piclist 2002\01\29\055137a >
Thread: Prolonging life-time of alkaline cell
picon face BY : Russell McMahon email (remove spam text)

I am using rechargeable alkalines (RA's) in a consumer product that is in
moderate volume production so far.
I don't like them but they were expressly required by the client.
You can make them leak enthusiastically if you abuse them.
At very low trickle rates such as Jinx originally proposed I would suspect,
but only suspect, that they would potentially behave much better.

It is important to note that not all RA's are created equal. My client tried
a number of brands and reported inferior lifetime performance from some
brands. While the patents are held by a Canadian firm they are made by a
number of manufacturers including Rayovac.
The brand my customer has currently chosen are Alcava made in Korea.

A good site with a reasonable amount of simple detail including performance
graphs under various operating conditions.


While these cells have a high open circuit voltage when new they droop very
rapidly in use and have very poor recharge lifetimes if deep cycled or
rapidly discharged. Rayovac publish an excellent document covering all
aspects of their product "The High Performance Alkaline Battery in a
Rechargeable Alkaline System" . 430 kB PDF - well worth downloading by
anyone ever liable to use them in any capacity.

While the literature usually proposes a charge/pause & test / charge
regimen, a simple charger that limits peak charge voltage and decreases
current as the terminal voltage rises will meet many requirements. This does
not violate the spec sheet charging requirements - despite the use of pulse
type charging in most applications. A simple way of achieving this is a
series resistor from a higher voltage supply and a zener diode clamp. A
slightly more complex scheme with a zener and 2 resistors provide a dual
slope charging curve and max terminal voltage control. This has a faster
charging time than simple 1 resistor chargers but less complexity than pulse
chargers (work out how to do it yourself :-) ).

I am not of the opinion that monitoring of individual cells is required for
typical applications. This may be desirable in high performance apps.

Rayovac target terminal voltage is 1.65v/cell.

I suspect that Jinx's idea of using one such cell with a very light trickle
current would be an acceptable backup solution.

I would query the claimed 10 year alkaline lifetime - this is certainly
longer than most domestic cells "use by" lives (about 5 years) although I
suppose they want these to have some useful life when actually used !.

Note that the ability to recharge STANDARD alkaline cells was widely
reported for some years before the rechargeable cells appeared and a recent
report suggests that a modern standard alkaline will in fact recharge every
bit as well (or as badly) as the custom built rechargeable alkalines.

I consider that they are not a nice battery for serious secondary cell use
unless the application heavily targets some of their advantages. This is the
case in my customers application where long shelf life behind charges and a
high terminal voltage per cell and lowish cost are very desirable.


                 Russell McMahon


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