Upload the Arduino Sketch to your Arduino Nano.
 
Once uploaded, you can open the Serial Monitor, and set the Speed to 115200.
 
Confirm the Arduino is operating as expected by monitoring the Serial Output.

Remove the foam backing from the board and place it to the side.

We will be reusing it, so try to keep the adhesive intact and dirt free.

Some heat on the PCB may help release the adhesive.

Remove IC2, IC3, and D3 from the board. They will not be needed.

IC2 and IC3 are the 7 segment display drivers, which we will be replacing with our Arduino.

D3 provides BATT+ with USB power, which we need to isolate.

Drill a small pilot hole, followed by incrementally larger holes in the location marked.

This hole must be big enough for your Arduinos reset button, so the exact size will vary.

This will cut through the trace between Battery and Voltage regulators for the previously removed 7 Segment Display drivers.
 
 
Next, drill a hole next to the anode pad of D3. We technically could use the 5V+ pin from above, but the Kaisi Board has a PTC fuse (R1), and it’s not a bad idea to utilise it. Drilling a hole here will give us a place to route a wire later on.

Depending on your level of Soldering Experience, you have 3 options here:
 
1 – You can remove the USB port from your Arduino Nano, and later reroute the USB Data pairs to the Kaisi’s Micro USB port.
 
We decided this would look nicer, and that Micro USB cables are much more common than Mini USB cables these days, so we opted for this route.
 
2 – You can drill/cut a hole in the Kaisi Ficture where IC3 was previously located (depending on version), allowing the Arduinos Mini USB port to protrude through.
 
3 – Remove the Arduino’s USB Port, and do nothing further. You will lose your ability to obtain Data via Serial, or upload an updated Sketch if one is provided.

As you can see, the Arduino’s reset button aligns with the hole we drilled, but there is some exposed copper from the Ground trace.
 
For this model of Arduino, that exposed copper sits uncomfortably close to the Reset trace leg of the button.
 
To avoid a short, resulting in the Arduino being stuck in a reset state, we covered both the exposed copper, and the buttons leg with some 5 minute epoxy and let it set.

The Arduino needs to be mounted face down. Exactly how you choose to do that is up to you. You could use adhesive tape, B7000, whatever you have on hand.
 
Much to the displeasure of some AVR fanatics (sorry, we’re not even sorry), we decided to use 5 minute epoxy on the ATmega328P itself. While they spit their coffee out and shake their fists, we’ll show you how that looks.

Using some tweezers or a blade, carefully scratch away the solder mask from the Ground and BATT+ traces as pictured.
 
The specific place on the Ground trace you expose, may depend on the width of your TP4056 board (they arent always the same size).
 
While you are at it, Scratch away the end of the HDQ/SWI trace.

Tin these 3 exposed areas with Solder.

Connect a TP4056 board to a USB cable.
 
Ensure that a Green or Blue LED illuminates, and not a Red one. This is the Fully Charged LED, and it’s not uncommon for them to be backwards. If it is, swap them over using hot air, or grab another TP4056 board and test again.
 
This isn’t too important. The LED’s won’t be all that visible through the PCB substrate, and it won’t effect the function of the board.
 
It’s always a good idea to test modules such as this before you solder them though, and make sure none of the magic smoke escapes.

Remove the Micro USB Port from the TP4056 board.
 
With a soldering iron and tweezers, carefully lift the CS leg of the TP4056 from the board.
 
Now solder a wire to it, ensuring is has good connection, and is not connected to the board at all.

The TP4056 board needs to be soldered face down, to the previously exposed Ground and BATT+ traces.
 
Be very careful to solder the BATT end, and not the INPUT end of the TP4056 board. This will require some fairly large blobs of solder. You might not get it right the first time.
 

There should be a gap between the components on the TP4056 board, and the Kaisi Board. It doesnt need to be much, but we don’t want the capacitors end caps to rub through and short against the Kaisi Boards ground plane. You can use a business card or a few pieces of paper as a spacer, to get an even narrow gap.
 
Soldering the TP4056 board like this serves multiple purposes. It provides good connection to the battery, a secure, low profile mounting method, and finally a heatsink.
 
The TP4056 IC itself has an exposed pad underneath it, which dissipates the heat into the boards Ground plane. By having a solid connection between the two ground planes, the heat dissipation is in turn slightly better.

Leaving some slack, solder the wire you previously soldered to the TP4056’s CS pin, directly to pin A5 on the Arduino.
 
This will give the Arduino the ability to enable/disable charging.

Again leaving some slack (we forgot to, and ended up breaking the wire later on), solder a wire from the HDQ/SWI trace on the Kaisi board, to pin A0 on the Arduino, via a 100 ohm resistor.
 
Because we used 0402 resistors, we soldered one side of the resistor first, and then soldered the wire to it.
 
If you are using through hole resistors, you dont need the wire, as you can just use the resistors legs.

OK, now it gets a little tricky.
 
We need to connect the 7 Segment Displays, starting with the segments.
 
The two displays need to have each segment connected in parallell, turning them into one 6 digit display.
 
Each segment needs a resistor to limit current. As stated earlier, we used 270 ohm.
 
Again because we used 0402 resistors, we soldered all 8 of them at once on one side, and then soldered the wires individually. In case you are curious, the 8th segment is for the decimal point.
 
If you are using through-hole resistors, you can solder them on as you go. Just make sure to keep the resistor between the Arduino and the segment pair, and use the leg of the resistor to connect the segments in parallel. Be careful to leave a gap between the legs. It may look like we are letting all of our wires touch, but they are enamel coated.
 
 
Starting with the bottom row, solder all of the segments as pictured.
 
Now do the same for the 3 segments on the top row.
 
Finally, the individual digits need to be connected. These dont need current limiting resistors.
 

Resistors:

Solder some heavier gauge wire to the anode pad of D3, making sure to leave the insulation as close to the pad as possible for isolation.
 
Next, feed the wire through the hole you made earlier.
 
Trim the excess and solder the other end of the wire to the IN+ pad of the TP4056. This provides the TP4056 with power. The IN- does not need to be connected, as its directly connected to BAT-, which we already soldered to ground.
 
Solder a short length of wire from the IN+ pad, to the 5V pin of the Arduino. This provides power to our Arduino.
 
Finally, Solder a wire from one of the Arduinos GND pins, to one of the Kaisi Boards Ground connections. We used the 5V- hole because it was close by.
 

TEST IT!

After a final close inspection, ensuring everything is connected correctly, connect a Micro USB cable and check the devices function. You should be able to read the displays, and obtain data from batteries. You will notice when the current/voltage mode rolls around, that the current is now measured in milliamps, rather than amps.

Any battery connected with a reported State Of Charge of less than 50%, will be charged using the TP4056. If the battery is above 50% SOC, the TP4056 will be disabled.

This behaviour can be changed in the Arduino sketch, but its worth pointing out that the TP4056 can only charge the cell to 4.2V. Therfore in the case of most iPhone Batteries (that can charge up to 4.35v), charging will be terminated at about 80%.

This is actually ideal in most cases anyway, because Lithium cells degrade less in storage with about 50% charge. If you plan to put them back into your inventory, you really dont want them fully charged, or flat.

The data provided including the voltage/current measurement, is now actually reported by the Gas Gauge IC itself. If no HDQ data is detected, the TP4056 will default to ON. This will recover any cells in a fault/flat state automatically, and still allow you to charge cells with no gas gauge (although, you probably wont wan’t to use them).

After confirming everything operates as it should, and patting yourself on the back, theres a few more modifications that can be made.

If you decided to reroute the USB data pins as we did, you’ll have to solder a couple of wires to the DM and DP pins on the USB Serial IC. Refer to the schematics for your particular IC for a pinout.

It’s worth noting that USB specifications generally suggest resistors for line termination. If your board has them, you should solder your wires there instead. If it doesn’t, it wouldnt be a bad idea to add some according to the datasheet of your USB-serial IC. Our Arduino Nano board didn’t have any, and we opted against adding them. With that said the PL2303 datasheet suggested 27 ohm resistors be placed as close to pin 15 and 16 as possible.

We made our DP wire longer to identify it at the other end. It may not have been necessary, but we also twisted the data pair together. This should help prevent crosstalk with our other wires.

The twisted pair was fed under the other wires, and through the hole in the corner of the Kaisi board, close to the Micro USB port.

After soldering the 2 wires to the corresponding pins on our Micro USB port, it was time for one more test to confirm USB communication.

D4 was way too bright for our liking. It actually hurt your eyes a little to look at.

On our board, R6 was a 1K resistor. On others its a 2K. Both seem to be uncomfortably bright, so we grabbed a 3.3K from our 0402 resistors and replaced R6 with it. It’s a little small, but we made it work.
 
 
Theres no visual feedback for HDQ data, but some of our TP4056 boards had a Blue LED on them.

We decided to steal one, and the resistor next to it, and solder them between the legs of S1.

Now we get a nice flashing Blue LED during HDQ communication.

Finally we finish where we started, with that foam backing that was put aside.

Do your best to mark the foam where the Arduino and TP4056 are located.

We advise cutting these out with a blade, but we burned a cavity into ours with an old soldering iron to accommodate the parts we added.

This creates toxic smoke, sets off smoke detectors, annoys the local fire brigade, and can make you blind. So, if you try something similar, you do so at your own risk. Remember, we told you not to.

 
 
Make sure you leave room for your wires ect, especially those that might be a little tight.

If you recall, we told you earlier to leave some slack on the wires. We didn’t, and in placing the foam back ended up breaking a couple.

If you wanted, you could leave the foam off, and use rubber feet instead. Or, you could encase the wires in epoxy or conformal coating to protect them.

The choice is yours.