Fixing one is not going to happen. I have a coworker who brings me things he breaks and can't fix himself. He bought a battery powered weedwhacker and leaf blower tool set from Sam's Club for about $75. It included 2 batteries and a charger. If you read Amazon reviews online about 1/2 the people that own this set or use the battery don't recommend it to anyone they care about. It works a couple of times and dies. More on that later. However, in trying to fix this I discovered that the cells inside are still good and are $10 each retail because they are top of the line Samsung 18650 lithium ion unprotected cells.
The cells are spec'd out here by Samsung. 22 amp max continous discharge and 2 amp hours of storage.
Battery pack replacements are about $75/pack. That is actually a fair price because of the cost and quality of the cells inside, but I would not recommend replacing them. The replacements won't last any longer than the originals. More on that later as well.
Step 1: Disassembly of the Pack
The makers of this pack made it literally impossible to disassemble the pack without it being known. If you look at the picture above you will see teardrop shaped plugs. There are 2 on each side covering 2 of the 4 screws holding each side piece in place. Both side pieces and the top and bottom assemble as a unit and you cannot remove those plugs without picking them out or drilling them out. Once you deform them it becomes obvious you tried or succeeded in opening the pack. Also, the pack uses #10 tamper proof torx screws. The charger has 2 recessed tamper proof torx screws that a bit won't reach so if you want to open the charger shell you need a #10 tamper proof torx screwdriver.
There are no screws under the foil safety label on the bottom of the pack, so it is not necessary to lift the label. The number on the label is E206252. This number is not the pack's ID number it is the label's ID number. The pack is a S12020a 20v Scotts sync battery.
The 18650 plugin wall battery charger in the upper left and the red and green jumpers were used to individually charge each cell and to test the pack. Fully charged, nothing worked. The proper charger refused the pack and the appliances would not run. More on that later.
Step 2: Circuit Board Pictures
There is a small thermal cutoff visible in a previous picture. I tried bypassing that cutoff with a wire and nothing changed. Then I replaced the wire with a 68 ohm 1/8 watt resistor and the battery charger would go into "testing the battery" mode, but refuse to proceed further. The pack still would not operate an appliance. I used a resistor because I correctly suspected before removing the board that the cells do not directly drive the appliance. A small resistor and a diode is needed to bias a transistor circuit that turns on the battery.
From underneath you can see that the positive battery tang has a direct connection to the positive end of the cell stack. The negative end goes through a circuit regulated by a pair of dc-dc mosfet converters MXP 4002AT TO263 2L.
Those are what regulate the drain on the pack. They are fried. I bypassed those mosfets and hot wired the stack and balanced the cells individually, then ran the pack in the leaf blower for about 1 minute. The cells went way out of balance quickly.
I cut off the weld strips with a handheld rotary cutoff tool (a Dremel if you like that brand name) instead of unsoldering 6 points on the board. You will need a high powered solder gun and solder wick if you want to remove the board nicely. Cutting the board off did not cut the cells apart.
Step 3: Hard-wired Test Results
Bypassing the circuit board resulted on the cells going from between 4.05v and 4.09v to
3.75, 3.88, 3.90, 3.91 and 3.94 in about 1 minute of run time. The low voltage cutoff Samsung recommends is 2.5v, which is pretty amazing. Most cells of this type should never be run below about 3v Using a Turnigy watt meter, I measured the draw at 320 watts peak/25 amps peak and 260 watts/15 amps running with the pack dropping from 20.75v to 15v and holding there. The pack voltage recovered to almost 20v after letting it rest a few minutes. That means the cells were pushed too hard during direct discharge.
I attempted to charge the pack as a string and the 3 strongest cells voltage went up over 4.5v, while one refused to come up over 3.6. Going over 4.2v is unsafe and will plate out lithium metal inside the cell and ruin it very quickly.
Step 4: No Room for Cell Expansion
Note that each cell is enclosed tightly in the plastic shell. This violates Samsung's published recommendation that there be room for 10% expansion to account for aging and heat. The original pack would probably work fine for an appliance that needed 5 or 10 amps. A battery powered weed whacker or leaf blower should have had a pack twice this size, but that would cost so much it wouldn't sell.
Both sides of the batteries are double strapped pulse welded with 4 spots. You can't weld cells with internal protection at the anode, so even if the cells' specs were not known, you would know that these particular cells have no internal charge or discharge protection. If you scrap the pack for the cells, you must insure these cells have both charge and discharge protection.
The automobile pocket sized jump starters use these kind of cells. 6 of them, three series by 2 parallel would easily supply the voltage and amps needed to boost a car with a dead battery. It's kind of scary to think you could jump start a car with something that fits in your shirt pocket. I'm beginning to wish nicads hadn't gone away. Those could easily handle a dead short and overcharging without the risk of fire these lithium cells have.