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Nimh charger ?

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curry87

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I have some questions to ask about charging nimh cells:


1.How do you tell when a nimh cell is fully discharged both open circuit current and closed circuit conditions ?


2.How do you tell when a nimh cell is fully charged on a open circuit ?


3.Will trickle charging nimh cells reduce lifespan when compared to a fast charge ?


4.Take a 200mah 8.4v nimh battery for example to charge it is simply a case of connecting a 7x 1.45 (10.15v) voltage supply across the battery with a resistor that controls charge current and waiting for the voltage to reach 10.15v before disconnection assuming fully charged?
 
NiMetal Hydride batteries have a similar voltage and charging characteristics as NiCad batteries. Nominal cell voltage is 1.25 vdc.

Open circuit voltage is not a great indication of state of charge but if the cell is below 1.1 vdc it is pretty much discharged. A fully charged NiMh will have a rest voltage of about 1.35-1.40 vdc. NiMh have a relatively high self discharge rate and after a few days to a week will need to be recharged to achieve full capacity.

Simpliest charging is a 14-16 hour constant current of 10% of mAH rating. NiMh can take the 10% rate for an indefinate time but it does degrade cells over a long period of time. For this charging rate for a six series cell battery of 200 mA-hour would be a resistor in series with a higher voltage source. For example, a 12 vdc power supply with a (12.0-8.4v)/0.02 mA = 180 ohm. If you want to leave them on charge all the time you should drop the 10% rate down to about 3 to 5%.

Faster charger run a higher current rate and look for step rise in voltage as battery approaches full charge to terminate charging. The cell voltage will begin to ramp up from about 1.35 vdc to 1.5 vdc near full charge. The voltage will depend on fast charge current.
 
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When NiMh cells are fully charged additional energy is converted to heat. Some chargers use this to determine end of charge. This may not work well or at all for a trickle charge.

When NiMh cells are fully charged the voltage will drop a bit from the peak. This should be true regardless of the charge rate.

Heat is what kills NiMh. So the worst is overcharging using a fast charge. If the cells get hot they are being damaged to some degree.

What is the application ?
 
The bit thats confusing is the charging time surely a 200mah nimh battery @10% charge current would take 200mah/10 = 20ma charge current ,200mah/20ma charge time = 10hrs why is speced at 14-16 hour charge for these cells ?


If a nimh battery is half discharged and a recharge is desired then how is charge time recalculated in order to not overcharge the battery ?


So to confirm a charge rate of 3-5%C for nimh cell can be used to charge indefinite without overcharging, 10%C would be pushing it if it was placed indefinite lengths of time in the charger ?
 
The bit thats confusing is the charging time surely a 200mah nimh battery @10% charge current would take 200mah/10 = 20ma charge current ,200mah/20ma charge time = 10hrs why is speced at 14-16 hour charge for these cells ?


If a nimh battery is half discharged and a recharge is desired then how is charge time recalculated in order to not overcharge the battery ?


So to confirm a charge rate of 3-5%C for nimh cell can be used to charge indefinite without overcharging, 10%C would be pushing it if it was placed indefinite lengths of time in the charger ?

Reason for 14 hours is charging is not 100% efficient.
10% is fine, just take off charge after a day. You can do it for partially discharged, it won't hurt it. If you want standby permanent charging then do the 3-5% (like cordless screwdriver, or hand vacuum cleaner).
 
Okay that cleared up thanks.


So for a simple nimh charger (and i mean simple) for a 8.4v pp3 nimh 200mah battery would the following be about right ?
Say a 12vdc supply with a (12v-10.15v) /0.02a = 92.5ohm resistor in series and then just wait for 14 hours until charged is it that simple ?
 
Your "8.4V" battery will not always be 10.15V while it is charging, its voltage might be only 7V when the charging begins. Then your 92.5 ohm current-limiting resistor will supply (12V - 7V)/92.5 ohms= 54mA which is a lot of current for a little 9V battery.
Maybe you should make a current regulator.
 
Your "8.4V" battery will not always be 10.15V while it is charging, its voltage might be only 7V when the charging begins. Then your 92.5 ohm current-limiting resistor will supply (12V - 7V)/92.5 ohms= 54mA which is a lot of current for a little 9V battery.
Maybe you should make a current regulator.

I have a really simple but fast charging circuit for a little PP3. My much promised but as so far missing casing/electronic design from this Forum. Electronic design is good. Reliability is exceptional. So far around 300 cycles for a little PP3 with no battery degradation. Totally flat PP3 is fully recharged within 2.5 hours. A year of testing and battery is still good for everything I throw at it. Like new @ day one.

My problem is the casing design for the finished product. I have to cut every groove and drill every hole. I have to polish this product.

Doing my very best here

Will let you know what happens

TV TECH
 
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The bit thats confusing is the charging time surely a 200mah nimh battery @10% charge current would take 200mah/10 = 20ma charge current ,200mah/20ma charge time = 10hrs why is speced at 14-16 hour charge for these cells ?


If a nimh battery is half discharged and a recharge is desired then how is charge time recalculated in order to not overcharge the battery ?


So to confirm a charge rate of 3-5%C for nimh cell can be used to charge indefinite without overcharging, 10%C would be pushing it if it was placed indefinite lengths of time in the charger ?


Hi there,

There's no such thing as calculating the time required to recharge, except when you just discharge the battery and you know how much charge you just took out of it, but even then it's not really the right way to recharge it.

There is no way to measure the state of charge without charging it up because there is nothing external about the cell that tells how much charge is left. Once you start to charge it however and it reaches full charge, there are a couple ways you can tell it has become fully charged.

One way that is used a lot is called the "Minus Delta V" method, where the 'V' stands for "Voltage". What happens during charge with a decent constant current (like 1 amp for a 2000mAh cell) is the voltage rises as time goes on, but then all of a sudden it starts to decrease even though you havent decreased the current (kept it at 1 amp all that time). This sudden decrease in voltage, which may be as little as 10mv, tells you that the cell is fully charged.

Then there is another method called "Zero Delta V", which looks for the time when the voltage slope decreases to the point where it is zero or almost so.

Another method relies on temperature instead of voltage, but you have to make sure the temperature measurement is done right. When a cell is charging it actually absorbs some heat so the temperature states somewhat cooler, but as it charges the cell temperature rises little by little. As the cell becomes fully charged suddenly the temperature slope increases, and that's the point where the charge is cut off.

So you see NiMH cells are a little more difficult to charge than say Li-ion or Lead Acid. To use the minus delta V method you have to be able to measure small changes in cell voltage, and with the temperature methods you have to ensure that the temperature probe never looses contact with the cell.
 
When a cell is charging it actually absorbs some heat so the temperature states somewhat cooler, .....
That is for a Ni-Cad battery, not for a Ni-MH battery.

Simply charge a Ni-Cad and a Ni-MH battery at the same time. The Ni-Cad battery becomes a little cooler and the Ni-MH battery heats a little. When they are both fully charged then they both heat a lot.
 
Hi audioguru,

Strictly speaking you are right about the NiMH cell not being endothermic but the NiCd is. However, the NiMH cell doesnt heat up very much at first depending on the current level.

Here's an actual set of plots from real life cells being charged at 1 amp constant current. The most temperature increase is seen as the cell nears full charge. This plot is nice to have because it shows the relationship between the temperature rise and the cell voltage.
 

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I have a really simple but fast charging circuit for a little PP3. My much promised but as so far missing casing/electronic design from this Forum. Electronic design is good. Reliability is exceptional. So far around 300 cycles for a little PP3 with no battery degradation. Totally flat PP3 is fully recharged within 2.5 hours. A year of testing and battery is still good for everything I throw at it. Like new @ day one.

My problem is the casing design for the finished product. I have to cut every groove and drill every hole. I have to polish this product.

Doing my very best here

Will let you know what happens

TV TECH
@TV TECH would you mind sharing that nimh charging circuit ?
 
Hi curry87

I would gladly share all with you. Problem is I am @ the final stages of completing my project.
The finishing touches and all.

Labeling in the right places and so on. Been one Year of hard work. With the flipping whole casing problem.

ElectroMaster and I spoke around a Year ago. He was happy to give me Prior Art on this great Forum.

It's a matter of days away now. I am almost ready to show my little design to you all. And I truly hope you will like it.

That is the best I can do right now.

Cheers
TV TECH
 
Ok thx tv tech.

Couple more questions:

What causes the voltage across a nimh cell to raise when it is charging i thought the internal resistance decreases when the cell is being charged so that it can provide more current without a voltage drop ?


So you got a really simple charging circuit which consists of a 12vdc, 180ohm resistor in series with the 8.4v nimh pp3 battery say you connect the battery when its at 8.4v and that causes 20ma to flow at the start of the charging process.As the voltage across the battery raises will it not reach 12vdc eventually and toast the battery ?


Is a typical nimh cell fully charged when it reaches 1.45v across terminals or when it has had 0.1C charging current for 14-16 hours ?
 
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What causes the voltage across a nimh cell to raise when it is charging i thought the internal resistance decreases when the cell is being charged so that it can provide more current without a voltage drop?
The chemical action of charging causes the cell voltage to rise and the internal resistance to drop during charging.


So you got a really simple charging circuit which consists of a 12vdc, 180ohm resistor in series with the 8.4v nimh pp3 battery say you connect the battery when its at 8.4v and that causes 20ma to flow at the start of the charging process.
As the voltage across the battery raises will it not reach 12vdc eventually and toast the battery?
Battery manufacturers show voltage curves on their websites.
Energizer shows that a Ni-MH cell charged at 0.1C (20mA for a battery rated at 200mAh) has its voltage rise to 1.41V then stops rising. A 7-cell battery (8.4V) will have a max voltage of 9.9V.
But in your case the charging current drops as the cell voltage rises so when the battery voltage rises to about 9.4V then its charging current is about 14mA.

Is a typical nimh cell fully charged when it reaches 1.45v across terminals or when it has had 0.1C charging current for 14-16 hours ?
The fully charged voltage varies with charging current and with temperature.
The voltage drops a little when a cell is fully charged and it heats so maybe the heating causes its voltage to drop a little.

Energizer shows that a single cell has a voltage peak of 1.45V when it is charged by a fairly high current of 0.3C and the ambient temperature is 25 degrees C. A charging current of 0.3C is too high for the tiny crammed-together cells in a little 9V battery.

My Energizer Ni-MH charger is a stupid simple timer. It fully charges dead cells perfectly but it severely over-charges cells that already have some charge.
 
If the maximum voltage rise of a nimh is 1.41v by the chemical reaction inside then how do "bad chargers" put greater than 1.41v across the cell and potentially reduce lifespan ?




I know at 3%C - 5%C charge rate you can basically leave the battery in the charger forever without any fear of overcharging but with 10%C if the minimum full charge time for a nimh cell is 14-16 hours then how much longer can you leave them in the charger for after minimum full time charge is reached without fear of overcharging?
 
If the maximum voltage rise of a nimh is 1.41v by the chemical reaction inside then how do "bad chargers" put greater than 1.41v across the cell and potentially reduce lifespan?
The max voltage of an Energizer Ni-MH cell is 1.41V only when it is charged at a fairly low current of 0.1C. Energizer shows a graph of a cell's voltage to have a peak voltage of 1.5V when it is charged at the high current of 1C. Some chargers charge at 4C and higher so the voltage will be higher.

I know at 3%C - 5%C charge rate you can basically leave the battery in the charger forever without any fear of overcharging but with 10%C if the minimum full charge time for a nimh cell is 14-16 hours then how much longer can you leave them in the charger for after minimum full time charge is reached without fear of overcharging?
Energizer says you can trickle-charge forever at 0.025C. They say that over-charging continuously at 0.1C reduces the life of a Ni-MH cell.
Panasonic says not to trickle-charge higher than 0.03C.
 
When designing a simple nimh charger what works best:

1.Voltage cutoff that terminates the charging when the cell reaches 1.45v ?

2.A timer so when 14-16 hours has passed during which time the cell has been charging at 0.1C ?
 
When designing a simple nimh charger what works best:

1.Voltage cutoff that terminates the charging when the cell reaches 1.45v?
A Ni-MH cell is 1.45V only when it is charging at a certain current and is at a certain temperature.
Look at a Ni-MH battery charger IC. It DOES NOT simply stop charging when 1.45V per cell is reached. Instead it detects the peak voltage or when the voltage stops rising. It also detects the temperature rise when the cell is fully charged.
 
The little E Block 9V cell likes a charge termination at around 10.05Volts. Thereafter, it can be left permanently on charge.

An LM317T plays the Voltage regulation role. Babies the battery. Looks after it always.

Never above 10.05V ever. For ever.

Cheers,
TV TECH

The LED's are babied too. Rated @ 20 Ma. Driven @ around 17 to 18 Ma.
This design has survived a year of testing. Not one failure yet....of anything.

I am tired. Seriously.
Thanks
 
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