In this posting I will describe how I built a cheap Version 2 DIY 18650 Lithium-Ion battery pack for my Yaesu FT-817. The Version 1 DIY 18650 Lithium-Ion Battery pack is available HERE.
I decided to go the cheap route by salvaging 6 x cells from a Samsung Laptop battery pack that was in my junk box. I tested each cell and discarded dead or bad cells. These cells are all Li-Ion batteries. The cell voltage of the batteries I used was in the 4.18v range. So that gave me 12,6V on a 3S pack (2 Parallel, 3 series configuration) which is in the specs of most standard amateur radio transceivers. The capacity of one cell relates to 2000mAh. With the current configuration I have more than enough capacity for the FT817 to make several contacts over a much longer period than the previous Version 1 battery pack.
Before connecting the cells in parallel, first, check the individual cell voltages. For paralleling the cells, the voltage of each cell should be near to each other, otherwise, a high amount of current will flow from the cell with a higher voltage to the cell with a lower voltage. This can damage the cells and even result in fire on rare occasions.
If you are using brand new cells, the cell voltage is near 3.5 V to 3.7 V, you can join them together without worrying much. But if you are going to use an old laptop battery, be sure the cell's voltage is nearly the same, otherwise, charge the cells to the same voltage level by using a good Li-Ion Battery Charger.
I used a simple tester to first test each individual battery. However this is not the way to do it but I had it on hand and thus far it has worked for me. (More information on the tester can be found in and earlier post available HERE.) I charged each cell separately using a one cell charger as shown in the photo. Once all six batteries were charged to within specs I used my old trusted Accucell 6 Turnigy Charger to charge the battery pack. The charger is depicted in the photos below. To prevent a short between the terminals of the batteries I have used self-adhesive insulating rings on the positive terminal side. It is not mandatory but will give extra protection.
We have to assemble the battery to get the desired voltage and capacity. In my case, the voltage requirement is 12.6 V.
The voltage level of a fully charged 18650 battery is 4.2V, to get 12V we have to connect 3 batteries in series (3S). To increase the capacity we have to add such series groups (3S) in parallel. Here I have used two such groups. So our final battery pack configuration is 3S2P.
If you need more backup time, add more parallel groups. In that case, the configuration will be 3S3P, 3S4P, 3S5P... and so on.
Number of batteries required to achieve 3S2P configuration = 3 x 2 = 6
Now we have to arrange the 6 batteries properly for making the electrical connection among them and with the BMS board.
Place the first parallel group of cells (2 nos) positive side up, then place the second parallel group negative side up, and then finally the last parallel group positive side up. For a better understanding, you can look at the images below.
I used plastic 18650 cell holders/spacers to assemble the 6 batteries. The main advantages of using these holders are:
1. You can make a custom pack of any size according to your requirement. It's like solving a puzzle.
2. It provides space between the cells, which allows fresh air to pass and the battery gets cooled easily.
3. It makes your battery pack solid and reliable.
4. It provides safety anti-vibration to your battery pack
Once I was happy with the outlay I used nickel plates on top of the 2 cells ensuring that it covers cell terminals, and then cut it. Connect the first parallel group negative terminal to the positive terminal of the second group and then the negative terminal of the second group to the positive terminal of the third group. Unfortunately I do not have a battery spot welder (yet) so I had to revert to using a soldering iron which is not the best idea to make up battery packs. However I used it and we will have to see how long the batteries will last.
I connected the 3 S BMS as shown in the wiring diagram. The BMS has four soldering pads: B-, B1, B2, and B+. You have to connect the first parallel group negative terminal bus to the B- and positive terminal bus to the B1. Similarly the third parallel group negative terminal bus to the B2 and positive terminal bus to the B+.
You can spot weld the nickel strips to the BMS or solder them to the PCB pad. I preferred to solder the nickel strips to the PCB for a sturdy connection. First, apply soldering flux to the PCB pads and end of the nickel strips. After that tin all the pads by applying a little amount of solder and then solder them together. After connecting all the cells, you can check the battery pack voltage to ensure you have connected it correctly.
I decided to build this battery pack as simple as possible with very little bells and whistles. Thus far it works flawlessly with the FT817. This pack can also be used for a variety of other power sources. You can either add a power up converter or a down converter for the voltage that you need.
I
still have several 18650 batteries in the junk box and will construct another battery bank for my Internet Router and Fiber connection.
The total cost of the project did not exceed +- R75.00 having used salvaged batteries. The items that made up the total cost was the battery holders, nickel plates, BMS and heat resistant polyimide tape
Once everything was in its place I gave the battery pack a full charge with the Accucell 6 Turnigy Charger. I charged the pack at 3.0 Amp with 11.1V (S3) as prescribed. The charge time was about 22 minutes with a charge supply capacity of 1834 and 12.6V reflected on the screen during the charge process. Unfortunately I did not save the charging data. This would have been helpful to also analyze the state of the battery pack. There will always be a next time. I am contemplating whether I should cover the battery pack with Battery Sleeve Heat Shrink Tubing for good isolation and safety.
Finely: I will test this battery pack with the Yaesu FT817 but might also use in for a back-up WiFi Router UPS. Once I have decided I will update this posting.
(Click on images for larger view)
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