[Milsurplus] Cells, Batteries, what's up doc?

[Alan Tasker]

Since Military portable devices employ batteries, I thought it would be=20
interesting to at least a few folks to see what insight we can gain from a=
=20
review of battery technology over the past 60 or so years. This article is=
=20
an attempt to put in one place a few facts about some of the more common=20
types, most of which have been used by the military. Comments are welcome.

In a subject this complex, a divide and conquer approach is preferable. The=
=20
first division is between Primary (use once, throw away) and Secondary=20
(rechargeable). A second division can be made between =93mainstream=94 and=
=20
=93specialty.=94

First, what are the relative advantages of Primary and Secondary? Primary=20
batteries offer the following two main advantages.
=B7       They have more energy density than secondary cells, so if long=
 life=20
between changes is a necessity, then this is the type to use.
=B7       The modern type (not those from the 40=92s and 50=92s) have low=
 self=20
discharge rates, so if the energy has to be there when you need it=20
(flashlight for instance), use this type. Rechargeables have a=20
self-discharge rate of between 1%/week and as high as 5%/day.

  Secondary batteries, on the other hand, offer the following two main=20
advantages.
=B7       When you need sustained high current output, to start a motor for=
=20
instance, secondary batteries are the way to go (but not all types are=20
recommended for this service).
=B7       Life cycle costs are lower because of the ability to keep charging=
=20
and reusing them.

Primary/Mainstream
Back in the 1940=92s, the =93flashlight=94 battery (technically, a single is=
 a=20
=93cell=94, multiple cells make a battery), otherwise known as the Leclanche=
=20
cell (1.5V/) was the mainstream primary product. It didn't offer such a=20
flat discharge curve, nor did it have a low self-discharge rate. Since that=
=20
time, however, the Alkaline cell (also 1.5V/) has greatly improved things,=
=20
and is the mainstream consumer product. After a flirtation with Magnesium=20
cells (1.75V/), which offers excellent high temperature storage=20
characteristics, the military moved on to Lithium Sulphur dioxide cells=20
(3V/). Offering about the same capacity of present day alkalines (but more=
=20
than the older alkalines), a super light weight, and a super low=20
self-discharge rate, their potential for releasing poisonous gas will most=
=20
likely be their downfall. They are capable of outputting high currents, but=
=20
cell heating is of great concern because of the possibility of cell rupture.

The new kid on the mainstream block is the Lithium Manganese Dioxide (3V/)=
=20
chemistry, offering a 25% to 50% increase in capacity over the Lithium=20
Sulphur Dioxide cell, and low toxicity. Both of these Lithium chemistries=20
offer superior low temperature operation, and very light weight. Capacity=20
testing, however, is problematical. There is another type of cell just now=
=20
appearing, the LiFeS2 (1.5V/). Maybe this is a lithium chemistry that can=20
replace the so-called =93flashlight=94 cells, i.e. alkalines?

Primary/Specialty
Back in the 40=92s again, the Mercury battery (1.35V/) was a specialty cell,=
=20
offering long (2-3 years) of shelf life. It was the chemistry of choice for=
=20
many years, especially in the rescue radio field. It was only replaced in=20
the relatively recent PRC-112 with LiSO2. But environmental concerns plus=20
better chemistry offers more today. Even the LiSO2 is being replaced, as=20
noted above, with the LiMnO2.

For completeness, we should mention water-activated batteries=20
(magnesium/silver chloride for example) as used in sonobouys and such.=20
Also, thermal battery types as used in missiles.

More on the commercial side, there are also a host of specialty cells for=20
watches, hearing aids, etc., such as silver oxide, zinc air, etc.

Secondary/Mainstream
Back to the WWII period again, we find the Lead-Acid cell (2v/) as the=20
chemistry of the moment. It has remained in its place as the leading motor=
=20
start battery, and it is hard to beat. The lead-acid cell works best when=20
it is kept charged in what is called a =93float=94 application, as in=
 vehicles=20
or in emergency lighting applications. Although there are deep discharge=20
types available, immediate charging is necessary in order to prevent=20
sulphation, which destroys the cell. There are battery=20
charging/reconditioning products on the market that claim to rejuvenate=20
sulphated cells. I have no first hand knowledge about the veracity of these=
=20
claims.

Perhaps it was the aircraft industry that fostered a lighter replacement,=20
and thus enters the Ni-Cad cell (1.25V/).  Unlike the lead-acid, these can=
=20
be completely discharged, and in fact, can give better service this way.=20
There are two types of plates used. Most of the non-sealed cells did not=20
use sintered plate construction. The sealed type does use sintered plate=20
construction, which yields a higher capacity cell. However, when the=20
sintered plate types are used in float applications, they tend to remember=
=20
this light usage, and do not deliver their full rated capacity (so called=20
memory effect). Even the airline industry is tiring of the NiCad problems,=
=20
and is looking at either lead acid or some form of Lithium Ion.

The above two chemistries offer the highest amps-in-a hurry of anything=20
available, both in discharging and in recharging. The following two types=20
are not so robust in these areas, at least today.

The Nickel Metal Hydride (1.25V/) (is rumored to have the memory effect=20
too, but it may just be a settling out kind of thing) and the Lithium Ion=20
(1.5-3.9V/ depending on exact chemistry) have about the same energy density=
=20
and were developed at about the same time. It appears that the early=20
leading but somewhat problematical NiMH will give way to the Lithium Ion.

Secondary/Specialty
The Edison cell (1.2V/) has just one overriding feature; you just about=20
cannot kill them. Railroads used them for years, just changing the=20
electrodes and/or electrolyte. Did the phone company use them in their CO?

The silver cell (1.5V/) has been used by both the Soviet and the US=20
military. They offer tremendous capacity at a fairly light weight. The=20
downside: They do not take many charge/discharge cycles (dendrite growth=20
causing plate-to-plate shorts). Thus, there is not much life over which to=
=20
amortize the high cost. The Soviets added some Lithium Hydroxide to each=20
cell, perhaps leading to a longer life???

In Space work, there is the rechargeable Nickel Hydrogen chemistry. Each=20
cell must be fitted with a =93bypass module=94 (contains diodes) so as to=
 allow=20
the battery to continue to operate (both charging and discharging) in the=20
face of an open cell.

Lastly, there is the battery with =93super cap=94 included. The capacitor=20
supplies the amps-in-a hurry, so that any suitable battery chemistry can be=
=20
used. Products are beginning to appear on the market.

With sincere thanks to Brooke Clarke for his helpful comments.

More information on Primary chemistries can be found at=85=85
http://electrochem.cwru.edu/ed/encycl/art-b02-batt-nonr.htm
http://www.powerstream.com/BatteryFAQ.html#AlAir

More information on Secondary chemistries can be found at=85.
http://www.powerstream.com/Compare.htm

Information on battery charging can be found at=85.
http://www.powerstream.com/tech.html


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