Original link from Oscarliang: https://oscarliang.com/parallel-charging-multiple-lipo/

In this article, we’ll be talking about the first set of these jargons, comprising of RTF, BNF and ARF.

Explained: What does RTF mean?

When you’re out to buy a drone for yourself, the same drone can often be purchased in various combinations and varying prices. Most often you’ll see a drone in its RTF and BNF versions. RTF stands for Ready-to-fly, which means that you don’t need to purchase any additional accessory to be able to fly the drone once you do take the plunge and make the purchase. Typically, an RTF version of a drone means that there is a controller included in the package. So, if you’re just starting out with drones, it is the RTF version of a drone that you need to get yourself.

Okay, what about BNF then?

In simple words, BNF stands for bind-and-fly. This means that the drone needs to be bound to a controller that is not included in the package. This also means that you’re assumed to be in possession of a higher-grade controller than the one which the OEMs ship with the drones. Typically, controllers are classified into two types: Hobby- and toy-grade. Hobby grade controllers are usually bigger in size, better in quality and have a vast range of controls that the bundled toy grade controller might not offer. Therefore, if and only if you own a hobby grade controller (which is known to be compatible with the BNF drone you’re looking at) should you go for a BNF drone. The fact that you’re reading this article probably means that you don’t have one yet!

What in the world can ARF mean, then?

Okay, so we’ve discussed the two most common versions that drones ship in – RTF and BNF. Now, if you’re one of those people that like to get their hands down and dirty (DIY, in other words), then ARF (almost ready to fly) is for you. ARF kits may or may not ship with a controller, but they most certainly have something in common – the drone isn’t in one piece, and often requires a good level of assembly on your end to make it ready to fly. In other words, ARF kits are drone manufacturers’ take on the DIY concept, something which is a bit of a hit with budding drone hobbyists.

Conclusion

To sum it all up, one can say that: (i) RTF kits are for those that are entering the drone hobby, while (ii) BNF kits are for RC veterans, who are usually in possession of one or more hobby grade controllers. (iii) ARF kit drones appeal to a different market altogether, the section of the market which comprises of DIY enthusiasts and the like.

The first 32-bit flight controllers for mini quad was the CC3D with F1 processor. F1 FC has the the lowest processing power in the four. Good examples of F1 flight controller would be the Naze32 and CC3D.

F3 FC boards were introduced in 2014 and currently have the X-Racer, Betaflight F3, LUX V2, and the KISS FC.

F4 Flight controllers were introduced shortly after the F3. They are also getting more popular, such as the Raceflight Revolt, BrainFPV RE1, and DemonRC Soul.

F7 is a newer generation MCU. F7 FC’s are very new to the market and at the moment there are only a handful of F7 flight controllers, for example AnyFC F7 FC.

So, Should I Get F1, F3, F4 or F7 FC?

Sure, you can get your multirotor flying fine with even F1 boards, but faster F3 and F4 can give you better performance and allow you to run resource intensive features. As FC firmwarecontinues to advance, F1 boards will miss out on future cool features due to the lack of processing capacity.

So really, the decision is down to F3, F4 and F7 these days.

As for F7 flight controllers, unless you use it with a 32KHz capable Gyro, and run 32KHz looptime, the main F7 benefit is just more UART ports available (8 of them). It’s not that big of an advantage right now, but it’s more future-proof for sure. In my opinion, F7 would be great for applications where a lot of serial devices are used, such as iNav.

One downside with today’s F7 FC is they are using the bigger F7 chip (F745VG) that takes up too much physical space on the board, it doesn’t leave much room for other components and connections. Hopefully smaller F7 chip variants (such as F722RE) can be used. It has the same package as the F3/F4 we are using, although the F745 does have more flash memory for the software and RAM for code execution though than the F722.

Anyway, it will take time for manufacturers to refine the hardware design, and for the firmware developers to figure out what they can do with the extra processing power and features. So there is no need to rush and get the F7 right now IMO.

If I was going to buy a new flight controller today, I would probably opt for a F3 or F4, because they are generally packed with features we need, and have very well thought-out layouts with mature and user-friendly designs.

Table of Content
·Channels
·Modes
·Frequency Technology
·TX / RX pairing
·What to look for when buying a transmitter
·Why invest in good TX
·Transmitter Recommendation
·How to choose receiver – RX
What are a RC Transmitter and Receiver?
A RC transmitter (aka radio transmitter, or TX) is a device that allows the pilots to control the aircraft wirelessly. The signal/commands are then received by a radio receiver (RX) which is connected to a flight controller.
If you are new and interested in flying drones, you should check out the beginner guide to mini quad racing.
Channels
The number of channels determines how many individual action on the aircraft can be controlled.
For example, throttle, yaw (rotating right and left), pitch (lean forward and backward), and roll (roll left and right), each takes up 1 channel. And so as you can see, four channels is the bare minimum to control a quadcopter (pitch, roll, throttle, yaw).
But for hobby grade quadcopters, you definitely want more channels.
Additional channels on a transmitter are often called AUX channels (Auxiliary), in the form of switches and pots (potentiometer or knob). You can use them to change flight modes or trigger certain function/features on the multirotor.
In general it is recommended to have at least 5 or 6 channels for a quadcopter. The extra 1 or 2 channels can be used to arm the quad and/or switch between different flight modes
Modes
There are 4 different TX modes – mode 1, mode 2, mode 3 and mode 4. These are basically the different configuration of the 2 control sticks.
Mode one configuration has the elevator control on the left joystick and the throttle on the right one.
Mode two is the most common for quadcopter because the stick represents the movement of your quadcopter. It has the elevator control on the right joystick and the motor throttle on the left one. The right joystick self centres in the both axis, whereas the left joystick only self centres in yaw axis (left/right direction) and clicks or slides in the throttle (up/down) axis in order to allow constant throttle.
Mode three – same as Mode one except Aileron and Rudder are swapped.
Mode four – same as Mode two except Aileron and Rudder are swapped.
Because of the identical gimbals configuration, in some TX, Mode 1 and Mode 3 are exchangeable, so as Mode 2 and Mode 4. This is achieved by swapping Aileron (roll) and Rudder (yaw) channels in user settings.
There is no right or wrong which one to use, just what you are more comfortable with. If you don’t know which mode to use, just go for mode 2 since majority of the pilots are using it, and it’s going to have a higher resell value later on.
Frequency Technology
The most popular RC frequency is 2.4GHz. Lower frequencies are available for longer range such as 27MHz, 72MHz, 433MHz, 900MHz and 1.3GHz but they are rarely used in mini quad.
Just in case you are interested, here is some brief and interesting technical background.
For those of you who have been around the RC scene long enough, will remember 27MHz and 72MHz with the frequencyannel crystals (crystals were tuned to specific frequency channels to transmit the signal to the receiver which had its own same channel crystal essentially binding them together). This technology has been around for decades, they allow longer range and better signal penetration. However you could interfere with others using the same frequency (even different brands). Another problem was the large size of the antennas as they could reach a few feet in length. Crystals used for channel selection were also inconvenient as they broke easily and could constantly and annoyingly change when flying/driving with others.
2.4GHz system is a newer technology, and it’s currently the most popular frequency for small RC ground and air vehicles. It becomes the RC standard after new protocols were created that introduced frequency hopping technology which allowed the user to not have to worry about picking up frequencies or channels from other pilots. Antenna is smaller and easier to carry, but usually with shorter range than the 27/72Mhz.
You may have also heard of others using 1.3ghz, 900mhz or 433MHz equipment, these are more commonly used for long range or on larger crafts.
All the transmitter manufacturers switched to the new channel hopping protocols which made RC very easy to maintain and use. The software running is constantly scanning for the best frequency to use and if it detects any interference, automatically switches to another available channel. It is doing this many times per second so you never experience glitches or radio interference which was a big problem in the RC industry for many years. Another good thing about channel hopping is that you can fly with many other people at the same time without getting interference.
Radio Receivers
A transmitter usually comes with a receiver. It’s important to know that a TX only works with radio receiver (aka RX) from the same manufacturer generally. For example, if you get the Frsky Taranis, you will have to use Frsky RX’s, or other Frsky compatible RX.
Binding the transmitter to the receiver is very straightforward, and is only required at the first time you are setting up a new receiver with the transmitter. Most decent TX will allow you to bind multiple receivers to the TX. You can even create separate model profiles for each aircraft so you can have different settings for each model. Check the instructions for binding procedures that comes with the receivers.
Once the TX and Rx are bound, the RX cannot be controlled by other TX.
How to choose receivers
Your preference in receivers will limits what TX can you get, such as availability, size, receiver etc. For example, Frsky radio system was made super popular due to their receivers having compact form factor. which makes them perfect for mini quad builds.
In this list we rounded up all the popular Frsky receivers for mini quads and micro quads.
There is also consideration to what receiver protocols are allowed and technologies used, such as PWM, PPM and SBUS. Generally speaking, SBUS is better than PPM because of latency, while they are both better than PWM because of the number of connection required. For more detail: Receiver RX Protocols and Technology.
What to look for in a RC transmitter?
Budget and Channels
The price range is huge, from as cheap as $50 to well over $1000.
If you have a tight budget and not committed to spend $200 for a Taranis, it would make sense to get a cheap 6-channel just to get a taste of flying. And you could upgrade to a better transmitter later on when you are more experienced and know what to look for. Otherwise I think you should just get one of the decent TX we recommend in the next section straight away.
Hardware and Features
There’s a lot to consider when it comes to choosing a good transmitter, such as the display screen (resolution, backlight etc), how the sticks feel (gimbals quality), multiple model memories and Training features (Buddy mode), and so on. Some may also like the expo and curve functions which allow you to change how the sticks react to your input by softening there feel at points you setup (although this can also be done in the flight controller software, which is actually preferred by many pilots so you don’t reduce the sticks resolution and inputs on the radio).
Some RC transmitters support programming and firmware flashing to enhance user experience. You can personalize them with music and voice recordings or whatever you’d like. Do your research before spending good money on it.
Ergonomics
Ergonomics very much a personal thing, no one can tell you which TX would feel good or bad in your own hands. Considerations such as the weight, the location of the sticks and switches, how large your hands are, how long your fingers are, all play a part in this.
I don’t think it’s a huge issue to worry about though with the TX we suggested here. These companies are brand names in the RC industry for years and they know how to make a good TX. If you are still try to find out more, I would suggest to go to a local meetup and try a few from other pilots.
Telemetry Support
The longer I am flying quadcopters, the more I value Telemetry. It’s an useful feature that allows RX to send flight data back to the TX, such as RSSI, battery voltage, current draw etc.
External RF Module Capability
With this feature you can install and use an external transmitter module rather than the built-in RF module. It allows you to transmit on a different frequency such as 433MHz rather than 2.4Ghz, or use a different RF systems/receivers from a different brand. For example, the Taranis with Orange module can bind with Spektrum receivers.
Why invest in a good transmitter
A decent RC transmitter is a long term investment.
With programs available such as betaflight, we can setup the additional channels to tune the quads PID and rates during flight. This makes having a transmitter with additional AUX channels a big benefit. Having the ability to save multiple models is an added benefit of having a better radio as this allows one transmitter to be used for multiple crafts.
Another “should have” feature is direct connection between TX and computer via USB, which allows you to use the TX for flight simulators without any other additional hardware or mods. Training in FPV simulators allows you to get used to the feel of the sticks/controls and build up muscle memory. Some  cheap transmitters can also do this but requires a lot more tinkering and  additional hardware. 

Original link from Oscarliang:https://oscarliang.com/choose-rc-transmitter-quadcopter/

ESC stands for Electronic Speed Controller. As the name suggests, they are there to control the speed of the motors. The ESC receives throttle signal from the flight controller, and drives the brushless motor at optimal speed by providing the appropriate level of electrical power. Quality ESC’s ensure a reliable and smooth flight experience.

Current Rating – Amperage

First thing to look at when choosing ESC is the current rating, which is measured in Amps. Motors draw current when they spin, if you draw more Amps than your ESC can handle, your ESC will start to overheat and eventually fail. A catastrophic failure can even end up with your ESC in flames! Three things that tend to increase your current draw and put more stress on your ESC:

·Higher motor KV

·Larger motor size (stator width and height)

·Heavier propellers (length and pitch)

There are 2 current ratings to an ESC: continuous and burst. Continuous current rating indicates the maximum amount of continuous current which the ESC can safely handle. As it is doubtful that you will use maximum throttle for extended periods, even when racing, ESC’s are usually designed to withstand a higher current for short periods of time (e.g. 10 seconds) and this is the “burst” current rating.

Sometimes beginners confuse the current rating of an ESC as Amperage that it would apply to the motor, but the opposite is actually the case. It is the motors that draw current from the ESC’s, so the ESC must be rated for the maximum Amps of the motor you are using. The fact is there is no benefit to simply using larger ESC’s, e.g. if you replace your 20A ESC’s with some 40A ESC’s you would actually see a drop in performance as your quad will just be heavier.

How to find out about current draw

You can test the current draw of a motor yourself on a thrust stand, with a power meter. Alternatively, there could be thrust test result available online, that gives you the current draw of your choice of motor and propeller combination. Some motor manufacturers even provide this data on their product page.

For example, if you want to use FPVModel 2206 Motor with 5030 propellers on 4S LiPo, it draws 10A at 100% throttle (as shown in my tests), therefore an ESC rated for 12 Amps should be more than enough. If you intend to use 6045 props with this motor however, max current draw could reach 20A, in which case it would be safer to use 20A ESC.

It doesn’t hurt to leave some margin for error, but there is no need to go overboard. You can use 30A ESC or even 40A ESCs on something that only draw 20A of current. It will work fine, but it’s an overkill, dead weight and more expensive (because bigger ESC cost more).

Thrust and Current draw are higher in static tests?

One thing to bear in mind is that most static thrust tests are very likely to show both higher thrust and higher current than can be expected in actual flight conditions. First of all, PID controllers in the FC always tend to leave a little headroom to stabilize the copter, so you will never actually see 100% throttle when in flight. Secondly, the motors won’t need to work as hard when moving forward through “free”air, thus drawing less current.

To verify this theory, I tested it with an OSD (on screen display) that reports the total current draw at all times during the flight. In my test I used DYS Storm 2207 2500KV with DAL Cyclones 5045×3 props. A static thrust test of this motor shows about 30A of current draw at 100% throttle. However in flight the highest sustained current is only about 22-24A.

Besides everyone’s flight style is different, maybe you don’t push full throttle very often, if so your overall current draw will be lower.

Firmware

SimonK and BLHeli

Two of the oldest ESC firmwares for multirotors are SimonK and BLHeli. Originally these were both open source, developed and maintained by RC hobbyists. In the old days, firmware written by manufacturers was not optimal, so hobbyists would tend to flash either SimonK or BLHeli onto their ESC’s. Slowly this became the standard of ESC firmware, and most ESC’s came either with BLHeli or SimonK firmware pre-installed.

The majority of the users choose BLHeli because of its range of features and user-friendly interface. For more detailed benefits of BLHeli and SimonK firmware, here is a nice discussion comparing these firmware. Anyway, I believe that SimonK has now become obsolete as it’s no longer being updated, so flash BLHeli whenever you can.

BLHeli_S

BLHeli_S firmware is the 2nd generation of the BLHeli firmware, developed specifically for ESC’s that have Busybee processors with hardware PWM. It also has a much more simplified user interface. Aikon SEFM 30A and DYS XS series are early adopters of the updated BLHeli_S firmware.

BLHeli_32

The BLHeli_32  ESC firmware is the third and most recent generation of BLHeli. It’s written specifically for 32-bit ESC’s and it has gone closed source for this iteration. These more powerful processors allow for smoother, more precise and reliable performance than previous ESC’s.

KISS ESC Firmware

The KISS ESC firmware is closed source and is exclusive to KISS ESC.

Processor

ATMEL and SILABS

There are three main families of processors (MCU or micro-controller unit) you need to know about for RC Multicopters. Currently the majority of multirotor ESC’s on the market use these processors: Atmel, Silabs and ARM Cortex. The different MCU’s have different spec and features that allows you to run different firmware.

·ATMEL 8-bit based ESC’s are supported by both SimonK and BLHeli ESC firmware

·SILABS 8-bit based ESC can run BLHeli or BLHeli_S only

·Ateml ARM Cortex 32-bit (More specifically STM32 F0) – can run BLHeli_32

ATMEL 8-bit ESC’s used to be more common before the market began to be dominated by SILABS. Silabs ESC’s tend to outperform 8-bit ATMEL ESC’s with the exception of KISS ESC’s. Now in 2017 the Atmel Arm Core MCU is becoming common on 32-bit ESC’s.

SILABS F330 and F39X

Within SiLabs based ESCs there are different processors that provide different performance, for example the 2 main ones currently being F330 and F39X (F390 and F396).

F330 has a lower clock speed, and may have issues running high KV motors. The F39X doesn’t have these problems, and also supports Multishot ESC protocol and Oneshot42 perfectly. Two well known examples are Littlebee 20A (F330) and DYS XM20A (F39X).

Busybee (EFM8BB)

Busybee MCU are the upgrade to the F330 and F39X. BLHeli_S ESC’s commonly runs

·BusyBee1 – BB1 (EFM8BB10F8)

·BusyBee2 – BB2 (EFM8BB21F16)

These are better because instead of using software PWM (pulse width modulation), they have specific hardware that can generate a PWM signal that is synced with the duty cycle of the processor, the result is much smoother throttle response. They also support the latest D-Shot ESC protocols. Examples of ESC’s that use these MCU’s would be the Aikon SEFM 30A and DYS XS30A.

In a nutshell, the overall performance ratings from best to worst:

1.BB2

2.BB1

3.F39X

4.F330

5.Atmel-8-bit

8-bit and 32-bit

Many ESC’s are still using 8-bit processors (F330, F39X, Busybee etc), but since 2016 some 32-bit STM32 based ESC’s started to emerge, such as the KISS 24A Race Edition, the V-Good Firefly, and the Gemfan Maverick. These powerful 32-bit processors unlock many new features that were not possible with the limited processing power and capability of the 8-bit MCU. Features such as the “ESC Telemetry” on the KISS 24A, or “Change Rotation Direction at Start-up” on the Firefly – to mention just a few.

ESC Protocols

ESC protocols determine how fast the signals can be sent from FC to ESC, which can have a big impact on your quadcopter’s performance. The original (oldest) ESC protocol – standard PWM, has delay up to 2ms, while the currently fastest Multishot has reduced latency down to only about 5-25uS.

Here is a list of current protocols used on quadcopters, from oldest to latest:

·Standard PWM

·Oneshot125

·Oneshot42

·Multishot

·DShot(DShot150, DShot300, DShot600, DShot1200)

·ProShot

Supports for Active Braking and Hardware PWM

There are a few key features in an ESC that make them perform great and are worth mentioning.

·Damped Light, a.k.a. Active Braking – Greatly improves responsiveness

·Hardware PWM – Improves smoothness and responsiveness, makes your quad noticeably quieter and slightly more efficient. It also allows for more fine control

·Dedicated gate driver – Cheaper ESCs use transistors to drive the FET gates, but using a dedicated gate driver improves active braking effectiveness

Size and Weight

Normally the size and weight of an ESC is proportional to the amp rating.

ESC’s that are designed for mini quad have fairly standard dimensions and weight these days, at around 4-6g each. It’s becoming challenging to make ESC’s any smaller and lighter without sacrificing performance and cooling. For racing you generally want to keep your quad as light as possible, however ESC is probably not the best place to look if you want to drop more than a few grams.

Smaller ESC’s tend to heat up faster and they can be harder to cool down, which leads to concerns of overheating with tiny ESC’s.

Input Voltage

Some ESC’s might support input voltage up to 6S, some might only do 4S maximum. Make sure the ESC is compatible with the LiPo voltage you want to use with your mini quad. Do check what the voltage rating is for your ESC’s, powering your ESC with excessively high voltage will fry them, and possibly your motors as well.

With or Without BEC – Opto ESC

Some ESC’s come with a built-in BEC (battery eliminate circuit) that outputs 5V (which you can use to power your flight controller RX and other components). Those that don’t have built-in BEC, are often referred to as “Opto” ESCs by marketers and manufacturers, despite this claim  though, these ESC’s might not actually use opto-isolators.

An opto-isolator is an optical component that transfers signals using light. It basically separates the high voltage circuit from the low voltage circuit, and prevents rapidly changing voltages from damaging the ESC electronics or interfering with the signals from the FC.

ESC’s that don’t have a BEC have the advantage of being lighter, smaller, and less noisy (since the motor control circuitry is optically isolated from the radio receiver and flight controller).

Without the 5V BEC however, your FC and RX will require a separate power source. (Note: ESC’s without a BEC don’t have the “red” servo wire, only the signal and ground wires)

Novice Question: Connecting ESC with Motor

I still remember when I started with quadcopters, I was staring at my ESC and motor, wondering how to connect the 3 wires. I still get this question occasionally from beginners.

Don’t worry about the order, simply hook up the three wires on one end of the ESC to the three motor wires in any order you’d like. If the motor spins the wrong direction, simply switch any two of the motor/ESC wires. You can also change the rotation direction setting in BLHeliSuite (if you are using this firmware). For KISS ESC users, there are 2 solder pads you can bridge to reverse motor rotation.

BLHeli ESC Configurator

As you might be aware, BLHeliSuite only runs in Windows, but there is a good news for users of other operating systems. There is now a Chrome APP called BLHeli Configurator you can use to flash and configure your ESC’s, which will run on any OS as long as you have Google chrome installed.

ESC Integrated Motors

We have seen motors with an ESC built into them such as the ZTW Black Widow. While it seemed to be convenient and space-saving, it was actually quite a controversial idea. In the case of either the motor or ESC failing, both would need to be replaced and costing more in the long run. Another issue with the motor and ESC in one is that neither can be upgraded individually.

4in1 ESC

A convenient option is the 4-in-1 ESC, which is basically four ESC’s integrated into one single board of the same size as an FC or PDB, which you can stack together, cleaning up your wiring. However damaging a single ESC means the retirement of the whole board. This is a trade off between risk and convenience.

To combat this compromise, some manufacturers have released a design using 4 separate ESC’s which join together to form a single board which can be built into the stack, such as the Quadrant 4-in-1. A 4-in-1 ESC also has benefits in terms of weight distribution on the aircraft, because the mass is more centralized there is less moment of inertia to the mini quad, which should improve responsiveness.

Original link from Oscarliang: https://oscarliang.com/choose-esc-racing-drones/

For us mini quad pilots, it’s easy to go through 10, 20 or even more LiPo batteries in a flight session. You can certainly buy as many batteries as you need for a single session, but there is a more cost effective way to do it.

By bringing a portable charging station with you, you can recharge used and empty batteries in the field. You can recharge whatever you need and therefore if you were leaving early you wouldn’t go home with as many fully charged batteries.

To sum up, the benefits of charging in the fields are:

·it requires fewer LiPo packs – more cost effective

·smaller chance of going home with fully charged LiPo

·the charger can be shared between buddies for a good long session

Portable Charger

As a good portable charger, it’s best to pick one that doesn’t have integrated PSU, so you can power it with low voltage DC power which is widely available on many portable power sources. Many chargers work with 12V DC, though one with wide input voltage would be preferable such as 11V to 18V.

Charger specification depends on what kind of portable power sources you use.

Light weight and compact are also a bonus when it comes to portability.

Some particularly popular chargers for field charging are the iSDT series (SC608, Q6, SC620), because they support a very wide input voltage (9V-32V) and come with a XT60 connector that allows you to use LiPo batteries as power source. They are also good for indoor everyday charging.

Here is my review of the iSDT SC-620. Product Page for Q6: http://bit.ly/isdt-q6.

To sum it up, you want the charger to:

·allow wide range DC input voltage that is compatible with your portable power sources

·light weight and compact for easier transportation

Portable Power Supply

To power the charger without access to a wall socket and electricity, you’ll need a portable power source.

High Capacity LiPo

The most popular and portable solution is probably using a large LiPo battery as the power source. For example, a 4S 16000mAh (16Ah) can charge at least 10x smaller 4S 1300mAh. Some prefer higher cell count batteries as they can store more energy under the same capacity, and thus can charge more packs. For example a 6S 16000mAh has 50% more energy than the 4S 16000mAh.

Energy stored (Watt-hour, wh) = voltage x capacity

However, make sure your charger supports the voltage of the portable power source as input.

C rating is not that important with LiPo batteries at such high capacity, and because we don’t tend to draw too much current at any instance. 5C or 10C are more than enough.

Other Portable Power Source

Although you can use your car battery to charge your batteries, we recommend using a separately deep cycle battery just for your LiPo charging so you don’t accidentally kill the battery in your car. (unless you are as smart as Walter White)

If you have a lot of batteries to be recharged in the field, or need to power multiple chargers, you may get a portable generator. They are powerful and often have DC as well as AC output which is compatible with a wider range of chargers. But generators are also noisier and more expensive than other options in our list.

Solar generators are another great option especially for sunny day camping/flying.

Horváth Csaba showed us how he use a parallel board to “reverse parallel charge” in the field, by connecting several spare 3S batteries he has as power supply for the charger.

Original link from Oscarliang: https://oscarliang.com/charge-lipo-batteries-in-the-field/

Errors when flashing FC firmware

Often when there is a driver issue with the flight controller in Windows, you might get errors when press the button “Flash Firmware”. One common error is:

Failed to open serial port

Then the FC COM disappears, and it appears again when you unplug the FC and plug it back in… The same problem keeps happening when you try to flash the FC.

Or you might be getting this error:

No response from bootloader, programming failed

If you are able to connect your flight controller to Betaflight GUI, but you just cannot flash firmware. In which case please follow these steps to resolve the problem.

If you cannot connect to GUI at all, please try the “bootloader fix” first to flash the firmware. If that doesn’t help then read on.

Fix FC driver issues using ImpulseRC Driver Tool

Connect your FC to the computer with USB cable.

Download and run ImpulseRC Driver Fixer Tool (it’s a 3rd party software, please scan with AV software before running it)

This tool will automatically search for your FC, and install whatever driver you need on your computer.

Once that’s completed successfully, the driver should be installed. Now fire up Betaflight Chrome App GUI, and you should see “DFU” next to the connect button, rather than the usual COM port in the text box.

You should now be able to flash the FC as you normally would.

Once done, your COM port will reappear in the text box. And you should be able to flash the FC from now on without any issues on this computer.

Original link from Oscarliang: https://oscarliang.com/fc-driver-issues-flash-firmware/

Buzzers for quadcopter is one of the most basic hardware in the hobby, but a few beginners are still getting confused. So hopefully this article can help clarify a few things.
Don’t forget to check out our quadcopter building tips and tricks. Feature image by Frank Unger from our Facebook group.
Purpose of buzzer on a quadcopter
·Lost model alarm – helps you to find your quad in bushes after it’s crashed
·Voltage low warning – buzzer goes off when battery voltage is low
·Quad status – the beeping can help you identify what state the quad is in, e.g. when the quad is ready to arm it beeps 3 times; when there is a problem with receiver connection, it beeps non-stop etc
Choices of buzzer
There are 2 types of buzzers, Active and Passive.
Basically, active buzzers beeps as soon as you give it DC voltage, while passive buzzers require an AC “sound signal” for it to generate sound. Quadcopter flight controllers usually support 5V 2-pin active buzzer/beeper. These buzzers are cheap, light weight and widely available online.
If you are still wondering which type of buzzer to get, just get active buzzers.
Matek makes some really cool buzzer designs. For example this “Loud buzzer” works as a normal buzzer, and can also be controlled via a AUX channel (PWM signal) at the same time.
And this “LED tail light buzzer” is like the last one, but combines with RGB LED’s which can be mounted on the back standoffs.
Don’t confuse yourself with LiPo voltage alarmecker with the type of buzzers we are talking about here. Voltage alarms are equipped with speakers but they are meant to connect to the balance lead and read the voltage. Other ways to monitor quadcopter voltage.
How to use buzzer on quadcopters
Most flight controllers have dedicated pins for connecting the buzzer, normally labelled as B+ and B-, or similar. 
How to make buzzer louder?
·Connect multiple buzzers in parallel, for example.
·Use higher voltage rated buzzer, and provide higher voltage to it, such as using 12V in this example.
Buzzer Alternative?
As a lost model alarm, you could setup your ESC beeping as an alternative to buzzer: Using ESC Beacon as a lost model alarm

Original link from Oscarliang: https://oscarliang.com/buzzer-quadcopters/