Dave VanHorn made the suggestion.


Charge             :                 :
+ o-+-----------------+-----------------+-----------------+
    |              :  |              :  |                 |       Output
    +-|>|--+-----+ :  +-|>|--+-----+ :  +-|>|--+-----+    +-|>|--+-----o
        ___|___  | :      ___|___  | :      ___|___  |        ___|___  +
       |   +   | | :     |   +   | | :     |   +   | |       |   +   |
       |       | | :     |       | | :     |       | |       |       |
       |  12V  | | :     |  12V  | | :     |  12V  | |       |  12V  |
       |       | | :     |       | | :     |       | |       |       |
       |___-___| | :     |___-___| | :     |___-___| |       |___-___|
           |     +--+        |     +--+        |     +--+        |
           |       :|        +----+  :|        +----+   |        |
           |       :|        |    |  :|        |    |   |        |
           |       :|        o    |  :|        o    |   |        o
           |       :|     (D) \(C)|  :|     (D) \(C)|   |     (D) \(C)
           |       :+-------o  o  |  :+-------o  o  |   +-------o  o
Common     |       :           |  |  :           |  |              |
- o--------+-------------------+  +--------------+  +--------------+
                   :<Repeating unit >:

  Notes:  Relays are wired for minimum voltage across contacts.  Power
diodes must be rated for full output voltage and current.
(D) = Discharge, (C) = Charge.
--
  Cheers,
        Paul B.