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Charging batteries
after reading some of the messages under "battery help",I thought I might
be able to clear up some of the popular misconceptions about batteries.
Many of the problems blamed on memory or shorted cells are often problems
resulting from improper charging or discharging.It's a very grey area and
clearcut answers are hard to find,you will soon find out why!
One question often asked about ni-cad's is;"should they be deep discharged
to condition the cells".Heres where that grey area starts,most manufacturers
suggest some form of deep discharge occasionally but never a total discharge.
Most devices(ie.laptops,camcorders)have internal circuitry to sense when
a battery has discharged to or near its rated capacity and disconnect either
partially or totally from the load,don't drop a load resistor across the
terminals and kill it,this will cause cell damage to one extent or another.
Most importantly begin charging shortly after you have completed discharge!
I will adress lead acid batteries later in this file,but by all means don't
deep discharge a lead acid cell,doing so will definetly harm it and possibly
destroy it.Again charging shortly after discharge is essential to long
cycle life of a lead acid battery as well.
Ni-cad chargers exist in many forms.They can be a simple as a transformer
cap and diode or as complicated as microprossesor based systems that cost
up and over $1000.The more extravigant,the better the results,usually.
The problem is when to terminate charge,overcharging a cell results in
venting.This is when a cell builds pressure internally as a result of
overcharging and releases elecrolite into the air.Over time this can cause
cell damage resulting in a significant decrease in cycle life.(# of times
a battery discharges and subsequently recharges to full capacity)
The following graph shows the voltage profile of a single ni-cad cell
during a charge cycle.
l x l x
l x l x x
l x l
l voltage x l
l x l
l x l
l x l
l x l
l x capacity l
l_______________________________________________________l______________________
0% approx 100%
Notice how the voltage peaks near full capacity and then starts to drop.
So a simple assumption would tell you that you could sense that peak
and drop (commonly referred to as the negative delta v) and terminate charge,
right?......wrong!
The promblem occurs when you put cells in series (a battery)to get appropriate
voltages.They all have slightly different voltage profiles during the charge
cycle,like this:
t1 t2
l x=cell 1 l l
l o=cell 2 l l
l t=cell 3 l o o xt x t l
l o x t o x t
l oxt o x t
lvoltage xot
l xot
l xot
l xot
l xot l ??? 100% ??? l
l xot l l
l xot capacity l l
l__________________________________________________l____________________l_______
0% l 100% l
As you can see,things get kinda grey!
This graph is unrealistic in the sense that it's impossible to determine
individual cell voltages when they are connected in series to form a battery.
What actually happens is you get a very mushy curve.If you terminate early
at position t1(see graph)most of your cells will be below full capacity,
terminate at t2 and most of them will be in overcharge.
There are a number of methods of charge termination,some simpler than
others some more effective than others,these are a few of the popular ones.
-temperature as a cell accepts current,it has a certain effeciency
at wich it turns incoming energy into stored chemical energy.(cell efficiency)
most cell systems have been optimized over the years to a point were they
are all very efficient.Were this comes into play is simple,energy in = energy
out.If a cell has reached full capacity,the incoming energy from the charging
system has to go somwhere,that somwhere is heat!So,what you can do is sense
that increase in temperature and terminate charge.sounds simple but,there
are numerous drawbacks.first of all,you have to have a temprature sensing
device in close proximity to the cells,this is impracticle in most cases.
Secondly you again have the problem of cells being different from eachother
you may terminate to early or maybe to late,your guess is as good as mine!
Lastly if your battery is located near your charging system,your near
transformers,transitors,diodes and other heat generating devices,how do you
compensate for this???
-negative delta v as discussed before,this has many drawbacks but is
the more popular among higher grade systems.
-coulometric control basically,a coulometer that keeps track of how
much energy gets discharged and then puts it back in,high cost and
inaccuracies in mesuring equipment keep this one in the ultra high
grade market.
-constant voltage don't even think about it!every manufacturer
strongly suggest anything but!
-constant current the most popular,requires alot of guess work
on the part of the manufacturer.you have to select a charge current
appropriate for capacity of the cells.To high and you overcharge,to low
and it takes you 12+ hours to reach full capacity,if at all.
combinations of any of these are also very common (exept con.v) and
can result in some nice systems.
So far I've told everything wrong with ni-cad chargers,to be truthfull
many of these can work ok.Most manufacturers claim ni-cad's can cycle up to
10,000 times if treated properly,The problem is that treated properly
means making many compromises that are unrealistic or impossible for
most applications.The last form of charging I'd like to discuss is algorithm
or "pulse" charging.It employs alot of the methods listed above,but with
a twist.
A basic pulse charge algorithm looks somthing like this:
l --------------------1 second------------------------l--next cycle-----
l l
l l
xxxxxxx----charge level 1 ---xxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxx
x x
0 charge level------------------------- x xxxxxxxx -----------------
x x
x x
xxxxx ----------- -2.5 discharge
To start this a constant current charger,the output is determined by
The capacity of the cells.Charge level one represents current being passed
to the battery at a specified level.Lets say for now that that level is 1
amp.(charge level 1=1 amp)For a period of time,lets say 985ms we are putting
current into the battery.(charge level 1)After that time expires we go to
part two.(-2.5 discharge)At this point,for a short period of time,say
5ms we discharge at 2.5 times the level we charged at.(or in our case,2.5 amps)
this serves one major function,"burping".When a ni-cad cell is charging one
of the gasses being generated internaly is oxygen,it forms and sits on the
two plates of the battery.The problem happens when crystals begin to form
in the cell and use these bubbles as a bridge between the two plates(anode
and cathode)of the cell,if they complete the bridge you now have a dialectric
short,or a dead battery!What happens during that 5ms pulse or "burp" can
save alot of trouble,it breaks the bubbles and allows the oxygen to be more
readily available for the continuing chemical reaction.This accomplishes a
couple of things,as a result of the oxygen situation mentioned above you can
charge at much higher levels without cell venting.(fast charge)secondly,you
prevent shorts,giving you longer cycle life.Just as a note of interest,the
crystals that form in the cell,shorting or not,cause the chemical conversion
that takes place during charging and discharging to slow down,somtimes to a
point where the cell can't function anymore.True "memory"has yet to be proved
and this is what is really happening when your battery quits!Best way to
prevent it is to keep your battery charged when not in use.
Getting back to pulse charging,the last segment in the algorithm is
called the quiet window.It serves two basic purposes,it allows the cell
to chemically recuperate after discharging and it provides a period of time
to collect information from the cells.(such as looking for - delta v).after
the quiet window the loop begins again at charge level 1,it runs continuosly
at 1 second intervals until terminated by some method of charge termination.
Once the cell reaches near full capacity some type of maintanance charge is
usually applied,such as running an algorithym at a lower frequency and
amplitude,this helps top off the cells and keep them fully charged.
This technology is fairly new and should be introduced into the various
consumer markets soon.My only advice to people who are stuck with cheap
ni-cad chargers supplied by manufacturers is to be more selective next
time!Spending an extra $100 at purchase time will pay for itself if one
battery last you 2 years as opposed to 2 cycles.
comments or questions??
send replies to glenn sahlin