4S- Li- Ion SOTA battery

2020-07-15 11.37.27

This battery consists of recycled Li-Ion cells type 18650.

Time and again, we have to deal with defective Li-Ion batteries in the repair café next door where I’m working in. Usually individual cells became defective or the battery management unit is broken. The (remaining) cells are sometimes still as good as new (i.e. close to their original capacity) – too good to throw away!

At some point I asked myself whether it would be possible to build a SOTA battery from recycled cells.
First I looked up the properties of a Li-Ion cell:

  • Cell voltage of approx. 4.1 Volts when fully charged
  • Cell voltage of less than 3.3 Volts when discharged (depending on the manufacturer‘ s specifications).

Next, I looked up the specification of my KX3: It tolerates supply voltages up to 15 Volts and less than 10 Volts (but only with reduced output power below 11 Volts).

Finally I had the choice:

  1. Using three cells in series (3S): The battery voltage will soon drop below 11 Volts during operation. Below this threshold, power output must be compromised (which can sometimes be tolerated).
  2. Using four cells in series (4S): The battery voltage will be above 16 Volts in the beginning. No portable transceiver will operate at this voltage. A voltage reducer circuit is needed which will cause extra power loss!

I went for the 4S version, because I prefer using the KX3 with 10 watts output power. It helps being heard when condx are marginal.

Design and construction:

In order to perform accurate voltage measurements I built a 10V- reference source before I actually started. With this reference source it was possible to calibrate my multimeters. Now I was ready to go.

Four cells, each with a capacity of at least 2400 mAh, are connected in series. Although this is not recommended, I have soldered the „soldering tabs“. Spot welding is unfortunately not an option for me.
A thin layer of foam is placed between the cells to compensate for slight variations in thickness. For balancing, the battery is equipped with a balancer cable and a five-pin plug. The cells are held in place with adhesive tape. Open contacts and conductive surfaces are carefully insulated. For protection, the cells are packed in a heat shrinkable tube. A fuse holder is integrated in the power cable which uses a PowerPole connector.

Voltage reducer

The voltage reducer consists of two 1N5401 silicon diodes connected in series. Each diode has a forward voltage drop of approx. 0.75 volts, almost independent of the load current. In „reduce“ mode, about 1.5 volts will drop across the two diodes
The circuit is assembled in a small plastic housing and equipped with PowerPole connectors at both ends.


When starting the activation, the voltage reducer is in „reduce“ mode. Once the battery voltage drops below the threshold voltage *) , it is safe to switch to „bypass“. Then the diodes are bridged with the small switch and the full battery voltage is available at the TRx.
*) For the KX3, which works up to 15 Volts, this threshold is 13.5 V (shown in the KX3’s display).

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