A beginning comment: With modern avionics drawing less power than their predecessors and LiFePo4 with greatly increased energy capacity, maybe only one battery is needed these days for most glider pilots. Just sayin'.
I agree that our batteries should not be run in parallel (without diodes). If one is of a significantly different voltage than the other, then the higher one will charge the lower one possible at a high rate (current) that you don't want. This can potentially blow one or both battery fuses (you do have a fuse on each battery, right???), or might cause one battery to fail in a bad way, either of which can leave you in a poor situation especially during flight.
I have a presentation on my thoughts on the subject of two battery systems at
http://aviation.derosaweb.net/presentations#wiring.
Here is my setup on my current glider.
- Two batteries (lithium)
- Each battery has a fuse.
- Two separate power buses with one battery powering each.
- One bus is for "communications" (radio, transponder, FLARM) and the other bus is for "navigation" (GPS, Vario, etc.).
- Each bus has a separate breaker (5A) which is also a master switch (a true breaker/switch by Tyco
http://www.aircraftspruce.com/catalo...clickkey=5950).
- But I do have Schottkey diodes in each bus. Why, you might ask, do I have diodes if I have two separate electrical buses? Because I can bridge the two buses with another switch for that "just in case situation" (belts and braces engineering over design). I have never have to use this switch but then again you never know ...
Thanks, John OHM Ω