batteries often in this case), I am just pedantic :thinking:

Click to expand...

This is quite a reasonable concern to have, but in this case I would not worry about it too much. Damage from polarity reversal can be an issue with high drain devices, but it is much less of a concern with low drain devices like calculators. When the current draw is small the batteries will tend to balance their voltages much better and any potential damage from reversal will be limited by the low current flowing in the system.

What I would do is simply monitor the batteries from time to time and recharge them when it seems like they need it.

Follow The Herd - Buy the Maha MH-C9000 First!

Cereal_Killer said:

Wow some great info here, you guys are so far above my head...

Click to expand...

If you care to read my "Assembled Best Answers", IMHO as a newbie years ago, CLICK on the GREEN LINK in my Sig Line.

Cereal_Killer said:

...I'm getting a BC-900 for Father's Day, I choose it OVER the maha c9000 for the individual displays and ease of use (ie programming all 4 cells at once vs having to program it for each cell))...

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I agree with Mr Happy and, IIRC, the majority of the AA/AAA NiMH users here. Start out with the C9000 - it's a *MUCH* better product. To charge 4 2000mAh AAs @ 1000mA (0.5C), you just insert the cells and press ENTER 8 times. And the 'Break-In' function is great (and not available in the BC-900 family).

The BC-900/9009/1000 family is good for *CRAP* cells that the C9000 rejects (ICV GT 2.10VDC). These are the cells that get reassigned to clocks, thermometers, remotes, children's toys, etc... The slot spacing in the BC-900 family is too close - charging 4 2000mAh AAs @ 1000mA usually results in a thermal shutdown since the cutoff voltage @ ~1.53VDC is too high. The C9000 cuts off @ ~1.47VDC and then 'Tops Off' the cells @ 100mA for 2 hours. MUCH gentler. I also use my BC-900 @ 1800mA to 'Fast Charge' (with constant monitoring) cells that I need 'Right Now' if my C9000 is busy (i.e. both sets of 4 AAs dedicated to my ancient digital camera are 'dead' - i.e. camera won't turn on - so I charge them just enough to take a few pictures).

As for one cell (for discussion, let's say Cell #3) dropping to 0.8VDC first, the C9000 will probably show a lower capacity and higher ICV on this cell. Plus, as pointed out in Mr Happy's graph, NiMH cells run at a 'plateau' for the major of their discharge period and then they quickly 'jump off the cliff', compared to alkaline and carbon-zinc cells whose voltage is constantly declining from Day 1. That's why the flashlights I give my wife to use are either 1 GOOD Cell or 3 *CRAP* cells since with the 3 cells in series, she never notices the drop in brightness when one cell has 'jumped off the cliff'.

Cereal_Killer said:

...I have another question-the radio has a 12v in and is capable of charging the batteries (in series) when plugged in. Is that safe to charge 4x NiMh AA's in series like that? I have no idea what sort of charging technique/ algorithm it uses, is that just overall a bad idea?

Click to expand...

It depends on the current. If it's low, like 200mA (0.1C), the cells that finish first can absorb the low current overcharging. If it's high, like 1000mA (0.5C), you will be damaging the cells that finish first.

Have fun reading and learning - the CPF Archives are just great!

Where Did You Learn THIS Theory???

hoffmyster86 said:

batteries in series..usualy the 'header' battery takes the full voltage so releases its energy more readily than the others with less combined voltage accross them.

e.g. first battery 1.2v second battery (also a 1.2v) is at 2.4v, third battery (also a 1.2v) has 3.6v running through it..as a simple exsplanasion.

as the voltage goes up current (amps) flows more readily, or 'easier', also the voltage drop could be seen as the more amps in store in the battery the easier it maintains its voltage, as it wares down theres less 'potential' so the voltage sorta sags a bit lower, the more amps coming out the more the voltage sags.(the voltage sag depends largly on the make of the battery, some are better than others...anything ultrafirey..well, they're ok for shooting at to see if they exsploid lol).

and back to your pack, i'd say at a gues the header battery is papped...it exsperiencing all the current flow at the highest voltage so it burns out sooner than the other two, as time goes on, it just gives up a lot easier than the other two untill 'you'd throw the battery pack away' with two good cells out of the three to it.

Click to expand...

Sorry, but AFAIK from what I've learned here, I disagree with your logic completely.

In a series circuit, each NiMH cell is at its individual voltage (~1.2VDC -> ~0.9VDC), while the current through each is *EXACTLY* the same, until it poops out. Then the current being FORCED through it by the remaining cells will eventually 'Reverse Charge' it and damage it. There are no 'Header', 'Footer', or 'Middle' cell(s) - each cell releases its energy equally depending on the LOAD placed on the BATTERY (a set of cells wired in series).

The CPF Gurus that I learned from can explain this more technically. But, briefly, this is how I understand it.

I'm going to revive this thread if I may because I can't find a categorical yes or no answer sorry

I have the same issue as the OP in my nitecore Eax hammer which runs 2p4s setup on nimh AA batteries and after use there is always a big difference between batteries when measured on a pulse load meter. Is this safe or am I risking the light exploding. I'm using 2850mah ansmann digital batteries.

Thanks

torchsarecool said:

I'm going to revive this thread if I may because I can't find a categorical yes or no answer sorry

I have the same issue as the OP in my nitecore Eax hammer which runs 2p4s setup on nimh AA batteries and after use there is always a big difference between batteries when measured on a pulse load meter. Is this safe or am I risking the light exploding. I'm using 2850mah ansmann digital batteries. Thanks

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First of all, use quality cells, like Eneloops, or Fujitsus (FDK Eneloop factory.)

What differences are you seeing in your batteries?

Are you getting reverse charging on any of your batteries?

They're man made devices, so there will be some slight variation, even your 'pulse load meter' has been built to a certain tolerance.

Chris

Oh...I naively thought ansmann were decent quality.

My zts differs from 20% on 2 cells to 60% on 4 cells. Though actually I have noticed the results it gives even change from one moment to the next on the same battery!!

I don't know how to tell if the cells are reverse charging. Could you advise me

Thanks for your help!!