The problem

For any UAV (Unmanned Arial Vehicle) redundancy is a key factor. Simple consumer drones usually don’t have a lot of redundancy built in, as they usually only carry the minimum amount of parts necessary. A small quadcopter typically only has one battery to supply the motors with power, which is fine for most consumer applications. However, as soon as the size of a drone increases, redundancy becomes more desirable, as a potential crash will at least be more expensive, but also cause much larger damage compared to a small drone.

Failure modes

Before trying to figure out on how to achieve redundancy, let’s take a look at possible failure modes of Lithium Polymer batteries.

• Empty battery

This is one of the most basic, but yet one of the most likely failures to occur: Installing an empty battery on the UAV.

While this might not be a big issue on single-battery drones, were the voltage-telemetry indicates the problem before taking-off, it definitely can be an issue on multiple-battery drones. Installing a full and an empty battery can cause all kinds of issues, ranging from less endurance to damaging the batteries themselves due to the different charge levels and the cross-charging happening.

• Lost connection

A bad solder connection or a loose plug: Losing connection to the battery inflight can occur and causes the battery to fail instantly or intermittently. Often resulting in loss of the drone.

• Open cell failure

This is the mostly likely fault that might occur with the actual battery-cells: An open cell failure.

These faults are usually occurring when a battery gets old and the internal connection between the individual cells breaks during operation. The result is the same like if you’d unplug the battery mid-flight and often ends catastrophic for the drone.

However, there is a bright side to the Lost connection and Open cell failure: No other battery aboard the UAV will be damaged. This is not the case for the next fault:

• (Internal) Short circuit

One of the more problematic failure mode of a LiPo battery is an (internal) short circuit. This likely means increased current draw and usually causes the battery to overheat, often resulting in a destructive fire. While a battery burning on a drone itself is a bad situation already, this form of failure can also affect other batteries, as they might get discharged to the shorted battery as well.

This is one of the situations where a dedicated circuit to isolate batteries comes in very handy.

The solution

To create a redundant power supply for functional drone, several things are necessary:

1. Multiple batteries

Obviously, if a drone should be able to fly with a faulty battery, it needs at least one replacement battery onboard the drone that can take over and allow for a safe landing.

There are several choices to be made here: How many batteries are necessary? In which order should they be used? How much additional weight is acceptable?

However, before deciding on how many batteries are necessary, first let’s figure out on how they should be used. There are several approaches, but for simplicity three will be consider here. All of them have advantages and disadvantages which need to be weighed against each other.

• Have a smaller sized spare that is not used during normal operation

For this approach, there is a primary battery that is used during normal operation as well as a (smaller) spare battery that is our backup. If the primary battery fails, a switchover to the spare must occur which will then allow for a safe landing.

However, this means that the drone always carries a battery around, that is not used in 99.9% of the flights. This therefore is mostly dead weight that is reducing the payload capability as well as the endurance. This is obviously not that great for most applications.

• Several batteries connected in parallel, and discharge them sequentially

Compared to the first method, having several same sized batteries that are used sequentially is already an improvement. A “sequenzer” allows all the batteries to be used after each other, and therefor reduces dead weight.

Designing such a “sequenzer” is not trivial, as we have to make a couple of design choices: How much capacity is left in each battery before switching to the next? How do we create a reliable way of controlling? What if the sequenzer itself fails?

Therefore this design choice is not ideal either, so let’s have a look at the next…

• Several batteries connected in parallel, and discharged at the same time

Having several batteries connected in parallel that all supply the UAV at the same time might be the best option. Some of the advantages of this approach are:

• For some failure-modes no electronics are necessary (for example if one battery fails open cell, the other will still supply the drone just fine).
• Batteries share the load current during normal operation, therefore increasing battery lifetime.
• The whole usable battery capacity onboard the UAV can be used, without having to leave a reserve for an eventual switch-back.

However, while a simple installation might be possibly without electronics, to cover for some more complicated failure-modes electronics are still necessary. Nonetheless, this is the most desirable design for the HEXACOPTER project I am working on.

Now the next question is: How many batteries are necessary?

The ideal amount of batteries on a drone from a reliability-standpoint would be very high. For example 3 batteries would allow for a dual failure, provided one single battery can provide enough current for the whole drone. However, this soon becomes unpractical for several reasons: Having several batteries means increased work to charge and maintain them. The weight also does not scale up well, as several small batteries are usually heavier than fewer larger batteries, given a fixed total capacity.

Given the reliability of properly maintained and stored LiPo batteries, having two batteries on an UAV is a great way to increase reliability without overcomplicating things, so therefore we will focus on that number.

2. Battery monitoring

Monitoring the batteries is necessary to utilize the benefits we achieved by utilizing a redundant design. Usually on a single battery UAV, there is one voltage/current sensor each. However, for our multi-battery design, we need a voltage/current sensor for each battery.

This allows the monitoring of each individual battery and if there is trouble with one battery, it is possible to take corrective action immediately. Additionally monitoring the power-consumption of each battery individually greater awareness of the overall endurance remaining is achieved.

The current and voltage sensors should be placed directly next to the battery, in-front of any battery switching circuitry that is present. Therefore it is possible to find out about an (impending) battery fault while the drone is still performing the mission.

3. Battery Circuitry

Having two batteries in parallel without any additional electronics already prevents a crash if one of the two batteries fails open circuit, like described in the failure-modes earlier. However, to prevent any issues with different charge levels or an internal short an additional circuit is required.

More details about the circuit can be found in Part 2 – Circuit.