Charging batteries
\n after reading some of the messages under “battery help”,I thought I might
\nbe able to clear up some of the popular misconceptions about batteries.
\nMany of the problems blamed on memory or shorted cells are often problems
\nresulting from improper charging or discharging.It’s a very grey area and
\nclearcut answers are hard to find,you will soon find out why!
\n One question often asked about ni-cad’s is;”should they be deep discharged
\nto condition the cells”.Heres where that grey area starts,most manufacturers
\nsuggest some form of deep discharge occasionally but never a total discharge.
\nMost devices(ie.laptops,camcorders)have internal circuitry to sense when
\na battery has discharged to or near its rated capacity and disconnect either
\npartially or totally from the load,don’t drop a load resistor across the
\nterminals and kill it,this will cause cell damage to one extent or another.
\nMost importantly begin charging shortly after you have completed discharge!
\nI will adress lead acid batteries later in this file,but by all means don’t
\ndeep discharge a lead acid cell,doing so will definetly harm it and possibly
\ndestroy it.Again charging shortly after discharge is essential to long
\ncycle life of a lead acid battery as well.
\n Ni-cad chargers exist in many forms.They can be a simple as a transformer
\ncap and diode or as complicated as microprossesor based systems that cost
\nup and over $1000.The more extravigant,the better the results,usually.
\nThe problem is when to terminate charge,overcharging a cell results in
\nventing.This is when a cell builds pressure internally as a result of
\novercharging and releases elecrolite into the air.Over time this can cause
\ncell damage resulting in a significant decrease in cycle life.(# of times
\na battery discharges and subsequently recharges to full capacity)
\n The following graph shows the voltage profile of a single ni-cad cell
\nduring a charge cycle.<\/p>\n
l x l x
\nl x l x x
\nl x l
\nl voltage x l
\nl x l
\nl x l
\nl x l
\nl x l
\nl x capacity l
\nl_______________________________________________________l______________________
\n0% approx 100%<\/p>\n
Notice how the voltage peaks near full capacity and then starts to drop.
\nSo a simple assumption would tell you that you could sense that peak
\nand drop (commonly referred to as the negative delta v) and terminate charge,
\nright?……wrong!
\nThe promblem occurs when you put cells in series (a battery)to get appropriate
\nvoltages.They all have slightly different voltage profiles during the charge
\ncycle,like this:
\n t1 t2
\nl x=cell 1 l l
\nl o=cell 2 l l
\nl t=cell 3 l o o xt x t l
\nl o x t o x t
\nl oxt o x t
\nlvoltage xot
\nl xot
\nl xot
\nl xot
\nl xot l ??? 100% ??? l
\nl xot l l
\nl xot capacity l l
\nl__________________________________________________l____________________l_______
\n0% l 100% l<\/p>\n
As you can see,things get kinda grey!<\/p>\n
This graph is unrealistic in the sense that it’s impossible to determine
\nindividual cell voltages when they are connected in series to form a battery.
\nWhat actually happens is you get a very mushy curve.If you terminate early
\nat position t1(see graph)most of your cells will be below full capacity,
\nterminate at t2 and most of them will be in overcharge.
\n There are a number of methods of charge termination,some simpler than
\nothers some more effective than others,these are a few of the popular ones.
\n -temperature as a cell accepts current,it has a certain effeciency
\nat wich it turns incoming energy into stored chemical energy.(cell efficiency)
\nmost cell systems have been optimized over the years to a point were they
\nare all very efficient.Were this comes into play is simple,energy in = energy
\nout.If a cell has reached full capacity,the incoming energy from the charging
\nsystem has to go somwhere,that somwhere is heat!So,what you can do is sense
\nthat increase in temperature and terminate charge.sounds simple but,there
\nare numerous drawbacks.first of all,you have to have a temprature sensing
\ndevice in close proximity to the cells,this is impracticle in most cases.
\nSecondly you again have the problem of cells being different from eachother
\nyou may terminate to early or maybe to late,your guess is as good as mine!
\nLastly if your battery is located near your charging system,your near
\ntransformers,transitors,diodes and other heat generating devices,how do you
\ncompensate for this???
\n -negative delta v as discussed before,this has many drawbacks but is
\nthe more popular among higher grade systems.
\n -coulometric control basically,a coulometer that keeps track of how
\nmuch energy gets discharged and then puts it back in,high cost and
\ninaccuracies in mesuring equipment keep this one in the ultra high
\ngrade market.
\n -constant voltage don’t even think about it!every manufacturer
\nstrongly suggest anything but!
\n -constant current the most popular,requires alot of guess work
\non the part of the manufacturer.you have to select a charge current
\nappropriate for capacity of the cells.To high and you overcharge,to low
\nand it takes you 12+ hours to reach full capacity,if at all.
\n combinations of any of these are also very common (exept con.v) and
\ncan result in some nice systems.
\n So far I’ve told everything wrong with ni-cad chargers,to be truthfull
\nmany of these can work ok.Most manufacturers claim ni-cad’s can cycle up to
\n10,000 times if treated properly,The problem is that treated properly
\nmeans making many compromises that are unrealistic or impossible for
\nmost applications.The last form of charging I’d like to discuss is algorithm
\nor “pulse” charging.It employs alot of the methods listed above,but with
\na twist.<\/p>\n
A basic pulse charge algorithm looks somthing like this:<\/p>\n
l ——————–1 second————————l–next cycle—–
\n l l
\n l l
\n xxxxxxx—-charge level 1 —xxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxx
\n x x
\n 0 charge level————————- x xxxxxxxx —————–
\n x x
\n x x
\n xxxxx ———– -2.5 discharge<\/p>\n
To start this a constant current charger,the output is determined by
\nThe capacity of the cells.Charge level one represents current being passed
\nto the battery at a specified level.Lets say for now that that level is 1
\namp.(charge level 1=1 amp)For a period of time,lets say 985ms we are putting
\ncurrent into the battery.(charge level 1)After that time expires we go to
\npart two.(-2.5 discharge)At this point,for a short period of time,say
\n5ms we discharge at 2.5 times the level we charged at.(or in our case,2.5 amps)
\nthis serves one major function,”burping”.When a ni-cad cell is charging one
\nof the gasses being generated internaly is oxygen,it forms and sits on the
\ntwo plates of the battery.The problem happens when crystals begin to form
\nin the cell and use these bubbles as a bridge between the two plates(anode
\nand cathode)of the cell,if they complete the bridge you now have a dialectric
\nshort,or a dead battery!What happens during that 5ms pulse or “burp” can
\nsave alot of trouble,it breaks the bubbles and allows the oxygen to be more
\nreadily available for the continuing chemical reaction.This accomplishes a
\ncouple of things,as a result of the oxygen situation mentioned above you can
\ncharge at much higher levels without cell venting.(fast charge)secondly,you
\nprevent shorts,giving you longer cycle life.Just as a note of interest,the
\ncrystals that form in the cell,shorting or not,cause the chemical conversion
\nthat takes place during charging and discharging to slow down,somtimes to a
\npoint where the cell can’t function anymore.True “memory”has yet to be proved
\nand this is what is really happening when your battery quits!Best way to
\nprevent it is to keep your battery charged when not in use.
\n Getting back to pulse charging,the last segment in the algorithm is
\ncalled the quiet window.It serves two basic purposes,it allows the cell
\nto chemically recuperate after discharging and it provides a period of time
\nto collect information from the cells.(such as looking for – delta v).after
\nthe quiet window the loop begins again at charge level 1,it runs continuosly
\nat 1 second intervals until terminated by some method of charge termination.
\nOnce the cell reaches near full capacity some type of maintanance charge is
\nusually applied,such as running an algorithym at a lower frequency and
\namplitude,this helps top off the cells and keep them fully charged.
\n This technology is fairly new and should be introduced into the various
\nconsumer markets soon.My only advice to people who are stuck with cheap
\nni-cad chargers supplied by manufacturers is to be more selective next
\ntime!Spending an extra $100 at purchase time will pay for itself if one
\nbattery last you 2 years as opposed to 2 cycles.<\/p>\n
comments or questions??
\n send replies to glenn sahlin<\/p>\n
