Sean,
   
  Here's what I've built. It may be too simple for your needs but it's very cheap.

I wanted to check the condition of 12v sealed lead-acid batteries in the 30 to 100AH range, 
  just to get an idea of the capacity and watch trends. The method is to apply a lamp load and 
  time how long it takes to discharge a fully-charged battery to a chosen end-point voltage.
   
  A heavy-duty relay switches the battery between its charger (normally closed contacts) and
  the load. The relay is driven by a transistor fed from an operational amplifier which compares
  a proportion of the battery terminal voltage (across the load) with a reference voltage to
  determine the end-point. Closing a momentary switch across the relay driver transistor 
  causes the relay to change over, disconnecting it from the charger and connecting it to the
  lamp load. The reference voltage is applied to the inverting input of the op-amp and a 
  proportion of the voltage across the load to the non-inverting input. When the battery voltage 
  appears across the load, the non-inverting input goes high, the output goes high and the 
  relay is held energised by the transistor. It remains in this state until the proportion of the 
  battery voltage falls below the reference, when the op-amp output goes low, the relay drops
  out, the battery is disconnected from the load and reconnected to the charger.
   
  A second, low-current relay has its coil wired in parallel with the first relay. This relay 
  switches 1.5v to a cheap quartz analogue clock when it is energised. By setting the clock
  to 12 o'clock at the start of the test, the elapsed time is recorded to the point where the 
  relays drop out. Choose a lamp load to discharge the battery in under 12 hours and the
  clock reading will not be ambiguous. I used various car lamps for my tests. I know that 
  lamp loads are non-linear but this doesn't matter for good/bad/getting batter/getting worse
  tests.
   
  Hope this helps!
   
  John
   
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