Why replace your batteries

  • Lithium batteries have great capacity, light weight, long lifetimes.
  • NiMH batteries are OK-ish, medium capacity, good voltage levels over discharge level, heavy weight, medium lifetimes
  • NiCd batteries suck. low capacity, normal weight, low lifetimes, bad for environment.

So I replaced my old battery packs with lithium-ion batteries from old laptop batteries.

Tips for battery pack design

  • Protection board (BMS) selection

    • Higher “max continuous discharge current” than expected, but not much higher than batteries max continuous discharge current. 20A or 30A depending on batteries.  I have used a 15A board and had to reset under heavy load.  I was able to avoid this problem most of the time by setting the drill clutch setting to the max number, but not the unregulated setting
    • Ideally longer “Over-discharge protection delay” time if possible. Most boards are around 0.2s but some are 2s
    • ShortOver-discharge protection release delay” time. 20ms if pretty fast
    • “Recovery state”: Automatic recovery is recommended
    • If you have “high discharge current batteries”(15, 20, 30A), you will want a board with thermal sensor/protection
    • They sell boards specifically designed for drills with ideal characteristics
  • Battery selection

    • Higher “max continuous discharge current” are best
      • “High discharge current batteries” (15, 20, 30A) are recommended, but increase battery temperature under heavy use, and will need a protection board with thermal protection
      • You can use batteries in parallel (P) to increase discharge current, and charge capacity, but will increase size and weight, so if you have high discharge current batteries. 2 in parallel are called 2P.
    • Battery voltage doesn’t have to be exact.
      • Select the number of batteries to be a multiple of 3.7V
        • Example: for 14.4V drill use, 4 batteries in series (4S) to get 14.8V average
        • NiMH and NiCad voltage discharge characteristics stay around the rated voltage then, drop quickly.  Lithium batteries discharge characteristics start at max voltage and fall at constant rate. Generally 4.2V max to 3V min
        • NiMH discharge curve
          • main-qimg-bba531169ebdb9ffa2086fd0d1d0cc44
        • Lithium discharge curve
          • components_tenergydischarge
  • Charger

    • You will need a “lithium battery charger”, you can not use you old charger.
    • You can buy a charger board and put it in the battery pack, or use an external battery charger and add a socket or connector to your battery pack.
    • “Balanced charger” will increase your drills usage time, and overall battery life


Save money by using batteries from old laptop battery packs

Dead laptop batteries often have only one or two dead cells or have been left uncharged for too long.  So I measure each cell and if the battier is above 2V I keep it, lower I recycle it.  Mixing Li-Ion batteries is not the best solution but I had to use various old laptop batteries, and it works.  Possible problems with mixing batteries are if 2 different batteries with different capacities, the lower capacity could discharge below minimum level and cause damage, lowering the capacity even more.  Not the end of the world.

Here is decent instructions on dismantlement a laptop battery.



Drills I have modified

Skil 2585

  • Voltage Rating: 14.4V
  • Original batteries: DW9061
    • C-cell, NiCd
    • Capacity: 1.2Ah
    • Load: forgot to measure before I recycled them
    • Cost: $50
  • New homemade battery configuration
    • 16-cell Lithium-Ion 18650 (3.7V, 2.8Ah, 4A discharge)
    • Configuration: 4S-4P
      • 4 is series gets pretty close to the voltage rating (4.2V x 4 to 3V x 4)
      • 4 in parallel,
        • I only had 4A max discharge batteries, so to get a high current without killing the batteries I used 4 in parallel.  This gave 16A max continuous discharge.  It made the battery pack large and heavier than normal, but also lasts forever without recharge.  If you have higher current batteries, you can uses 2P or even 1P.
    • Voltage: 14.8V (12-16.8V)
    • Capacity: 8Ah (2Ah with my used batteries)
    • Measured Load: forgot to measure
    • Protection circuit board: 15A 4S Li-ion Charger Protection Board
      • Max average current: 15A(Natural cooling 10.A, add heat sink 15A, the maximum instantaneous current 30A)
    • Add-on: 4S Lithium Capacity meter indicator/display


Dewalt DW952

  • Voltage Rating: 9.6V
  • Original batteries: DW9061
    • 8-cel, C-cell, NiCd
    • Capacity: 1.7Ah(DW9061 XR)
    • Load: forgot to measure before I recycled them
    • Cost: $50
  •  New homemade battery configuration
    • 6-cell Lithium-Ion 18650
    • Configuration: 2S-3P
      • I was worried that 3 in series would make the voltage too high (9-12.6V)
      • I decided to put 3 in parallel to supply the high current @ 2C max recommended current would be 18A
    • Voltage: 7.4V (6-8.4V)
    • Capacity: 6Ah (3Ah with my used batteries)
    • Measured Load: 5A(no load), 20A(extreme load)
    • Protection circuit: 2 S 15A 7.4 V Li-ion  Battery Protection Board
      • Maximum continuous discharge current: 15A
      • Over current protection: 20.8A
    • Add-on: 4S Lithium Capacity meter indicator/display


Batteries used


  • Nominal Voltage: 3.7V
  • Size: 18650 (Cylindrical)
  • Capacity: 3000 mAh
  • Chemistry: Lithium Ion (Rechargeable Li-Ion)
  • Brand: LG


LGABC11865 / ICR18650C1

  • Nominal Voltage: 3.7V
  • Size: 18650 (Cylindrical)
  • Capacity: 2800 mAh
  • Chemistry: Lithium Ion (Rechargeable Li-Ion)
  • Brand: LG
  • Max Charging current: 2.7A (1C)
  • Max continuous discharging current: 4A (1.5C)
  • Max peak discharge current: 7A, for less that 4s (theoretical)
  • LG ICR18650C1 datasheet
  • List of lithium battery specs

Sony US18650GR (G6 date code)



Protection boards I have used