On Mon, Nov 14, 2005 at 09:06:52PM -0600, Lee Estingoy wrote: | This could go on forever.
Perhaps, but there's still value in the discussion. | Proper cycling and observation of cells will indicate whether they are to | be trusted. True, but even so, even batteries that `are to be trusted' fail occasionally. And then there's human error, where you forget to charge your pack, or charge the wrong plane, etc. Is there any reason why you couldn't have both a BEC and a seperate RX pack, both hooked up in parallel? If the BEC provides 5.0 volts and the four cell RX pack starts at 5.6 volts, then the RX pack will provide most of the current to the servos, but as the RX pack voltage drops, the BEC will provide more and more of the current, and if the RX pack fails completely, the BEC will provide all the current and vice versa. With the BEC providing only 5.0 volts, it won't charge the RX pack signifigantly, even if the RX pack was completely dead at first, so that wouldn't be a problem. (If the BEC provides 6.0 volts, then things will be complicated, as 6.0 volts *is* enough to charge the RX pack, so it may overcharge the RX pack and overload the BEC. A diodes would be required -- either that, or a five cell RX pack.) The normal failure mode for a battery pack is either 1) to go dead, which usually means that the output voltage becomes too low for use, or 2) for a cell to fail open, which basically means that the internal resistance of the cell increases greatly. Neither one of these failure modes would cause any problems with a BEC in parallel. I only see two possible problems here -- 1) how will a voltage regulator respond if the voltage at the output is slightly higher than it's normal output voltage? (Hopefully nothing happens.) and 2) is the normal failure mode of a BEC to fail with the output shorted or the output open? (Open would be better, and it sounds more likely thinking about how it works.) All of these possible problems could be corrected with appropriately placed diodes, but they'd reduce the voltages, and so you might want a five cell RX pack. That, and I'm wondering about keeping it simple, and diodes would provide another thing to fail. Just an idea. For my larger planes, I'm now using two smaller RX packs rather than one large one, along with a low battery alarm so I'll know if something is really wrong. But none of these planes are electrics, so I haven't really tried replacing one of the packs with the BEC. | However, using motor cells for RX has more mechanisms of failure due | to the higher amp draw/heat/Motor/Controller failure/more joints to | fail/etc. Could be something as stupid as the firewall coming | unglued and jamming the motor/prop/ Of course, if the firewall comes unglued, the motor may fall off the plane completely by pulling the wires loose, and then the CoG of the plane will be so screwed up that it's unflyable. Not pretty -- ask me how I know! :) (Actually, it's not that unpretty. The plane fell, but with the fuse level so it was a relatively gentle crash. Never did find the engine, however.) But you're right -- there's lots of things that could go wrong. | I'd go with simplicity, the weight savings are minimal, perhaps de | minimis, in perspective of the size of ship that we are talking | about. For a Pike, yes. For a 1.5 meter electric glider, maybe not so much. -- Doug McLaren, [EMAIL PROTECTED] "Wrinkled.... was not one of the things I wanted to be when i grew up" RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
