From: John Tserkezis 
Reply-To: Fidonet AVtech Echo 

David Drummond wrote:

> Bob has only recommended "thick" :-)

  It's just a matter of minimising loss.  When you're dealing with low 
voltages, what might be a "little" voltage drop is major,
relative to six volts.

> The cells do have an "emergency" vent.

  They're better than just a normal air pathway to the outside world like 
normal lead acids.  The only orifice is a dense sponge-like material, where gas 
can flow back and forth, but with a bit of resistance.  They do it that way, so 
if there is a spark, only the gasses on the outside of the battery case go pop, 
the flamefront cannot travel back into the battery case making it go bang.
  Because if they were to go bang, they go bang in a big way.

> I thought it charged harder when the cells were flatter.

  Yes and no.  If the cells are REALLY flat, as in abnormally flat, it starts 
with a very slow rate.  Hitting dead cells with full current will pretty much 
kill them.

  This slow rate persists till the battery reaches "normal flat".

  From here, it is charged with a voltage limited, current limited supply.  The 
voltage is limited to about 2.4 volts per cell, depending on temperature. (this 
is why the temperature sensor is there).  If the cell is flat, it is likely 
this level of voltage will result in an unsafe (for cell life) level of 
current.  So the current is limited too (initially resulting in a lower charge 
voltage).

  Towards the end of charge, it stays at the maximum charge voltage, and the 
current slowly drops to nothing.

  Once the current drops to nothing (or near nothing), it switches to a 
"trickle" charge mode, which in essence is exactly the same as
the bulk charge 
mode, but at a slightly lower voltage- about 2.3 volts per cell.

  It maintains this point indefinitely (or at least till you start to draw power).

  Exactly what charge mode (initial, bulk or trickle) is used, as well as the 
maximum current, and maximum voltage supplied is, all depends on temperature, 
how much charge is in the cells, and how much juice the sun can supply.

  Now THAT is an intelligent charger.

  Bob is right to be skeptical of "intelligent" chargers though,
I say there 
are different levels of "intelligent", varing between smart and brain-dead.

>  JT>   Oh yeah they do.  It's classed as abuse- they do release gas.  
>  JT> Actually, it's part of the normal charge cycle.  They are charged to 
>  JT> higher than the recommended cycle charge point, (higher than long term 
>  JT> voltage) and kept there for a short period of time.  Then the charge 
>  JT> voltage is brought down to the normal float point.

> OK.

  That went over your head?  Should I attempt further technical explanation 
like above? :-)

>>> Nowhere to stick the hygometer....

>>  Ahem.  This is AVTech.  Need I remind you where it needs to be stuck?

> I thought of that, but I don't think it will indicate the state of charge of
> the cells under discussion.....

  It would if you were to lick the battery terminals at the same time.

>  JT> Tell Bob how much they cost.  Let him do the solar economics math 
>  JT> with a life reduction of more than half.  I would not recommend in 
>  JT> anything that would reduce their life more than normal operation.  It 
>  JT> will cost you dearly.

> I bought the cells at mate's rates for $1260.40 for the six.

  Bargain.  You have nothing to worry about then. :-)

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