Idea Log

a record of ideas I've come up with


Hardware hacks

Replacing cordless power drills batteries with lithium batteries

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


Power strip controlled by computers on/off status

So i wanted to have some of my peripherals turn off when my computer is off. Speakers and lamp and such.  I made a simple one using USB power to switch on/off a power strip but modern PCs have USB powered on always.  So without some intelligent circuitry, this will no longer work.

So what i use now is a power strip controlled by the PCs power supply.  Essentially I have a relay that switches when the computers power supply is on.  But with some added protection, so I wont damage anything inside the PC.


The PCs power supplies “12V” output is hooked into an isolation DC-DC converter, then a linear regulator.  This protects the PC if there is a short or if the external wires touch a high voltage by accident.  I put these all on a card to make it pretty, and more convenient.  After the protection circuit a cable connects to the power strip.  Inside the power strip a relay is controls the hot line of the power strip.  I used a small power strip to save space and to remind me not to plug too much into it.


Parts List:

  • Molex 8981 – Disk Drive Power Connector
  • VB1212LS-1W – Isolated 12V-12V DC-DC converter
    • VB1212LS datasheet
    • You can use any isolated DC-DC converter that allows Vin=12V and Vout>6.5V and max power > 0.1W
  • 5V linear regulator
    • You can use any 5V linear regulator or LDO that allows Vin=12V
  • 5V relay
    • You can used any 5V relay with switch voltage>=110Vac
    • Note that the max switch current of the relay is the max current you can use on the power strip
    • I used a 4.5V relay, because that is what I had.  If you use a 5V relay remove the diode, “diode1”
  • diode
    • Can use any switching or rectifier diode. I used 1N914
  • 1uF capacitors
    • I used 1uF electrolytic capacitors
  • Power strip









hacking bluetooth headphones by adding a 3.5mm headphone jack

bluetooth_headphones_openSo i go through ear buds like crazy.  Cheap ones last me two weeks to a month, high end ones last me 6-12 month.  I though this may be because of snags. So i bought a cheap set of bluetooth headphones.  It lasted around 90 days.  the hardware works fine, but the wires to the ear buds wore out.

I liked the functionality of the bluetooth thing so i decided to add a headphone jack to it so i can plug in any headphones i want.



  • bluetooth head phones
  • 3.5mm TRRS(4-pole) headphone jack



  • opening it up
  • removing the ear bud wires
  • drilled holes for the headphone jack
  • soldered wires from board to jack
    • [R+] to [right]
    • [R-] to [ground]
    • [L+] to [left]
  • added so glue to re-enforce the wires contacts
  • closed it up and added some glue to hold it together
  • plug in a set of headphones
  • zip tie excess wire from plug to buds



  • test with 3-pole and 4-pole headphone jacks before closing it up
  • use thin wire so it doesn’t pull off the PCB, I used 30AWG wire wrap wire.
  • be careful not to pull off contact from PCB.  I used super glue to re-enforce contact


Useful info:

from  CableChick
Example of a 3.5mm TRRS 4-pole cellphone headphone jack, from mklec



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