[Elecraft] Two battery questions - K2 portable operations

[G3VVT at aol.com]

In practice it is possible to use 12V batteries in parallel without  any 
problems apart from a possible one of charging where one battery could hog  the 
charge current at the disadvantage of the other.
 
Had number of 48V solar powered communication systems at my place  of work to 
maintain for many years that had banks of batteries in  parallel. One of 
5400AH capacity used 4 series banks of 6 V cells in  parallel and another of 
7500AH capacity had 2 series banks of extremely large 2V  batteries in parallel. 
These get over the problem of current sharing when  charging by applying a time 
limited equalise charge on a daily basis when  the normal charging cycle was 
complete (can be 120A on a clear day with the  larger system!). The only 
concession to separating the banks were that isolation  switches fitted to each 
separate bank of batteries for maintenance  purposes.
 
The problem that does happen in the end is that as the batteries reach the  
end of their life they can either go high internal impedance which effectively  
removes that particular battery out of the chain and drops the system  
capacity or internally S/C on individual cells which can drag down the whole  
battery bank voltage. However this is normally after much usage. The better you  
maintain the batteries by careful charging and not over discharging, the less  
likely this will happen. Telecom sites normally have a low voltage  disconnect 
(LVD) to protect the batteries from being over discharged. With a K2  or 
similar it would be a matter of monitoring the battery voltage to ensure that  it 
does not fall below 1.75V per cell or 10.5V on a 12V battery.
 
Even with placing batteries in parallel not being a problem, it would  
probably be better to go for 2 X 6V batteries in series particularly if a 6AH  
version could be obtained with the ease in charging with this  configuration.
 
If there is any interest by anybody using 12V battery power, I have made up  
a simple, small LVD circuit with a relay, transistor, zener diode(s) and a few 
 other bits and pieces on strip board that removes the battery from  circuit 
when the 12V battery voltage falls to 10.5V. Have fitted them to the two  
repeaters I maintain here in the UK to protect the back up  batteries from being 
over discharged under power failure. Have the scanned  schematic available for 
e-mail if anybody wants to build such an  item. 
 
Bob, G3VVT
K2 #4168
IRLP node 5140