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Topic: Need help understanding NiMh charging (Read 1 time) previous topic - next topic

waqaszahid

Hello,

I have built an NiMh constant current charger that can charge a single AA cell upto a 12 AA cell pack connected in series, I did extensive testing in the start but I never faced such an issue. Right now when I am trying to charge a single AA cell the charge terminates after only 15 minutes into the charging after the charger has detected a 2mV dip in the terminal voltage of the cell when I am charging at 1C where as ideally the charge should have terminated after around one hour, initially I though the cells were false peaking and after doing a bit of research I realized that the NiMh's only tend to false peak when they are subjected to a fast charge cycle of 1C after they had been discharged completely and that the false peaks only tend to appear within 10 minutes of the start of the charge cycle.

Based on this I programmed a function in the my charger that ignores any peaks during the first 10 minutes of the charge cycle but after 15 minutes or so into the charge the battery peaks again and charge terminates, now had this been a false peak the battery would not have been hot/warm to touch, temperature change for an NiMh is a far reliable indicator of charging than negative delta V termination therefore I can say that once the charge terminates I can tell by the battery's temperature that it has indeed been charged to full capacity, had the battery been cold then that would have been another story.

Now these single cells that I am talking about have been lying around for say 2 months and I lastly used the same charger to charge them but I had just left them until recently.

Secondly I have 8 AA cell 9.6 volt pack which seems to have held up nicely and still takes around one hour to charge at 1C using the same charger.

The single cell NiMh AA cells that I was talking about arent delivering the rated 2.1 Ah as well, when I connect the cell across a 1 ohm resistor the terminal voltage drops to 0.8Volts in a matter of seconds, have my batteries gone bad? I believe they have been tossed around and dropped a few times and I remember reading somewhere that this can damage the NiMh. I am looking for answers online and til now my best guess is that my batteries have lost capacity.

Thanks

Pigeon-Kicker

Just a quick knowledge question here.

Do they have a charge base similar to the Lithium Polymer batteries ?
They are wired like this in the battery pack that i have.


If so, i was told that to overcome the signal peak i should put a capacitor between the Vcc and Gnd.
That this would even it out so to say.

I have not tried this yet, LiPo batteries tend to get hot quickly.

MorganS

Quote
i should put a capacitor between the Vcc and Gnd.
There's no Vcc or Gnd shown in your schematic. Honestly a pencil sketch badly photographed with a phone is better than showing us a colourful schematic for someone else's equipment.

Personally I don't like NiMh because they self-discharge so badly. After a month on the shelf, I need to charge the battery again. I won't do that unless it's a machine I use more often than once a month.

waqaszahid, what data logging do you have? If you are researching how to charge batteries then you need to have good logs so you can go back and examine all the peaks and weird things. An SD shield on your Arduino makes it easy to store lots of data.
"The problem is in the code you didn't post."

waqaszahid

#3
Apr 20, 2017, 05:34 am Last Edit: Apr 20, 2017, 06:05 am by waqaszahid
Hello Morgan,

During my initial testing I took the voltage readings through a serial monitor, once the charging was done I plotted the voltages wrt to time in an excel sheet, for the same AA's that I am talking I received zero false peaks and a near ideal NiMh charging curve with the battery peaking at the end and the charger cutting off the cycle once the charger detected 2mV drop after the peak had occured.

Right now I have an lcd which displays the voltage every second while charging and yes it does false peak in the beginning but since I have the peak ignore enabled for the starting 10 minutes it does start to pick up charge again, once the 10 minute ignore time expires it peaks again shortly and then the battery is already hot to touch unlike the first time it peaked.

Can all NiMh batteries be charged at 1C? I am beginning to think these particular ones werent meant for it and my testing has decreased their capacity, although I have put in alot time building the hardware and writing the code but I am afraid I am lagging behind on basic NiMh chemistry knowledge, I am actively looking online and hoping could get some help from here as well.

srnet

Personally I don't like NiMh because they self-discharge so badly. After a month on the shelf, I need to charge the battery again. I won't do that unless it's a machine I use more often than once a month.
Just use the readily available low self discharge NiMh. 
$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

waqaszahid

I think I might have a theory on this, I believe it all comes down to substandard battery holder contacts, while in the holder and connected across a one ohm resistor the voltage directly across the resistor drops down to 0.8 volts, leading me to believe the battery is under charged and doesnt hold as much energy as it did before, however holding the resistor directly across the battery terminals shows a terminal voltage reading of 1.2V, obviously the batteries werent discharged before I reconnected it back to the charger having them to peak under 15 minutes.

This makes sense I think I have to look for good quality battery contacts and see how that goes.

srnet

The single cell NiMh AA cells that I was talking about arent delivering the rated 2.1 Ah as well, when I connect the cell across a 1 ohm resistor the terminal voltage drops to 0.8Volts in a matter of seconds, have my batteries gone bad?
Your missunderstanding what the 2.1Ah means.

The battery may well be capable of delivering 210mA for 10hours (= 2.1Ah), but that is completely unrelated to the current the battery can deliver into a load.

That the voltage drops to 0.8V at a load of approx 1A is not far off what you might expect.

There are NiMh designed for high current applications, but your average high capacity NiMh is not good at high current.
$50SAT is now Silent (but probably still running)
http://www.50dollarsat.info/
http://www.loratracker.uk/

waqaszahid

Hello Srnet,

Thanks for your response, as far as I know the NiMh terminal voltages remain fairly constant through a discharge cycle and drops exponentially as the capacity is reached, for batteries I work on a very simple formula, when the battery is connected across a load it will try to drive a current through that load that will help maintain the same voltage (1.2V) across resistor as well.

The battery has a rated Ah on it and I think it should be able to provided that much current if the load is low enough, for example I am connecting a 1.2 V AA cell directly across a 1 ohm resistor the battery is rated at 2.1Ah but in order to maintain a 1.2V across the load the battery should be able to drive 1.2A of current through it which is well under its rated capacity, at this rate the battery should take roughly 2 hours to be consumed fully, now I am only getting 0.8 volts across the resistor once the battery is mounted inside the holder as compared to 1.2V when the battery is connected directly across the resistor, this lead me to conclude that voltage is dropping across holder contacts.

For a 0.8V drop across the 1 ohm resistor the holder contact resistance comes out to be around 0.5ohm or 0.25ohm per contact that is an expected value and this is how I got to this conclusion.

Noobian

Personally I don't like NiMh because they self-discharge so badly. After a month on the shelf, I need to charge the battery again. I won't do that unless it's a machine I use more often than once a month.
They fixed that issue a decade ago with the low self discharge variants starting with eneloop. They also have 4x the charge cycle compared to a ICR cell.




slipstick

The battery has a rated Ah on it and I think it should be able to provided that much current if the load is low enough, for example I am connecting a 1.2 V AA cell directly across a 1 ohm resistor the battery is rated at 2.1Ah but in order to maintain a 1.2V across the load the battery should be able to drive 1.2A of current through it which is well under its rated capacity
You are confusing CAPACITY (measured in Ah) with discharge current capability (measure in A - no h). There is no reason to assume that a 2.1Ah capacity battery should be able to deliver 2.1A or anywhere near that. If it delivers .21A for 10 hours then that is its full capacity of 2.1Ah. Some batteries can sustain discharge current of 1C (2.1A in this case) or more but many small consumer batteries cannot.

OTOH if you see MORE volt drop from the battery in the holder than when directly connected you ALSO have a problem with poor contacts.

Steve

waqaszahid

Thanks Steve, that makes sense! I got the capacity and C-rate mixed up there

waqaszahid

Soldered wires directly onto the battery terminal and its now working, problem solved. Bad contacts were the last thing that I had expected.

Castingflame

Just a quick theory question.

Say a battery Pack is rated as 12v, 2.1Ah, 1C.


If 2.1Ah  = capacity of battery = 2.1 Amp in 1 hour


and

If a 1C rate means that the discharge current will discharge the entire battery in 1 hour


does this mean that the battery can provide 12 volts at 2.1 Amps of current for 1 hour (or close)?

slipstick

That's describes what might happen but it's missing the point of the C-rating. The purpose of that is to tell you the maximum current that the battery can deliver. So 1C is saying don't even try to take more than 2.1A.

If the battery was rated 2.1Ah @ 10C then you could (in theory) take 21A out but if you did it would be empty in only 6 minutes (1/10th of an hour).

OTOH we were talking about NiMH batteries and they very rarely specify a C-rating.

Steve

MorganS

The manufacturer sells more batteries if the Ah number is higher. The Ah is higher at lower discharge rates. 1/20 C is very common. (Also called the 20-hour discharge rate.)

1C may be the rate at which the 2.1Ah was achieved or it may be the peak rate for a short period - possibly not even 10 minutes. Without seeing a datasheet it is impossible to tell.
"The problem is in the code you didn't post."

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