11.7V High Current Back Up Battery

Hello,

I have a DJI 2 Phantom 2 Drone which operates with a 11.7V 5.2A/h Li-Po battery. I want to design aback up battery circuit and load my second battery on drone to increase flight time. It will switch batteries when main battery falls below critical level. I searched a lot but most designs are for low power, like LT1579 from linear tech. Do you have any suggestions or example circuits?

Thank you

You would be better off doing load sharing between the two batteries using a pair of LTC4359 or similar.

If you really want to switch over on low battery, use a pair of diodes to generate a reliable low power rail, then use comparitor and a voltage reference to detect the voltage threshold on the primary battery where you want to do the switch and enable the LTC4359 on the seconday battery only after the primary one is discharged. If the secondary battery gets very low, the ideal diode function of the LTC4359 will share the load between both batteries until you get on the ground (one way or the other).

Thank you very much. So you are suggesting a circuits as below. I can simply add a microcontroller and monitor the voltage of batteries using ADC and turn on back-up when necessary. As you said because of LTC4359 behavior they can both stay on and share the load.

Yes, but you must use the circuit in figure 16 if you are going to use the shutdown to operate the UAV from the primary battery until it is flat. This is because the N channel mosfets actually operate like a diode (source to drain) when they are off and a short between source and drain when they are on. If you use a single mosfet, then the one that is off (the second battery) will have a higher voltage than the active battery, the second battery mosfet will conduct as a diode (which BTW has limited current capacity so you would have issues there too) and charge the primary battery and power the UAV. The charger current would not be limited by anything which would also be very very bad.

In reality, what would actually happen with figure 5 and the bottom ideal diode disabled is that the UAV would be powered by both batteries even though the bottom ideal diode was disabled once the top battery is discharged to the bottom battery voltage minus the voltage drop across the mosfet body diode on the bottom mosfet. The bottom mosfet would then (quickly) be destroyed by the current flowing through the body diode and catastrophic failure would likely follow.

So, if you are not going to manipulate shutdown and just share the load across the two batteries, use figure 5. If you are going to use shutdown, use figure 16 with the two mosfets. You can get dual mosfets in a single package for figure 16.

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