NiMH battery question

[charlielinn]

Just curious..does anyone know the reason that NiMH (nickel metal hydride ) batteries are rated at
only 1.20 or 1.25 volts output, compared with 1.5 volt rating of alkaline, nicad, etc???
Must be a reason, but damned if I know what it is.

 

[W2NJS]

I believe you've made a false assumption about what kind of battery has what for a design center voltage. The design center of NiMH and Nicad batteries is usually understood to be 1.33 volts per cell at full charge or even a shade higher. The difference between these batteries' cell voltage and the old zinc-carbon batteries is attributable to the chemical makeup of each cell. The voltage of a single Lithium Ion cell is about 3.6 volts.

 

[charlielinn]

Thanx...and I just noticed that my charger is rated way lower ( current) than is recommended for
the batteries in question..so will upgrade it and see what happens. thanx again for the comeback..C

 

[ST-Bob]

What do you mean "rated lower" than your batteries? The MaH rating of the battery is its capacity (think size of the bucket). A charger with a low charge rating, say 50 Ma, is just going to charge them slower (think a smaller hose filling the bucket).

You usually don't want to repeatedly charge any battery at its maximum rated recharge rate as it will get hot during charging. Heat kills batteries. Even those rated as 1-hour charge type don't really like it very much. You're much better off with an intelligent charger which takes 5-10 hours to recharge them.

 

[CoolCat]

Just curious..does anyone know the reason that NiMH (nickel metal hydride ) batteries are rated at only 1.20 or 1.25 volts output, compared with 1.5 volt rating of alkaline, nicad, etc??? Must be a reason, but damned if I know what it is.

The reason why is quite simple; Different elements that makeup the cell have different chemical reactions which produce slightly different output voltages.

Alkaline and Zinc-Carbon cells are ~1.5v, NiCd and NiMH cells are typically ~1.2v. That voltage "rating" is basically the cell's fully charged / starting voltage. A new zinc-carbon or alkaline cell starts off at ~1.5v but that starting voltage tends to drop fast as it discharges. A fully charged NiCd or NiMH cell starts off at ~1.2v and tends to hold close to that starting voltage longer as it discharges. So while an alkaline cell starts off at a higher voltage, in many devices it will fairly quickly drop below the average voltage of a NiMH cell.

 

[nanZor]

The other interesting thing is where did the 1.5 volt standard come from?

It started in 1866 with the "LeClanche" wet-cell, which was popular with low-voltage household items like doorbells, etc. The replacement for this cheap little wet-cell turned out to be the common zinc-carbon later on....

So here we are 145 years later using that same voltage standard.

http://en.wikipedia.org/wiki/History_of_the_battery

I think all of us here would be just as fascinated with batteries as our great-great-grandparents were.

 

[ST-Bob]

It's not like someone pulled a number out of their hat and it was 1.5 volts. The voltage is a result of the chemical process and materials used. Other popular chemistry combinations used in batteries are Nickel-Cadmium (1.2 volts per cell), Lead-Acid (2 volts per cell), Nickel-Metal-Hydride (1.2 volts per cell) and Lithium-Ion or Lithium-Polymer (3 to 3.7 volts) and lithium-copper or lithium-lead chemistries (1.5 to 2.0 volts per cell).

Cell voltage varies all over the map with the lithium variants and the devices they're used with are designed around the battery, not the other way around. "Voltage-Compatible" Lithium chemistries are often used in one-time-use cells to provide direct replacements for carbon-zinc or alkaline batteries in high current uses like digital cameras etc. or where long storage life and low self-discharge are needed.

 

[nanZor]

It's not like someone pulled a number out of their hat and it was 1.5 volts. The voltage is a result of the chemical process and materials used.

Precisely. Although back in the 1800's voltages varied from about 1 to 2 volts depending on the make and chemistry of these consumer type cells. I can't imagine mom and dad maintaining them - what a chore!

Even better than Wikipedia is a book I recommend:

"The Battery - How Portable Power Sparked A Technological Revolution"
by Henry Schlesinger
Harper publishers
2010
ISBN 978-0-06-144293-3

Fascinating look at how we got where we are. Or where we are just stagnating.