From: jfh@cs.brown.edu (John F. Hughes)
Subject: Re: 2 Batteries in charge
Keywords: not auto, full display
Organization: Brown University Department of Computer Science
Date: Mon, 14 Sep 1992 10:38:13 GMT

In article <1992Sep11.175141.15850@infonode.ingr.com> scott@bell.b17a.ingr.com (Scott Bell) writes:
>
>
>  In a strange compromise between auto and manual charging of 2 batteries,
>  I came up w/ the following circuit. It is NOT auto switching, but does
>  provide an indication of how much charge is going to which battery.
>
>
>                         Manual Switch Battery Charging
>                         ------------------------------
>
>  
>                                       \  SPDT Switch
>                                    ~X  X  X~
>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   ~   ~~~~~~~~~~~~~                      
>     ~                                  ~               ~
>     ~                                  ~               ~
>     ~                 Voltmeter~~~~~~~Ammeter          ~
>     ~                          ~       ~               ~
>     ~                          ~       ~               ~
>     ~       ~~~~~~~~~~~~~~~~[-]~    [+]~               ~
>     ~       ~               -----------                ~
>     ~       ~               | Power   |                B2
>     B1      ~               | Source  |               Light
>    Light    ~               | From    |                ~
>     ~       ~               | Engine  |                ~
>     ~       ~               -----------                ~
>     ~       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~          ~ 
>    [+]   [-]~                               ~[-]    [+]~
>    Battery 1                                 Battery 2
>
>
>        
>      The Voltmeter and Ammeter measure the charging difference between
>      the selected battery and the power source. This could also be hooked up
>      for multiple batteries using some kind of rotary switch.
>
>      The lights indicate which battery is being charged/selected.
>
>      Ammeter is hooked up in series, voltmeter in parallel. '~' indicates
>      a circuit connection.
>
>      Let me know if this will work, or torpedo me( sarcastic grin ;) ) if
>      you have any other ideas.
>
>     Thanks!
>     Scott

(1) The light in series with the battery will cause the voltage across the
battery to be lower than you think. I think you might want a DPDT switch, with
centers called C1 and C2, and outputs A1 and B1 and A2 and B2. When the switch
is in one position, C1 connects to A1 and C2 to A2. In the other position, 
C1 connects to B1 and C2 to B2. 


One "side" of the circuit should then be:


       

	 C1   A1 ----- light -------ground
         |
	 C2   A2 ----- Battery 1 --- ground
	 |
	 |
      Engine +

    (ALTERNATOR)

      Engine -
	|
	|
	|
      Ground

I've left off the voltmeter and ammeter, which seem OK as drawn.



An alternative to the Ammeter, which is likely to be a -60 -- 0 -- +60 style
meter, which basically is not very accurate, and has to handle a hell of a lot
of current, is to put in a *shunt*, which is a very low resistance plate of
metal. What's nice about it is that its resistance is known quite exactly, and
your charging circuit now looks like this (simplified)



                            -------(V)---------
	                   |                   |
	                   |                   |
       Engine + -----------=======SHUNT=========-------+ battery - -----ground

       Engine -

	  |
	  |
       ground


The shunt provides, perhaps, 1/1000 ohm resistance. If 30 amps are passing
through it, then E = IR tells you that  the voltage across it is 30/1000 V, i.e.
.03 volts. Hardly enough to matter to your battery, but easily enough to
measure with an accurate digital voltmeter. You can get a very accurate
charging-rate reading this way. The power dissipated is I*I*R, which is 
900 * (1/1000), i.e., about 1 watt. Not too bad...

There's an article in an old issue of Practical Sailor that describes this in
gory detail. Someone from rec.boats faxed it to me a while back, and I'll dig
up the reference at home if others care. 

-John