[Elecraft] KBT2 charging question

[Don Wilhelm]

Doug,

I finally looked at your reference, and find that your quoted voltage is
only a portion of the charging spec.  While a float voltage at 13.6 to 13.7
volts is OK, it will not fully charge the battery until applied for a very
long time (many tens of hours, maybe even hundreds of hours).

Notice the 'float voltage' you quoted is only the last part of the
recommended charging cycle:
1) charge with a constant current of 580 ma until the terminal voltage
reaches 14.4 to 14.7 volts.
then 2) Hold the terminal voltage at 14.4 to 14.7 volts until the current
drops to 29 ma.
3) After that, stop charging or hold the float voltage at 13.5 to 13.8
volts.

The KBT2 uses a series resistor to limit current, and the series diode
voltage drop must be considered too.  The KBT2 instructions correctly state
that a voltage of 14.0 volts is recommended.

For those who want to fully charge the internal battery and maintain that
condition properly, I refer you to the SmartCharger article on my website
http://w3fpr.qrpradio.com which shows how I connected an external 3 state
charger to my internal battery (fused for safety).  I operate my QRP K2 from
the battery at all times (no other power source connected) and the 3 state
charger keeps the battery topped off.  One could use a solar panel
controller to charge the battery in place of the AC powered SmartCharger
that I used - I am thinking of adding a solar controller here.

73,
Don w3FPR

> -----Original Message-----
> Take a look at the charging specifications from the datasheet:
>
> http://www.powerfactorinc.com/Batteries/ps-1229.htm
>
> It appears that the 100 - 200 mv drop across the shockey diode is
> acceptable. This lowers the 13.8 volt output from the power
> supply to 13.6
> to 13.7 volts. The float voltage can go as low as 13.5 volts across the
> battery terminals.
>
> Doug (KE7GYQ)
>