🚀 go-pugleaf

R.Bramley@minerals.csiro.au (R.Bramley) wrote:

>will1000@ix.netcom.com (Will Stewart ) wrote:
>>(Bernard Debreil) writes: 
>
>>OK: to your opinion, what, on an EV, is a replacement for the fuel 
>>gauge of an ICE ?  On the only EV that I have seen from close, it was 
>>a mere voltmeter. 
><SNIP on how to use a voltmeter as a energy guage> 
>>Besides, it is more 
>>complex than that, since the voltage versus % charge curve is nothing 
>>close to a straight line : there is a long flat around 2.1 volts or 
>>so, the length of the flat, and its range of co-ordinates being 
>>subjected to changes as the cell wears. When all 60 cells of the 
>>battery are new, this system is likely to work fine. But, the problem 
>>with batteries, is that they wear, they deteriorate, unevenly. Some 
>>elements die a lot sooner than others  (this is even worse with NiCad 
>>elements). Therefore, after awhile, the voltages to be relied upon for
>>full capacity, half capacity, or 1/4 capacity, will change, and the 
>>system will not know what figures to rely upon. 
>
>If the battery is discharged at constant current then the discharge curve is 
>fairly close to a straight line. 

All depends on what curve you are talking about. If it is that of

			y = f(x)

where "y" is the voltage of the battery in open circuit, and "x" is the
Kilowatt hours having been used since the battery was fully charged,
then the discharge curve is not a straight line, by a long shot. You
surely know that a lead battery cell has an almost flat discharge at an
almost constant voltage for most of its discharge, the voltage changes
occurring only during the 10% 


 2.2v   .
             .
               .
  2.1 v          .
  2.0 v                   .        .        .        .     .      .     

.
  						     .
  1.7 v						        .
						          .		

          100%       80%     60%     40%     20%     10% remaining capac


   
>However since electric vehicles draw 
>different currents at different times (high currents when accelerating, low 
>when going down a hill) the battery voltage can not be used as a fuel guage 
>since the battery voltage will drop lower with greater currents being drawn. 

It is true that, upon trying one such vehicles, I haven't noticed that
the "gauge" (which I thought was a mere voltmeter) moved when
accelerating... However, I may not have watched carefully enough... I
tried this only once, and for a few minutes...
>
>On the other hand if the batteries do not have a load across them (ie sitting 
>at the lights) then the battery voltage can be used as an indication of 
>roughly their state of charge. If the battery voltage is 1.9 V then you know 
>to charge it when you get home. 

If I am lucky enough to get there in time...
>
>The state-of-charge of the battery bank in the car is calculated by measuring 
>the amps being drawn from the battery, multiplying this by the time (100 amps 
>for 3.6 seconds is 0.1 ampere-hour (Ah)). This Ah value is subtracted from the 
>total capacity of the battery bank. So if you have driven a car with a 
>capacity of 500 Ah for 1 hour at constant power (50 amps) then you have used 
>50 Ah out of 500 - your batteries are 90% charged. Again due to the nature of 
>batteries (chemical changes etc.), this should only be used as a good 
>indication.

Yes, it surely is a much better idea than simply measuring the voltage,
since nowadays, computers can easily integrate the amps having been
drawn. However, do you know that such process is, in fact, what is being
used by at least some EV manufacturers ?

Much better. Still, there is a lot of uncertainty on the total
kilowatt-hour capacity of the battery. This indeed is a parameter which
will greatly vary while the battery is coming of age, it will also vary
a great deal according to the rate of charge to which the battery has
been submitted. In any case, it will be a lot more difficult to figure
how many miles can be driven at a given time... and this knowledge is
all the more important when the autonomy is small, which is the case
with EVs.
>
>
>>Besides, batteries do not hold charges forever, and, after a bit of
>>wearing, they will not hold charge for more than a few days. This mean
>>that one will not be able to rely on how many miles have been driven
>>since last recharge, especially if these miles have been driven over
>>several days.
>
>Batteries do self-discharge but this would not be a problem with electric 
>vehicles as the vehicles would be used, thus charged regularly. Self-discharge 
>is only a problem if batteries are left for months without charging. If a 
>battery is self-discharging over a few days then there is a serious problem 
>such as a shorting.

I can tell you that, at least in the case of Ni/Cd batteries, it is not
uncommon at all to have them fully self discharging over a few days,
even when the batteries are only three months old...
>
>A few  points which also need to be taken into account when discussing 
>electric vehicles and ways to extend service between charging are:
>
>
>Improved charging algorithims - by altering the charging methods from constant 
>current - constant voltage to pulsing when the battery has reached 
>top-of-charge-voltage allows charge to be replaced quicker.
>
>
>Solar cells can be made flexible so that a sheath of solar cells can be placed 
>on vehicles while still maintaining aerodynamic efficiency. Current solar 
>cells have fixed narrow bandwidths accepting energy in a narrow wavelength 
>range. Improved solar cells are almost availble which utilise much greater 
>amounts of the available light by accepting larger bandwidths.

Oh well... Have you calculated how long it would take to recharge an EV
battery, with solar cells covering the surface of the car ?   I haven't,
but I just remember a sporting competition last year in Australia, with
solar powered EVs. They were very light vehicles, almost the weight of a
bike, covered with a huge surface of cells... and it was done in the
Australian Summer... Interesting though, for sport purposes... but not
practical. If you do not agree with this, please show figures and
calculations.
>
>
>Regenerative braking systems covert the energy normally dissapated during 
>braking back to electrical energy which can then be used during acceleration, 
>thus reducing the load on the battery bank. 

Sure. This is a good thing with EVs. Also, they don't use any power at
all while stopped in the traffic jam or at traffic lights... However,
don't expect  the energy being recovered in downhills to be any close to
100% of that needed to climb the same hill...

>This has been proven to increase 
>the distance of an electric vehicle by roughly 30%.

Sure... but when they say that such vehicle can drive 60 miles between
reloadings, these 30% are included in it...

> (A study of Lead-Acid 
>batteries under simulated electric vehicle service - Journal of Power Sources, 
>10 - 1985).
>
>
>Regards
>
>  R.Bramley                 _--_|\     R.Bramley@minerals.csiro.au
>  Division of Minerals     /CSIRO \    http://www.dmp.csiro.au/
>  tel +61 03 9647 0372     \_.--.*/    MRA# 321         DoD# 900R
>  fax +61 03 9646 3223           v
>  Energy Storage Group.                I speak for only myself.
>
>Reality is a cop-out for people who can't handle drugs.

Bernard DEBREIL
---------------
Le Chesnay (France)
Home Page: http://www.teaser.fr/~bdebreil/
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