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Originally Posted by Matt Krass
We used it, the only problem we had was a slight trickle through of power to the RC when the breaker was open and the backup battery connected, it wasn't even really a problem, somewhat amusing is all. I don't think we ever had to remove our backup battery and everything worked great for us, it gets a thumbs up from me.
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There are two problems with the schematic from IFI. The first one is that if the 8.7V zenner comes off or breaks, you risk supplying a full 12V to your RC and backup battery.
ANother problem is that of the main battery is dosconnected, the backup battery will end up "replacing" the main battery. It will supply power to the motors through the 43 ohm resistor. At 7.2 V, the power dissipation inside that resistor is about 1.20W. You will end up frying the resistor.
My recommandation is that you add a diode(any diode that can take over 1A will work)before the output.Disconnect the "+" ouptut, and connect the anode of the diode to it. Connect the cathode to the wire you just disconnected. Or just build a new circuit with the extra diode. This will prevent power from the backup leaking back if the main power drops too much, or the main battery is disconnected. However, this modification will not protect the backup if the 8.7V zenner fails.