Introduction: Repair a Thrift-store Drill (BLUE RIDGE BR2101U)
I got this drill from a thrift store for $5. It didn't work, but I was confident that I could simply create a charger of the appropriate specification and it would resume normal operation immediately. This was not exactly the case, but I was ultimately successful in reviving it.
Supplies
BLUE RIDGE BR2101U (dead battery and missing charger)
Kungber lab power supply, 0-30V, 0-10A
Assortment of barrel jacks (such as would come with a universal power supply like this)
Soldering iron (I'm using a MINIWARE TS100 -- since replaced by the TS101)
Solder (SN100C alloy, 3% NC601 flux, 0.032" diameter)
Superglue (cyanoacrylate)
Step 1: Preliminary Troubleshooting Steps
The first thing I tried was blindly connecting the 12V 800mA 120V power converter to the best-fitting barrel jack I had available, wired center-positive, and plugging it into the drill to see if any lights turned on. This is unwise for a variety of reasons -- while barrel jacks are *usually* center-positive, it is not guaranteed, and furthermore, the no-load output of an unregulated linear power supply like the one I was using is not 12V -- it is about 17V. So that's two opportunities to damage my device under test, right there: reversed input voltage polarity and excessive input voltage. Luckily these did not come to pass. However, the drill did not accept a charge.
Step 2: Opening It Up
One common problem that I have previously found in lithium-ion powered devices, in particular, in a GameBoy Advance SP (frontlight variant), is that the battery will discharge below its safe minimum operating voltage (3.0V to 2.5V at the lowest), and then the charge controller will refuse to allow the battery to charge at all. Directly charging the battery itself from a lab power supply will oftentimes allow you to start using the device's built-in charging system again (once the battery voltage goes back above the internal charge circuitry's "safe threshold"). Many sources of information note that it is unwise to revive lithium-ion cells that have been over-discharged in this manner, citing a risk of the cells "likely becom[ing] chemically unstable, creating a risk of a short circuit ... [and a] risk of them blowing up." I'm rather surprised that the venerable PowerStream Battery FAQ doesn't mention this.
In any case, I opened the drill and found three Li-Ion cells in series, which measured a combined voltage of 1.8V (0.6V per cell). Very safe, yes. So naturally, I set my handy-dandy lab power supply to output 12.6V current-limited to about 0.3A (300mA), and directly wired it to the cells in the drill's battery pack. Those of you following along at home will note that the "safe" "recommended" charge rate for an unknown battery is 1/10C, that is, one tenth of the battery's Ah or mAh rating, but in amps or milliamps respectively. So, for example, a mystery battery printed with a 1000mAh capacity can be safely charged with a current limited to 100mA.
After some time directly charging the internal battery, lo and behold, lights illuminated, and I was able -- after a while -- to pull the trigger on the drill and cause its shaft to spin. Furthermore, after charging the internal battery directly for a while, I was able to get the drill to accept charging through its barrel jack power input port!
Step 3: Looking Up the Documentation
You might think, dear reader, that I would look up the documentation -prior- to merrily unscrewing the drill's case and tapping into its battery bank directly with my lab power supply. This is not the case. However, once I had ascertained that the drill would turn on at all, I finally found the time to look at what was printed on it:
BLUE RIDGE BR2101U 10.8V 14.04Wh 12V Max Lithium Ion 1.3Ah 3INR19/66
For use only with BLUERIDGE JLH031350300U charger.
So I looked up the charger in question, which was rated to provide 13.5V 0.3A (300mA).
Step 4: Conjuring a Replacement Charger
Using the power of electromancy, we are now ready to conjure an appropriate replacement charger.
The schematic is listed above. The first step is to solder an LM2596 buck regulator module in-line with the 12V 800mA power converter I happened to have on hand.
Step 5: Tuning the Replacement Charger
Having attached the LM2596 buck regulator module in-line with the power converter, now we may tune the output of the regulator module to provide the appropriate 13.5V. The regulator has a multiturn potentiometer which may be adjusted using a screwdriver until the output voltage is correct. After setting the output voltage to 13.5V, use hot glue to hold the potentiometer screw in place so that it doesn't get jostled into a different output voltage over time.
Step 6: Mechanically Securing the Replacement Charger
After correctly configuring the LM2596 buck module's output voltage, the LM2596 buck module may be affixed to the 12V 800mA power converter, and the input and output wires elegantly secured with fanciful knots and hot glue.
Step 7: Attaching and Testing the Output Barrel Jack
The barrel jack may now be soldered to the output wires coming from the LM2596 buck regulator module. After verifying the output polarity (center positive, for this particular drill), the newly-crafted charger may now be plugged into the drill to verify that it receives a charge.
Step 8: Mechanically Securing the Output Barrel Jack
Once you've verified that the circuit charges the drill, it's time to mechanically secure the output barrel jack. I did this by surrounding the pins and solder with hot glue to create a strain relief. After it cooled, I coated the hot glue with superglue in order to create a hardened outer layer, better capable of withstanding abrasion and rough handling.
You may view the resulting operation of the drill -- in living Technicolor -- here, if desired.
