This article is copyrighted (c) 1996 by Bill Cheek 
Here is another angle to broaden the perspective.  It's true that batteries 
are "current" devices, in a sense.  But it is impractical to feed current to 
batteries in another sense.  In this physical real world, current is much 
more a RESULT of something, than a something in itself.  Current is a lot 
like motion where it just happens when you apply an imbalanced force to a 
mass. Likewise, resistance and voltage are the everyday "somethings" and when 
put together, current is the result.  From that perspective, let's peer 
deeper into the basics of recharging ANY battery.
  --------------              + | | |  _
  |          D |o---------------||||||-----
  | Recharger  | +            C | | |  B  |
  |            | -                        |
  |          A |o--------------------------
  --------------
1.  Correct hookup for recharging is always (+) to (+) and (-) to (-).
2.  The recharger's terminal voltage should be higher than the battery's
    terminal voltage so that current will flow from A into the battery
    via B; out C and back to the recharger into D.
    A.  If the recharger is 8.600 volts and the battery is 8.600 volts,
        then there is a state of equillibrium where current will not flow.
    B.  If the recharger is 8.600 volts and the battery is 8.599 volts,
        then condition (2) above is met, and the battery will recharge
        until condition (2A) is met.
    C.  If the recharger is 8.600 volts and the battery is 8.601 volts,
        then the battery will discharge through the recharger until
        condition (2A) is met.
3.  The actual current flow that results from an imbalance of voltages
    between battery and recharger is dependend on the SUM of all series
    resistances as follows:
    A.  Internal resistance of the battery
    B.  Internal resistance of the recharger
    C.  Resistance of the path from A to B and C to D.
    D.  Any physical resistors in the path.
    E.  factored by any diodes in the path.
4.  The below figure is more of a real world situation:
                                          EB
 --------------             RL   Ls   + | | |   Rb  -
 |   D /\/\/--|o---------/\/\/---((((---||||||-/\/\/-------
 |      Rr    | + ER                  C | | |        B    |
 |            | -                Ls                       |
 |   A -------|o----|<-----------))))----------------------
 --------------     diode
Where:  ER = Recharger voltage
        EB = Battery voltage
        Rr = internal resistance of recharger
        Rb = internal resistance of battery
        RL = physical resistor to limit current flow
        Ls = series RF chokes (have some resistance, too)
     diode = prevents discharge of battery into recharger
5.  The MINIMUM voltage required of the recharger (ER), then, is more than
    the full recharge terminal voltage of the battery (EB).  How much more
    is determined by choosing the desired recharged voltage of the battery
    and by choosing a safe trickle current charge AFTER the battery is fully
    charged and solving the following equation:
    ER =  EB + Ed + (It)(Rr) + (It)(RL) + (It)(Rb) + (It)(Rls+Rls)
where: ER = Recharger voltage          Rb = Internal resistance of battery
       It = Trickle charge current     EB = battery full charge voltage
      Rls = resistance of choke(s)     Rr = Internal resistance of recharger
       Ed = constant diode drop (0.6v) RL = physical current limiter resistor
6.  It is impractical for hobbyists and consumers to calculate the internal
    resistances of the recharger and the battery.  This is not necessary so
    long as we measure the voltage of the recharger under actual connected
    conditions.  For example, a recharger may produce 17-v under no load, but
    drop to 12 volts when producing a current of 100-mA. We can neglect the
    internal resistance of the battery since it is very low in comparison to
    the real resistances in the circuit.  The useful equation becomes:
    ER =  EB + Ed + (It)(RL) + (It)(Rls+Rls)
    and if we add .5 to 1 ohm per choke to the RL, then it gets simpler:
    ER =  EB + Ed + (It)(RL+)
Thus if we want a 60-mA trickle charge and a battery pack voltage of 8.60v,
and if RL+ is 23 ohms, then solving for ER is easy:
    ER =  8.60 + 0.60 + (0.06)(23)
       =  9.20        + 1.38
       =  10.58v
If the recharger is less than 10.58v at 60-mA, then the battery will 
undercharge.  If over 10.58v at 60-mA, then the battery will overcharge.  The 
final variable here becomes RL.  Change RL to meet the needs of your battery 
packs and rechargers!  RL is usually located inside the scanner in the 
recharging circuitry, and is typically 22 to 33 ohms.  You can jumer it out 
in the scanner, (short across it), and use a desired series RL at the plug 
that inserts into the RECHARGE jack on the scanner.
This article is copyrighted (c) 1996 by Bill Cheek 
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