[Elecraft] SLA Battery charge level

[G3VVT at aol.com]

In a message dated 12/12/04 00:35:05 GMT Standard Time, spituch at ev1.net  
writes:

When the  battery is exactly at 13.8 volts, there would be
no  current.



In practice found over the many systems worked on professionally this is  not 
entirely correct.
 
After the charging system comes out of the absorptive charge phase where  the 
voltage has been allowed to rise above the normal bulk charge voltage for a  
limited time and now reverts to the float voltage, the battery indeed does  
not take any charge. It can even supplement the  supply voltage and current for 
a short while after the main charge  cycle.
 
However if one monitors the battery over a longer period after the float  
charge period has started the battery will start to take a small charge that  
eventually settles on the 25 to 50 mA region for small SLA's. It all depends on  
the float voltage and the size of the SLA, but with a float voltage of 13.8V 
and  a 12V SLA does definitely occur.
 
Security and fire alarm technicians when carring out routine maintenance  can 
note the current taken by SLA's in the alarm panels under float conditions  
as this can give an early indication of the forthcoming failure of the  SLA. 
For critical systems under a maintenance contract they sometimes replace  the 
SLA's at 2 to 3 year intervals to ensure system reliability under all  
conditions.
 
A good source of SLA's that still have a useful life left in them if you  can 
make contact with one of the technicians and I did!
 
Bob, G3VVT