Harbor Freight Battery Pack Salvage:
Introduction: 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.
Supplies
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)
Step 1: The Battery Pack:
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.
Step 2: Battery Pack Circuit:
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. 267
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.
Step 3: Li-Ion Battery Placement:
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.
Step 4: Motor Current Draw:
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
Step 5: Cell Selection:
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. 331