Thanks everyone for the in-depth replies! Everything from schmatics to personal experiences. I don't know why I've been so thick - I'm pretty sure someone has told me around the airport that you can't connect in parallel such dissimilar batteries. Then again, others (with my level of understanding I now know) said they didn't see any problem as long as both batteries were fairly well charged - the old two pails (one big, one small) of water theory.
In designing my back-up system I took care to completely isolate both systems . . . except for the occasions when I might want to connect them . . . which would be to charge them concurrently, either in the hanger or in flight. I thought I was being clever in allowing for this, but I was just being dumb. I'll take the switch completely out and there won't be any further possibility of harming one battery or the other. Good thing I didn't damage my AGM - it cost $75 and I did some shopping. With the gel cell down to 5.6 volts I've probably destroyed it. They aren't designed to be discharged so completely. I don't think any battery is, from what I've read. The inline voltmeter on the back-up circuit is only on when the back-up bus is turned on - and my idea was the only time that would be on is when I'm charging the battery or actually using the back-up bus. The simplest way to fix this problem is to stick to my original idea and just have a completely separate system. Which is what I've got. Except for that switch. In getting rid of this switch I'm avoiding what could turn an incident into a true emergency. If my main battery were somehow very depleted in flight and I were to turn on the secondary system in parallel with the main as a "boost", the current from the smaller battery being drawn by the big would certainly burn some wires and maybe even start a fire. It would certainly cause some smoke and scare the heck out of me at the very least. And so we learn. Thanks again everyone. Mike ____________________________________________________________ Save $10 on Flowers and Gifts! Shop now at www.ftd.com/16714 http://thirdpartyoffers.juno.com/TGL2141/fc/Dh4GRr5bId0OdXFuWenvsNIBhDOEYLvJNsVvTSuOA1DJ3gZLgUUkva4tEic/
