Intro: Making a Functional Replacement for a Scotts 20V Lithium Pack
In another Instructable I showed how to disassemble a 20v Scotts lithium pack. I still had the weed whacker and leaf blower laying around and not wanting to throw them away decided to attempt making a substitute pack that would actually work. I already knew that the original pack was supplying more than 300 watts and that wasn't going to be easy to do with the leftover used cells. I had one original bad Scotts packs disassembled and 2 three-pack 18650 cell holders and some spare 18650 cells of unknown capacity.
Step 1: Reusing the Original Pack As a Sabot
A couple of years ago I bought some 18650 lithium cells and holders for testing. Since the Scotts pack uses 5 cells, 2 three packs was enough to make a test replacement pack. I soldered wires to the back of the original pack battery tangs.. This cannot be done with a pencil iron shown, too much heat is required. I already owned a Craftsman solder gun with a 200 watt rating, and even then it required careful tinning and some patience to get the wires well soldered to the back of the battery tangs. Then I reassembled the now empty original pack.
Step 2: Adding the New Batteries
I drilled and screwed the 2 three packs to the back of the original and added a wire to bridge the two in series. I originally only wired the packs to 5 in series like the original. After charging the cells with 18650 wall adapters chargers I tried the blower outside. The street I live on is covered in limerock dust from recent roadwork. The substitute pack worked well for about 5 seconds and then one connection began to glow red, so I shut it down and let it cool and went inside to investigate. The spring holding one of the original cells had melted from the current demanded. I resoldered that contact point and stretched out the spring and went outside to test and it glowed and melted again.
I guessed that there was too much current being demanded (20+ amps) for a spring loaded plastic contact holder to work and be safe. To check, I swapped out on of the original cells for a lessor quality replacement and that stopped the overheating since that cell simply choked the current flow. That also slowed the blower down.
Step 3: Power Isn't Just Current
I know the motor needs 300+ watts to run full bore, but power is volts times amps. If I couldn't push the current needed, choking the current with lessor cells and adding one cell to boost the voltage might help. I resoldered the negative lead to the end of the pack and added one more of the lessor cells. By now I had figured out that 2 of the original cells are basically no good and had substituted for those. Now I have a pack with 25.2v open circuit instead of 21.0v. When I flick the switch on the blower or weed whacker the voltage will drop very quickly as both devices have no electronics. They both just use switches and a fixed magnet dc brushed motor.
Step 4: Testing Again
Back outside with the 6 pack and same result. The pack overheated in less than a minute. Most of the cells became too hot to touch and the plastic shell melted behind one cell. The original blower design is basically not sane. A 300 + watt 20v battery powered hand held lawn appliance is pushing the edge of what's possible. Maybe that's why I see 40v and higher battery packs on portable tools now. Power goes up with the current, but resistance losses rise with the square of the current, so boosting the voltage is the simple fix. Unfortunately this motor was not designed for higher voltages, that's why the battery packs send pulsed regulated dc to the tools. I can't easily duplicate that. There is no point in trying to set this up to run from a car battery as a leaf blower needs to be truly portable to be useful. I have a drill, circular saw and cutoff tool that run from my car but those tools don't need the same walking around portability as the Scotts tools do.
None of the cells were discharged much in the test. They all stayed above 4.07 after beginning the test at 4.2, and they all cooled down after a few minutes.
After waiting a day, I put some fresh line in the weed whacker and used the half-discharged pack as a test on it.
It works amazingly well trimming weeds. No overheating issues and lots of battery life. I would like to get the blower going, but that will depend on if I can modify the motor to draw less current.