I saw this on Ebay while researching what's out there for chargers. 
  An American made charger for 36 volt/20 amp runs about $260 new.
  Anyway, there was also this little gage for a golf cart.
  
  I'm wondering what it measures. Probably volts because it only has two 
  wires but it brought to mind the next phase of my project which will be 
  batteries.
  Everybody has their own idea of what's best for them, and right now I'm 
  leaning toward the Trojan golf cart ones. They have a good track record and 
  are pretty affordable. The plates are thicker so can take more cycles than a 
  basic marine deep cell.
  Would be nice to get a set of Li-ion but they're still pretty 
  pricey.
  The Trojans advertise 700 cycles and I've used the same ones on a solar 
  system to run my cabin. They were still working after 13 years. Of course, the 
  application is different. These were kept charged usually where a sub or golf 
  cart will pull them down regularly but golf carts are made to do this.
  Anyway....my question is, anybody got any new info on batteries ? I know 
  there's been a lot of new developments lately. Frank 
D.