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