NiMh Charging

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Hi.

I have to charge the battery for my RC remote, it is 8cells 2000mAH. Problem is i forgot my charger at home so have to use a desktop power supply.

I have it connected with current regulation and voltage regulation, and i am monitoring voltage continously.

Am i correct asuming that the voltage will keep increasing untill it reaches a cell voltage of ~1.4V, assuming constant current. At 0.5A it will take roughly 4 hours to complete the charge, right?

Just want to make sure as i have no plan on having it blow up in my face.

Thanks
Kim

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I had done this few times. I charged them to 1.5V/cell, but that depends on temperature and current charge. I used C/10 and I measured (I had nice graphs, but that was a previous life) that usually 12 to 14 hours were needed to complete the charge process. At C/4 I be you will ned someting in the range 4.5 to 5.5 hours.

Voltage limited also sets an 'automatic finish' when full charge voltage is achieved.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Thanks Guillem,

Feels a bit safer now :)

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If you monitor the temperature closely, you'll be alright.

Interesting factoid: NiCd/NiMh chargers detect a full battery because the voltage no longer raises (or even falls for NiCd cells). The reason the voltage no longer raises is that the battery gets warm as the energy pumped into it from the charger does no longer convert into chemistry and that the cell has a lower voltage when warm. The lower voltage due to warmer cell compensates the higher due to charging enough to be a reliable detection mechanism.

So the 'normal' way to cut of charging is by indirect temperature measurement !

Markus

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So in other words, when i feel that the temperature of the battery is above room temperature, it is fully charged?

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Yes, Markus is rigth, but depending on charging current, you can feel them warm even when they are half charged due high current flowing through the internal (really small) resistor.

Standard NiMH/NiCd chargers use different termination criteria, and commonly few at same time. Temperature rise (also called Delta-T) speed and voltage drop 9(also called minus Delta-V) after certain voltage had been reached are commonly used, being the first one the 'safety' criteria, while the later one is used as the 'standard'.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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kherseth wrote:
So in other words, when i feel that the temperature of the battery is above room temperature, it is fully charged?

Unfortunately it is not that simple. The charging process is around 80%-90% efficient, so your battery will heat up even while charging normally. The faster you charge, the more heat you get. When your battery is fully charged all incoming energy will become heat, not only 10%-20%.

So it is normal that a charging battery gets somewhat warm. The secret is to know how much is too much and this depends very much on the type of battery, how it is packaged, how fast you are charging, etc.

Markus

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I read to look for a temperature rise of 2C per minute.

The largest known prime number: 282589933-1

Without adult supervision.

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Charge it with constant current 1/20 of 2000mAh (100mA approx.), untill it reaches the 1.45V/cell. After this charge it with a trickle charge current over 1/100 2000mAh (20mA). This will prevent the stored charge not to be consumpted due to the internal battery loss. Also this small trickle charge current will keep the NiMH temperature at low levels.

Michael

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

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Lots of input here, i forced ~1A trough it until it got warm and cell voltage got too about 1.4V. I know the battery is close to dead anyways, the insulation was punctured a year ago while flying, so the battery was shorted. Luckily got the plane down, but it is a nervous time when seeing the battery monitor dropping 0.1V ever 3sec.

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A safe charging current is C/10,but you have to calculate the output voltage of the charger.If this voltage is reached the current flowing to battery drops to 0 and the charging stops.Output voltage is 1.2v per cell x 1.17.

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There is no problem with how you charged your battery. Your battery was designed for fast charging and discharging (modeling batteries are) and I presume you don't care for it that much anymore as it is already damaged.

The main problem you can have is that you forget it while charging manually and it catches fire and damages the surroundings, worst case setting your house on fire. Here the risk is much bigger with LiIon batteries as lithium, a main component can burn quite aggressively as many youtube videos with burning laptops prove.

Markus

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Normally you don't know how many amperes you can charge with, so at least C/10 is safe value, maybe even C/5.

So for a C=2200mAh battery, it charging current I=C/10=220mA and you need to charge it up to 140%, so T=1.40*C/I=14 hours. Approximately.

Several things to look when battery is full:
- stop immediately when cell voltage starts to drop or it has stopped rising( dV/dt zero or negative)
- stop when temperature rises quickly! (don't know how much dT/dt)
- stop when temperature reaches a limit (don't know how what T)
- stop at certain max cell voltage (1.4V or 1.5V is it with NiMHs)
- stop when time is more than enough like 140% plus some margin of error, like 160% or 170%

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How do you find dv/dt if you are regulating for constant current? Do you have to stop charging and sample periodically??