Harbor Freight Battery Pack Salvage:

A while ago the battery pack for my Harbor Freight cordless, 5½“, circular saw died. Figures 1, 2, and 3 are of the saw and battery pack. At that time Harbor Freight had also discontinued the saw and the battery pack and it has sat on a shelf collecting dust since then. The Instructables “Trash to Treasure Contest” gave me the inspiration to salvage the battery pack by converting it to a Li-Ion battery pack. I would then have my saw working again.

Tools:

Soldering iron and various other hand tools.

Spot welder

Materials: 5 Li-Ion power cells such as the Samsung 18650-25R or the Epoch 18650-3000P

Nickel Strip 0.015mm thick x 8mm wide

50 Amp, 5S, PCM with cell balancing. see this link

https://www.amazon.com/gp/product/B07RHZMXPD?ref=...

14 gauge stranded wire, short lengths

51K, 1%, 0805 Resistor, Digikey # YAG3742CT-ND

Zener diode, 22V, SOD80 , Digikey # 112-TZMB22-GS08CT-ND

Kapton tape

Double back foam tape

Small piece of 1/16: MDF (see text)

Figures 4, and 5 are of the open battery pack case and of the battery pack after it was removed from the case. The battery pack is 15, 1500mAh Nickle Cadmium, NiCd, cells connected in series. The nominal voltage of a NiCd cell is 1.2 volts with a nominal operating voltage range of about 1.3 to 1.0 volts. This provides a nominal pack voltage of 18 volts (15 x 1.2 Volts) with an operating voltage range of 19.5 Volts to 15 Volts.

A Li-Ion cell has a nominal voltage of 3.7 volts and a nominal operating voltage range of 4.2 Volts to 3.0 Volts. Five Li-Ion cells will provide an 18.5 Volt pack with an operation range of 21 Volts to 15 Volts. A pack of 5, series connected, Li-Ion cells, will have the about the same operating voltage range as the NiCd pack.

The battery pack has three terminals. A positive and negative terminal and a charging terminal as shown in Figure 6 and 6B, while Figure 7 and 7B shows the terminals’ wire connection inside the case. The charging terminal has a thermal breaker, in series with it, to disconnect the charger if the battery overheats during charging. Figure 8 is the schematic diagram of the battery pack.

Figure 6 shows 5, 18650, Li-Ion cells fitted into the bottom of the battery case with space above the cells for the protection and balance circuits.

A factor that needs to be considered is the motor starting current, or inrush current, which is needed to select the battery’s Protection Circuit (PCM). Figure 10 is the schematic of a circuit to measure the start and no load operating current of the motor. Figure 11 is an implementation of the circuit. The 5S 18650 pack shown was assembled from Sony-Fukushima US18659GR GP4 cells. These cells have only a 4 amp maximum continuous discharge current and are only being used to measure motor current. The measured current values will be used to select cells that have a maximum discharge current specification required for this motor.

Figures 12 and 13 are the oscilloscope pictures of the battery voltage and current during motor starting. The peak current, the blue trace, is 31.06 Amps (1.62V/0.049ohms). The battery voltage drops to 5.0 volts during starting. The motor’s resistance is 0.102 ohms ((5.0-1.62)/33.06). The internal resistance of the battery is 0.151 ohms (5.0/33.06) or 0.03 ohms per cell.

The peak current measurement of 31 amps is low because of the relatively high resistance of the 0.05 ohm current sense resistor.

The no load motor current is about 3.1 amps

The Li-Ion cells for this project require a high current continuous discharge current rating as well as a high current pulse specification. The current flow through the motor will vary between the no load current of and the stall current measured with the starting current measurement. These types of cells are classified as power cells.

The Samsung INR18650-25R would be an ideal cell for this application, but are in short supply. I could not locate any to purchase in time for this Instructable.

I did locate Epoch 18650-3000P cells. These are 3000mAh cells rated at 15 amps continuous, which I purchase for this project. Unfortunately, I could not locate a peak surge current specification for these cells. Additionally, Epoch has not responded to my requests for a complete datasheet or the peak surge current specification.

I assembled a 5S pack using the Epoch cells as shown in Figure 14. The cell are glued together with a bead of hot glue. The electrical connections are 0.15mm thick by 8mm wide Nickle strips spot welded to the cells terminals. Figures 15, 16, 17, and 18 are the motor circuit’s currents and voltages using the Epoch cells, with the circuit shown in Figure 11, and the value of the current sense resistor reduced to 0.0245 ohms. The cells were about 95% charged with the cell voltages at about 4.08 volts per cell, 20.4 volts for the five cells in series.

The peak current, see Figure 17 blue trace, is 100.4 amps (2.46V/0.0245ohms). The voltage across the cells dropped to 12.8 Volts, or 2.56 volts per cell, see Figure 16 yellow trace. The motor’s run current is about 3.3 amps (0.080Volts/0.0245ohms). See Figure 18.

These measurements point to two problems using the 5S Epoch cells. The inrush current is high. The second problem is the cell voltages dropping below the over discharge voltage threshold of the battery protection circuit (PCM).

A 5S PCM with a 50 amp short circuit current limit was selected, which is shown in Figure 19. An additional feature is an over current with two delay values to allow for high inrush current without activating the over current disconnect feature.

Figure 20 is a schematic of the PCM connected to the cells with the load and charger connections. Figure 21 shows the locations of the over current disconnect delay. Figure 22 shows the PCM, cells, and the saw wired together for testing. The current sense resistors, with a resistance of 0.001243 ohms, on the PCM is being used to measure the current waveform.

The initial test was with the over current delay jumper open, which is shown in Figure 23. The blue trace is the current waveform and the over current shutdown activates in about 12 milliseconds with a peak current of of about 105 amps (130mV/0.001243 ohms).

The test was repeated with the over current disconnect delay pads jumpered, see Figure 24. The measurements for the battery voltage, the yellow trace, and the current, the blue trace, are displayed in Figures 25 and 26. The over current shutdown delay is extended to about 120milliseconds, which provides time for the motor current to drop below the 50 amp threshold. Repeated starting and no load run testing showed reliable operation.

The charger that was supplied with this battery, Harbor Freight model number 68859, is a constant current charger. Measurement of the charger’s parameter indicated that several modifications would be needed. The charging current was measured at 1.31 amps over a battery voltage range of 15 volts to almost 29 volts. The current should have turned off at around 18.75 volts (15 NiCd cells x 1.25 volts/cell). A second problem was the open circuit voltage of the charger was about 41 volts, which is excessively high for charging this type of battery pack.

Figures 27 and 28 are pictures of the charger's PCB. Figure 29 is the schematic I drew by tracing out the PCB. I am not going to explain the circuit operation but point out the two components I changed to make the charger compatible with the Li Ion battery.

The open circuit voltage was reduced to about 26 volts by replacing the zener diode ZD2 with a 22 volt zener diode.

The battery voltage during charging is monitored by the AD6610. Charging for the Li Ion battery is complete when its voltage is about 21.25 volts (4.25 volts per cell). Resistor R71 was changed to a 51K resistor. Figure 30 highlights the locations of the resistor and the zener diode

With these two component changes the Li Ion battery was was charged successfully using several clip leads to connect the charger to the battery. The charger was then reassembled.

Three wraps of ¾” kapton were wrapped around the 5S pack as shown in Figure 31. A piece of 1/16” MDF, slightly larger than the PCM was attached to the battery using a strip of kapton tape. The MDF provides an electrical and mechanical isolation of the PCM’s PCB and the battery. In Figure 32 two strips of ¼” kapton tape are wrapped around the PCM and battery to hold everything together. Figures 33 and 34 are of the completed battery and PCM assembly.

The following steps mount the battery is the case and connect the wires to the battery pack’s terminals. A 1” wide stripe of double back foam tape is placed in the bottom of the plastic case as shown in Figure 35. The battery is positioned in the case towards the end opposite the latching mechanism location. See Figure 36.

The charging terminal has a thermal breaker in series with it that will disconnect the charger if the battery gets hot during charging. The thermal breaker is attached to the battery with a strip of kapton tape and the a 24 gauge wire is used to connect the charging pad on the PCM to the terminal of the thermal breaker. See Figure 37.

As shown in Figure 38, the positive wire from the battery is connected to the positive terminal of the case and the P- terminal is connected to the negative terminal of the case. The wire junctions are covered with shrink tubing.

The case is reassembled, see Figure 39.

The battery was placed on the charger and charged to completion, see Figure 40. The battery’s voltage after charging was 20.95 volts, or 4.19 volts per cell.

The battery was then attached to the saw and tested its operation cutting wood. Figure 41 shows some of the results cutting a 1”x3” board and a strip of ½” plywood.

This project successfully salvaged a 5 ½” portable circular saw from the e-waste scrape heap by converting the dead NiCd battery, that was discontinued by Harbor Freight, to a Li-Ion battery with twice the capacity of the NiCd battery.