This post is for those who want to increase their battery life on the Horus. The stock 2s Li-ion Battery @ 2600mAh is a little short lived, especially if you intend to run TBS Crossfire. The modifications presented here will most likely void your warranty and should only be attempted if the user is competent in battery pack assembly. Do so at your own risk.
Place your transmitter on your workspace in a way that relieves strain/pressure on the gimbals and switches. Locate and remove the 4 screws on the back case.
Once these screws are out you can slide the back case off gently. If you want to adjust your gimbal tensions/frictions now is also a good time.
You can now remove the battery by disconnecting it from the board and unclipping it from its holder. Next remove the two screws that hold the usb circuit board in at the top of the cradle. Put these screws in a separate spot so that you dont mix them up with the others. Once you have removed the circuit board you can remove the four other screws holding the battery cradle in.
If you are a crossfire user now may be a good time to modify the main board for 400k baudrates. You can find the steps to do so on Philipp Seidels site below, with some excellent pics for reference.
https://blog.seidel-philipp.de/fixed-inverter-mod-for-tbs-crossfire-and-frsky-qx7/
At this stage we are going to cover the transmitter up and prepare our battery pack. We will use 4 x 18650 cells with the highest genuine capacity available. Note that alot of cheaper cells are incorrectly rated. Anything over 3500mah for an 18650 is generally false. The cells I am using can be found here:
https://www.phaserfpv.com.au/products/imren186503500mahbattery
Also the Genuine NCRs are good value too.
https://www.phaserfpv.com.au/products/boruit4000mah18650li-ionrechargeable37vbatteryforfatshark
It is advisable to assemble your pack in a clean environment free of conductive elements as to limit the chance of accidental short circuits. Also consider discharging the batteries to a low state of charge to mitigate the potential discharges if mistakes are made.
Make sure all cells are within 0.1 volts of each other with your multimeter.
We are going to make a 2 series 2 parallel pack (2s2p) to nearly triple our capacity over stock. The first step is to carefully scrape or scuff the surface of the batteries terminal so that we can tin it with some solder. Start on the negative terminals.
The ideal way to attach a lead would be to use a spot welder or buy 18650s with solder tabs already welded to them, but as you can see from the stock Horus battery soldering the leads on gives an acceptable finish. If you can source copper tabs or you find them hard to work with use some silicone coated multistranded wire, like AWG 18.
Try to use a reasonably hot iron and minimise the time spent contacting the cell, we dont particularly want to overheat our cells. Once you have 2 of the cells negative terminals tinned you can make your first connection. You may want to temporarily tape the cells together to help support an align them.
The next step is to repeat the process on the positive terminals. Please note that the cells outer case is the negative pole and take sufficient precautions to insulate it from the positive terminal.
Seen above with copper tabs, though you may find it easier to use flexible wires. Once you have completed this you can repeat the process on the other two batteries. The process is to connect them in series to the original 2 we have just placed in parallel, so you double check your connections are correct and join positive terminals of the first parallel arrangement to the negatives of the next two. Add a center tap wire at this connection to connect to the cell monitoring circuit, seen below in yellow.
There is very minimal space in the horus case so its important that the battery pack be assembled flat and with minimal added thickness. Even a couple of layers of electrical tape or heat shrink may make it too thick, and the back cover of the Horus wont fit neatly.
Ive used some hot glue to insulate and support the center of the pack in the above photo. The electrical tape and plastic tubes are temporary and are only there to splint the pack while I finalise assembly. Next add a lead to the negative and positive terminal and you are ready to add the protection circuit.
You have a choice here as to whether you remove the protection circuit from the original pack or source another. I will detail the process of removing the circuit from the original pack.
Remove the blue shrink carefully and note the stock connections to the small PCB. You will mimic these connections on the new pack.
Note the small black coated wire that extends from the circuit.
I believe this is the thermistor and is responsible for monitoring temperature. Also note that its in close proximity to the cells themselves, presumably so it can accurately measure heat radiated. Place it in a similar way, making sure not to insulate it and hinder its ability to function correctly. Leave the connectors in place, they will plug back into the Horus PCB.
Copy the connections across and solder your new pack to the boards correct terminals. At this stage you might try to measure the output pins on the connector and find that it reads almost no voltage.
Dont worry, you just need to jump some pads to retrigger the circuit back into operation. Bridge the P- and B- pads to activate the circuit again once all reassembled.
Confirm it has worked by measuring the voltage on the plug, note that the center tap is there for monitoring the cell and wont actually give you a reading, not like a true balance lead would.
Finally, attach your standoffs to the back of the 3d printed cradle which can be ordered from PhaserFPV here:
https://www.phaserfpv.com.au/products/horusx10sbatteryupgradecradle
Screw your circuit board back into place at the top and insert your battery, being careful not to bump the large electrolytic capacitor on the main board. I ended up using clear packing tape to wrap my pack to insulate it with the minimum change to its thickness.
Plug your pack back in and carefully replace your back cover taking care to check the alignment of the usb plug. Charge it up (it will take significantly longer than the stock battery did) and enjoy!