My apologies to the original thread starter who was talking about a cheap servo. The thread morphed. It is my fault. Not to worry; I'll get tired soon and go back to building.
ORIGINAL: Flyfast1
Leo,
According to information that I've read elsewhere, such as Red Schoefield's Battery Clinic and some testing results that were posted somewhere here on RCU, it's not a problem to use two packs without diodes. According to the testing that was done, a failure in one of the packs caused only a very slow drain on the second pack. My recollection may not be correct, but I thought that after an hour, the remaining "good" pack was only drained by 10% or so. The conclusion of the testers was that with regular checks of battery voltage, e.g., between flights, or every other flight, that the risk of a failed pack draining the other pack and bringing down a plane was extremely low. I generally use two 6v receiver packs into two ports on a receiver on all my planes and am interested if you have more information on this approach. I would like to better understand the scenario you are thinking about.
Thanks,
-Ed B.
Ed,
Excellent information. I would agree depending on the type of cells you use and the level of charge you have left in the batteries before you take-off. 10% would be a good rule of thumb for a Nicad on the first flight of the day but it will go up considerably with each flight. It is more of a concern on the 4th or 5th flight.
Here are some quick tests that show 10% of a fully charged packs capacity would be a good ballpark for NICAD & NIMH cells. Not for Li-Ion cells with lower impedance like A123. Let's look at some quick test data for cells I have in the shop. I just did a 2 cell to 1 cell charge on sub-C size NICADS, AA size NIMH, and A123 cells. They read 154 mah, 162 mah, and greater than 10 Amp respectively. It blew-out my meter. You can see the pictures. I knew the A123 would be something outrageous because they have very low impedance. My meter was fuse protected to 10A. (Huh? Figure that. Fuse protection.) I saw an initial reading of 6A then it blew. So it is something greater than 10A and I have nothing to check any higher.
10 years ago I used to fly with single 1200 mah Nicad packs. I would use about 250 mah per 15 min. flight and I would fly 4 times on a plane in a day. In hind-site not very smart because I only had 200 mah left at the end of the day. The battery would test good and still be on the plateau before the last flight but I really didn't have much reserve energy in the cells. A weak pack or a stalled servo and it would have crashed. It's amazing that I got away with that.
About 6 years ago I thought I was smart and I converted to dual 700 mah Nicad packs plugged directly into the RX (1400 mah total). No diode isolation. Took my total mah up slightly. In hind-site not too smart because on the 4th flight of the day I would test the battery voltage and everything would look good and in theory I was leaving the ground with 650 mah left in the combined packs. But here is the problem. If I dropped a cell after takeoff (which I have seen happen, and you will too if you fly enough) then I would unknowingly be down to a theoretical 325 mah left in the remaining pack for a flight that will consume 250 mah. Add to that the drain of the low pack would consume another 40 mah and it would be all over. I amazingly got away with that for another 3 years on roughly 3 airplanes.
About 3 years ago I boosted everything up to over 4500 mah total capacity. I have more than enough reserve capacity for just about anything. Some of the planes are equipped with dual A123 cells which must have diode isolation if you use them on one common RX. At a greater than 10A charge rate a lost cell will cook the buss on the RX they are connected to. However, with the dual RX, I don't need that diode isolation but I still use it to drop the voltage 6.6V down a bit.
Leo