NiMH vs. NiCd Batteries Question

[the jeenyus]

Hello. I am new to batteries, and upon the connection of a 6V 1700 mAh NiCd battery to my $50 robot, one of my servos and the microcontroller fried. I initially used a 6V 1400 mAh NiMH battery, but I am not sure what the difference is. I read somewhere that NiCd batteries have a higher output than NiMH, so could that be the reason that my poor little servo fried?

I am also seeking advice for future projects. I would like to know your personal preference battery-wise, or if they each could be unique to a particular situation.

NiMH or NiCd?

[SmAsH]

id say something else in the circuit is wrong if the mcu fried. it should be protected by the voltage regulator.

[the jeenyus]

But it already worked. It only worked while using the 6V 1400 mAh NiMH battery. It immediately fried when I plugged in the 6V 1700 mAh NiCd battey.

[TrickyNekro]

something else happened for sure....

[GearMotion]

James makes a reasonable assumption. So what could have gone wrong? Possibly your new battery pack polarity is different? Did handling the circuit cause a mishap such as pressing two connections together?

[SmAsH]

if it fried the servo it would have been over 7V2 or a bit higher. as for the mcu, wtf? maybe a bridge somewhere?

[TrickyNekro]

This reminds me polarity switch

[the jeenyus]

Ok, I will check and let you know the results.

[Webbot]

NiMH or NiCd?

My preference is NiMH

[airman00]

I once heard that NiMH batteries get hot when charging, but NiCd batteries actually get colder while charging

[superchiku]

sumthing is serverly wrong in the circuit...battery wont fry away until it is really rated high..

[SmAsH]

I once heard that NiMH batteries get hot when charging, but NiCd batteries actually get colder while charging

wow, who would of thought?

[TrickyNekro]

I once heard that NiMH batteries get hot when charging, but NiCd batteries actually get colder while charging

wow, who would of thought?

It's all about internal chemistry... If NiCd batteries charge is based on a exothermic phenomenon, meaning the while the
deliver current they heat up which is more common to be know then
charging would be the reverse phenomenon....
Even while current flows through the battery heating it up due to internal resistance, the charging effect is a
endothermic phenomenon, which means that the battery draws heat to be charged...
But charging while heating isn't possible at all and will in fact destroy the battery,
cause for a reaction to take place certain conditions must be meet (like certain internal energy of the reactants and such)
So you apply current to the battery to start some reaction to start charging up....
But charging is actually caused by drawing heat from environment.....

I'm not sure for what I wrote here, correct me if I'm wrong...
It's really been a while, since I last read about who NiCd work.....

[Soeren]

Hi,

Both NiMH and NiCd heats up when charged.
NiCd can supply a much higher current (relative to its capacity). NiMH packs a much higher capacity per cubic measure, but still the NiCds can provide higher surge currents.

However, a circuit only draws what current it needs, so unless it was current starved on NiMH, this has no chance in a Hot Place of being the problem.


It would be helpfull for most people building robots, to really know their battery technology inside out; how they're charged, the pros and cons of each chemistry etc. as it is what drives your robots in every way - a bad match between battery and load means you're loosing out on something that would be cheap to maximize.

[TrickyNekro]

Hi,

Both NiMH and NiCd heats up when charged.
NiCd can supply a much higher current (relative to its capacity). NiMH packs a much higher capacity per cubic measure, but still the NiCds can provide higher surge currents.

However, a circuit only draws what current it needs, so unless it was current starved on NiMH, this has no chance in a Hot Place of being the problem.


It would be helpfull for most people building robots, to really know their battery technology inside out; how they're charged, the pros and cons of each chemistry etc. as it is what drives your robots in every way - a bad match between battery and load means you're loosing out on something that would be cheap to maximize.

Ahem ahem.....
The safe temperature range for a NiCd battery in use is between −20°C and 45°C. During charging, the battery temperature typically stays low, around 0°C (the charging reaction absorbs heat), but as the battery nears full charge the temperature will rise to 45–50°C. Some battery chargers detect this temperature increase to cut off charging and prevent over-charging.

From Wikipedia

[Soeren]

Hi,

Ahem ahem.....
The safe temperature range for a NiCd battery in use is between −20°C and 45°C.

Which has nothing to do with charging them.

During charging, the battery temperature typically stays low, around 0°C (the charging reaction absorbs heat), but as the battery nears full charge the temperature will rise to 45–50°C.

Total baloney, or the tens of thousands NiCd packs I have had between my pinkies (and on my charge analyzers) must have been really atypical 

But don't trust me, get the lowdown from Isidor Buchmann over at Battery University, from which I quote
"A battery should always remain cool during charging because high temperatures shorten battery life. Some temperature rise with nickel-based batteries cannot be avoided."

And

"Charging at high and low temperatures

Rechargeable batteries operate under a reasonably wide temperature range. This, however, does not automatically permit charging under these same temperature extremes. While operating batteries under hot or cold conditions cannot always be avoided, the user has some control over charging. Efforts must be made to charge the batteries at moderate temperatures.

Nickel-cadmium: In general, older battery technologies are more tolerant to charging at temperature extremes. Nickel-cadmium can be fast-charged in an hour or so, however, such a charge should only be applied within temperatures of 5°C and 45°C (41°F and 113°F). More moderate temperatures of 10°C to 25°C (50°F to 77°F) produce better results.

Charging below 5°C requires a reduce charge rate of 0.1C (one tenth of the rated current). This is in line with the rate at which the oxygen and hydrogen can be absorbed within the cell. Because of the decreased combination rate at low temperatures, too rapid a charge would cause excessive cell pressure, which would lead to cell venting. Such a battery would never reach full charge state under these conditions.

[...]

Nickel-metal-hydride is less forgiving than the nickel-cadmium if charged under high and low temperatures. Nickel-metal-hydride cannot be fast charged below 10°C (45°F), neither can it be slow charged below 0°C (32°F). Some industrial chargers are designed to adjust the charge rate to existing temperatures. Price sensitivity does not permit elaborate temperature sensing on consumer chargers.

At higher temperatures, the charge acceptance of nickel-based batteries is drastically reduced. A battery that provides a capacity of 100% when charged at moderate room temperature can only accept 70% if charged at 45°C (113°F), and 45% if charged at 60°C (140°F). This demonstrates the poor summer performance of some vehicular chargers."

Some battery chargers detect this temperature increase to cut off charging and prevent over-charging.

There is a temperature rise from the moment you start charging, it just goes skyrocket while approaching the 100% charge and that is detectable by measuring the dt/dT (temperature change per time unit).

Please go through (all) the pages at Battery University, that will be time well spent.

From Wikipedia

Edited by who?  Do you trust an anonymous person that you have absolutely no background on?
If I wrote a Wikipedia page of building your own nuclear reactor, would you follow my plans (and get the exclusive Darwin Award)?
Always know your sources!

And if you still don't believe me, drop me an email with your postal address and I'll send you some NiCds that you can experiment with and get first hand knowledge about charging them 
(Regular offer!)

[TrickyNekro]

Hey, I was playing with NiCd since child....
Well.... if you want a full answer then...


Batteries, as all devices in this world are more or less a combination of capacitor and a resistor...
As current flows through the battery, the battery is charging....
Charging NiCd is based on an endothermic reaction causing the pack to lower it's temperature...

BUT....
How much, a conductor (resistor) or anything that flows current through, is heated up is based if I remember right on this formula....

Q= R*V*I²


Charging them with a constant current rate means that how much the battery is heated depends on its internal resistor...
I do consider dV and dI negligible.... or very very small for the period of testing...

So as happens with almost every chemistry the battery internal resistance to charging raises when battery gains more and more charge....

So... Q is getting larger... And as the charge take place that Q is getting greater than the Q of the endothermic reaction at the cells...
When battery charges up the resistance is so great the most of the energy is felt as heat and not charge....
Meaning at the end of the charge the battery can get around 40 - 50 ^oC
as written above....

I didn't say the when charging a NiCd you better get some of your beers around to freeze...



And hey.... please don't play around with only some of my phrases.... it's kinda irritating.......... really.....

[the jeenyus]

I tested my things that need to be tested... and the microcontroller was fine. For some reason that I do not understand (negligible experience level...) one servo fried. I replaced it, but I am afraid of plugging NiCds in to any robot.... I am going to stick to NiMH.

The NiCd was only in for 1 sec, so 6V at 1700 mAh = (1700/60) /60 = ~.5 A in 1 sec.

The NiMH at 6V 1400 mAh would only be about .3 A in 1 sec...

I dont know if this matters much, I am only a beginner, but it doesnt seem like there is much difference in the current at all. Why would it fry my servo in 1 sec?

[Soeren]

Hi,

The NiCd was only in for 1 sec, so 6V at 1700 mAh = (1700/60) /60 = ~.5 A in 1 sec.

The NiMH at 6V 1400 mAh would only be about .3 A in 1 sec...

It is not that simple. The current you draw from a battery is not controlled by its capacity.

A 1700mAh NiCad will have no problem delivering 20A (or much more) and most R/C-planes used in competition actually milks their NiCds in a few minutes, due to very high current motors.
The maximum current a battery can deliver is controlled only by its Ri (Internal resistance), which is very low in NiCds.

Capacity tells you for how long you can draw a specified current, calculated by discharging them at C20, which is 1/20 of capacity, for 20 hours. If you double the current, you get less than 10 hours and if you halve the current, you get more 40 hours (assuming a battery that hold its specified capacity).

It is NOT using NiCds that caused your problem, but if you want to stay away from them for now, then do, but reconsider when you are getting more into the whole thing.

[Soeren]

Hi,

So as happens with almost every chemistry the battery internal resistance to charging raises when battery gains more and more charge....

I'm not really sure what you mean here?
You're not talking about Ri (Internal Resistance) of the battery, are you?  Because that goes down when the battery charges.

I didn't say the when charging a NiCd you better get some of your beers around to freeze...

Nah, I prefer Whiskey myself 

And hey.... please don't play around with only some of my phrases.... it's kinda irritating.......... really.....

Err?
I only quote what I answer, the rest can be read in the original post.
This has been considered good netiquette for as long as people has communicated via the .net, originally to save bandwidth, but it's just as much to avoid confusion about what is actually answered. What changed?

[SmAsH]

hah,why does this always seems to happen between you guys?

[TrickyNekro]

hah,why does this always seems to happen between you guys?

It's in a friendly manner so you don't need to worry....
Arguments exist for the shake of humanity...

I'm not really sure what you mean here?
You're not talking about Ri (Internal Resistance) of the battery, are you?  Because that goes down when the battery charges.

Why did I have the the though that
Charging Ri goes up?
I'm not really sure about it but whatever...

Nah, I prefer Whiskey myself 

How about a kamikaze or a Havana on the rock with some cola???


Hey, Soaren I was in a very bad mood this days... maybe....
I read something wrong so please neglect that quote thing I said....

Whatever... Both have our points... So... it;s good to say hello!!!

[SmAsH]

hah,why does this always seems to happen between you guys?

It's in a friendly manner so you don't need to worry....
Arguments exist for the shake of humanity...

thats good to know

[the jeenyus]

So anyway...

What would be an accurate way to calculate the current coming out of a battery over a time interval then? Is there a tutorial that says anything about it? I would like to know.

Also, how specific does this have to be for energy requirements?

My guess on this: (working backwards) Voltage * Current = Power (Watts), Watts = Joules/second, Power * time interval (in seconds) = energy required

Am I right?

If not, I would like to know. I only know introductory physics, but I am very interested in learning more advanced concepts and would definitely like to learn a lot more about robotics. I am only in high school and do not have these sorts of things there, but will be attending college next year and join a robotics club!!!! yay!!! I reeeeeally want to learn.

[Admin]

Power * time interval (in seconds) = energy required

ya, it equals joules