Battery question

[jdriller]

I don't know much about alkaline & Nimh, hopefully you guys can help.

I tested the voltage on a regular battery, about 1.57 volts. Then I tested a Nimh out of the charger, 1.42volts. It then dropped to 1.38 volts, very quickly.

Why the difference? Almost 1.6 for a duracell and 1.4 for a Nimh. Some people say not to use Nimh in a light. Why? Is is because of the mA? Do the Nimh have much more mA? I'm just looking for a basic explanation.

Also, is 1.4 volts correct for a newly charged Nimh.

Thanks.

 

[markus_i]

It's unfortunately a bit more complicated than that. What you did was measure the voltage of single cells without load. Which doesn't say anything about the voltage on the cell when it's loaded.
Alkalines have a nominal voltage of 1.5 Volts and a relatively high impedance (or internal resistance), which means that the voltage drops fast enough with increasing load.
OTOH, NiXX cells have a lower nominal voltage (1.2 Volts) but also a lower internal resistance, which means that under load the voltage doesn't drop as fast.
So, while without load the voltage on an alkaline will be higher than on a (charged) NiXX, at a specific load the voltage (and therefore the current) with NiXX may be higher than with alkalines. Where this point is will depend on the load.
Especially if you use a direct drive LED light (which relies on the internal resistance of the cells for current limitation), it may well be that you blow the LED instantly with NiXX while it works fine with alkalines (incandescents usually don't blow quite that fast).

 

[Zeppert]

1.4 volts for a Nimh is great. They usually run at 1.2v. Alkalines run at 1.5v but drop off steadily over the duration of thier usage. Nimh are 1.2v but maintain a lot more consistancy, then they die off quickly at the end. So it depends on the light application. I have many white round press lights around the house and they work perfect on 4AA Nimh 1.2v. Now on the other hand if you have a HIGH volt DEMAND light then stick with the Alkalines because you'll get greater output (higher V's.

 

[Zeppert]

Look, Markus and I were writing at the same time! /ubbthreads/images/graemlins/ooo.gif

Great minds must think alike,... I guess!! /ubbthreads/images/graemlins/yellowlaugh.gif

 

[INRETECH]

Different battery chemistrys have different ESR (Effective Series Resistance)

For example, a typical 1.5v Alkaline may read 1.5v - but when you attempt to pull 4A thru it, the battery voltage will suddenly go down since the internal resistance of the battery starts becoming a part of the circuit

NiCads although having a slightly lower voltage (1.25v instead of 1.5v); have a lot lower ESR and as such are better for high current applications

You can sometimes find the ESR of a battery in the datasheets, but you can also measure it as well

 

[Ferrous]

Alkaline 1.5V is a misnomer that's here to stay. They start at 1.5V per cell, but aren't considered dead until they reach 0.9 Volts per cell. Their average voltage over the life of the cell is 1.2V. NiCd may be "hot" when they come off the charger, but quickly come down under 1.3V and are considered spent when the reach 1.1V. Want more? Look at the side of the bulb in your torch. For two cells it'll be 2.4V, not 3.

 

[FalconFX]

I've driven some of my NiMHs down to .8V with the ArcLS with no repercussions in recharging them. Initially, they can start out as high as 1.41V... The CCrane can bring them up to 1.55V in charging them.

 

[emitter]

jdriller- there's quite a good evaluation of most every known NiMH AA on this page:
http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM
The methodology is fair and consistent, the reviewer tests packs of 4 down to 4V under ~1A loads. This is supposed to simulate digital camera use, which is the reviewer's primary interest.


(yeah- long time listener first time caller...)

-ned

 

[jdriller]

Thanks everyone /ubbthreads/images/graemlins/thumbsup.gif

I knew I could get an answer here. I can see that there are much more variables than voltage.

I read the "battery shoot out". Great resource for the best batteries.

Cheers to all. /ubbthreads/images/graemlins/smile.gif

 

[shiftd]

It is best to runs Nixx just until .9 especially if you are using multi cells compartment. This way, you should be able to prevent reverse charging and thus increase the battery life quite a bit. While on ALkaline, it is better to leave it when it reach 1.0 V, too prevent leakage.

 

[obeck]

[ QUOTE ]
jdriller said:
I don't know much about alkaline & Nimh, hopefully you guys can help.

I tested the voltage on a regular battery, about 1.57 volts. Then I tested a Nimh out of the charger, 1.42volts. It then dropped to 1.38 volts, very quickly.

Why the difference? Almost 1.6 for a duracell and 1.4 for a Nimh. Some people say not to use Nimh in a light. Why? Is is because of the mA? Do the Nimh have much more mA? I'm just looking for a basic explanation.

Thanks.

[/ QUOTE ]

Basic explanation...

Batteries are an electro-chemical reaction. Usually, they consist of two compounds that are ideally suited to swap electrons across a medium. Galvanic action is like this. There is a "potential" between any two metals or battery formulations. Alkaline batteries have a 1.5 volt potential, and a whole slew of devices and methods were built around this 1.5v "standard". In the last twenty years, companies have been trying to build rechargeable batteries... batteries capable of swapping the electrons back when a higher external voltage is applied.... that conform to this 1.5v standard. The first real success was Nickel-Cadmium batteries. They max out at about 1.25 Volts and dont have a whole heck of a lot of capacity. Additionally, they are poisonous to the environment. Nickel-metal hydrides have a higher potential still (though still not 1.5V), have greater capacities, no memory, etc.
So thats why the different voltages. Neither nicads nor nimh batteries have a 1.5 V potential, but they come close enough for most uses. Additionally, see the other note that talks about battery resistance (when current is flowing through the battery, the battery acts as a resistor and some of the batteries energy is spent by the battery itself).