[Elecraft] NiMH Battery Question

[G3VVT at aol.com]

In a message dated 04/07/05 19:56:32 GMT Daylight Time, cbw at io.com  writes:

There  are some small charge controllers out there, but a 
relay configured to  open when the battery reaches full charge is the  
simplest.


Reply:------------------------------------------------------------------
 
The problem with using a relay for solar charge control is the current  
needed to activate the coil of the relay. This could be as high as 100mA and  with 
a charge budget available of only 250mA you would have to select the relay  
used with care. If a solar charge controller was seen to be necessary I would  
have thought a solid state version similar to the type Don Brown is making the  
kit for would have been a safer bet for minimising power losses in the 
control  mechanism (pity these are only available in the USA as I could have  used 
one).
 
The usable solar power available for solar panels is usually calculated at  
8hrs per day, though does depend on the seasons further away from the equator.  
With 8hrs @ 250mA that would equate to 2000mA per day which is the total  
rating of the NiMH cells being used. The charge needed would depend on the  
initial state of the batteries and what the consumption of the radio was to  offset 
the available charge. Some device to monitor the battery voltage and an  
on/off switch would also work as a charge controller in this case at probably  
even less cost.
 
With regard to the point when full charge is reached with NiMH  cells, I was 
checking out a motorcycle racing ignition pack last week and using  a 
commercial charger built for the purpose, the end voltage for a 6 NiMH pack  was 
8.40V, which equates to 1.40V per cell. These guys can get exceedingly upset  if 
they are leading a race and the batteries die, so had to ensure that all was  
OK. The point with this type of cell and many others including gel cells is that 
 the initial voltage at the end of charge is higher than the nominal  voltage 
of individual cells, though the voltage soon droops with use to the  nominal 
voltage of approx 1.20 to 1.25V per cell. This carries on pretty  well 
constant until the battery is almost empty then there is a rapid fall  as the battery 
dies. Under these circumstances using NiMH cells it would be  better to 
maintain the charge to prevent the voltage falling as was recommended.  Understand 
with the earlier NiCADS it was better to take them as far as possible  before 
recharging otherwise cell capacity could reduce.
 
Bob, G3VVT