Improved RX Battery Scheme, Part 2
I was chatting with Kevin Sung, who is a pattern flyer, the other day about his new RX battery setup. He now uses two RX batteries for a level of redundancy and increased safety. Current regulator products on the market enable this RX battery setup in a convenient way.
Some time ago, I did an article called “Dual Battery Setups, 2/2011” that is available at www.eastbayrc.org under Tim’s Tips that discussed use of a separate receiver (RX) battery for an increased level of safety. I call this article Part 2 because it builds on that basic idea with further increased reliability.
As background, there are a couple ways to power the RX and servos in our aircraft.
- Use BEC (Battery Eliminator Circuit) in the ESC (Electronic Speed Control). This method is attractive due to weight and size.
- b.Use separate RX battery to power the RX andOn larger aircraft where the investment is higher and weight is not such an issue, I use a separate RX battery. The idea is that power separate from the motor battery yields enhanced safety.
Clearly, either of the schemes mentioned above is susceptible to single point failure. Using a BEC, a failure of the motor battery will cause the loss of the plane. Using separate RX batt, the loss of the motor battery or ESC will not cause the loss of the plane since you retain flight surface control, but loss of the RX battery will result in the loss of the plane.
If redundancy could be added to an RX battery setup, then loss of an RX battery will not result in loss of the plane. Certainly, this is a step toward increased reliability and goodness.
The enabling technology for this is a product named PowerBox Sensor from a German company called PowerBox, manual is here. http://www.powerbox-systems.com/e/powerbox_systeme/powerbox_sensor/start.php .
This can also be ordered from Chief Aircraft at this link, http://www.chiefaircraft.com/radio-control/power-systems/switches-sensors/dur-804000.html .
Also available from the Duralite guys here https://duraliteflightsystems.com/index.php?_route_=powerbox-systems/powerbox-systems-switches&product_id=99 .
The PowerBox Sensor has a separate connector for each of the two RX batteries and also dual receiver connectors and circuitry that handles the necessary switch over so that the redundancy (for RX power source) is maintained at all times. This switch/regulator combo costs $120 which may seem expensive, but it is a small price compared the cost of a plane.
I’ve been very happy with another product from PowerBox in my 51 Slick which is a linear regulator; nice operation, size and weight.
Here is a pic of Kevin’s very clean install, note the two thunder power RX batteries stacked under the yellow velcro.
Kevin Sung’s dual RX battery setup using Power Box Sensor
More technical detail.
There are a couple of issues this sort of product needs to solve.
- a.Each battery must be able to individually supply the needed current to support the RX andThis happens from a steering mechanism between the battery regulators and the servos/rx(load). The current from the selected battery needs to supply this current. The selection mechanism for this is that whichever battery has the higher voltage output will supply the current. If the voltage is the same for both batteries, then both batteries will share the load. As you may know the cell voltage between two lithium cells may be very close but is seldom identical.
- b.If one of the RX batteries were to fail, the regulator needs to switch to the good battery for continuous RX/servo power; this switching needs to be seamless and without aAlso, the circuitry needs to ensure that the failed battery does not act as a load to the good battery. Without this isolation, the purpose of redundant battery would be defeated since the load imposed by the failed battery would ultimately take the entire system down.
Below is a block diagram of how this thing works taken from the users manual. Note the decoupling diodes 1 and 2. These are connected in a diode OR configuration which allows current to be pulled from either battery/regulator(diode is forward biased, conducting) and also prevents a bad battery from affecting the RX/servo supply(bad battery diode is reverse biased, blocking). This diode OR is the heart of the redundant connection and solves the issues outlined above.
Power Box Sensor Block Diagram
Regarding consumption on the RX battery, I’ve found that each flight consumes approximately 50maH from the RX battery on my 51” size plane which uses digital servos. This was determined by how much charge was put back into the battery after the flight so includes charging losses. On my 51 Slick I use a 2S610 battery and get at least 6 flights per charge. On my 70” class machines, also with digital servos, the consumption is about the same (60maH) and I use a larger battery 2S1350. I think Kevin uses 2x 2S480 batts on his pattern ship to save weight.
So, if you’re looking for a way to make your plane safer, this is a significant element toward that goal.