🚀 go-pugleaf

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. 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. 

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. 

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.


>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.


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.


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. This has been proven to increase 
the distance of an electric vehicle by roughly 30%. (A study of Lead-Acid 
batteries under simulated electric vehicle service - Journal of Power Sources, 
10 - 1985).


Regards

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