DIY Portable 12V Battery Charger From Laptop Batteries

This project shows you how to reuse 18650 lithium-ion cells from old laptop batteries to create a cordless battery charger capable of reviving dead car batteries, motorcycle batteries, and even vintage 6V batteries. With adjustable voltage output from 0-30V and current limiting features, this is an essential tool for any electronics enthusiast or DIY mechanic. What You'll Build: A portable 3S lithium-ion battery pack (11.1V nominal) with a buck-boost converter, capable of outputting 14V for charging 12V batteries or adjustable voltages for various applications.

Difficulty Level: Intermediate

Time Required: 3-4 hours

Cost: Approximately $15-25 (using salvaged batteries)

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Electronic Components : 9x 18650 lithium-ion cells (salvaged from laptop batteries or new) 3S BMS (Battery Management System) with balance charging Buck-boost converter module (adjustable, minimum 3A output), 2A toggle switch, Female power connector (XT60 or similar), USB to 12V charging module (for recharging the pack), Heat shrink tubing, Battery charging cables with alligator clips.

Hardware and Consumables : 3D printed battery holder (3S configuration) , Wire (16-18 AWG for main connections), Zip ties, Hot glue gun with glue sticks, Solder and soldering flux, Electrical tape or insulation material.

Tools : Soldering iron (60W recommended), Wire stripper, Wire cutter, Multimeter, Clamp meter (for current measurement), Heat gun (for heat shrink)

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Check Your Batteries (5 minutes)

What: Test each battery to make sure it's good

1. Use your multimeter
2. Good batteries show 3.0V to 4.2V
3. Throw away any battery showing less than 2.5V or looking puffy/swollen

Why: Bad batteries can catch fire or blow up

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Simple Version:

Think of it like organizing soldiers in a formation:

1. Take 9 batteries
2. Group 1: 3 batteries sitting side-by-side (like friends holding hands) = 3 in parallel
3. Group 2: 3 more batteries side-by-side
4. Group 3: 3 more batteries side-by-side
5. Then connect these 3 groups one after another (like a chain)

Why this setup:

1. 3 batteries side-by-side = more power, more capacity
2. 3 groups in a chain = higher voltage
3. Result: 11.1V (perfect for a car battery charger)

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What's soldering? Hot metal glue that connects wires to batteries

Steps:

1. Cut wire into pieces
2. Strip (remove plastic coating) from both ends - about the width of a pencil eraser
3. Tin the wire (apply a tiny bit of hot metal to the bare copper)
4. Use 2 wires to connect between batteries (this is safer - more like having 2 ropes instead of 1)

Connection Order:

1. Positive side of Group 1 → Negative side of Group 2
2. Positive side of Group 2 → Negative side of Group 3
3. Connect the overall positive and negative terminals with 2 wires each

Important: Always check polarity (+ to - connection). Reversing this causes fires!

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What's BMS? A tiny circuit board that acts like a bouncer at a club:

1. Stops overcharging (like saying "enough already!")
2. Stops over-discharging (like saying "save some energy!")
3. Stops short circuits (like jumping out of the way of a crash)

The BMS has 6 connection points:

Label : What it is

B-

Main negative (ground)

B1

Connection between Group 1 and Group 2

B2

Connection between Group 2 and Group 3

B+

Main positive

P+

Positive output to your devices

P-

Negative output to your devices

Connection Order (VERY IMPORTANT):

1. First: Connect B2 (between groups 2-3) and solder
2. Second: Connect B+ (main positive) and solder
3. Third: Connect B1 (between groups 1-2) and solder
4. Fourth: Connect B- (main negative) and solder

Why this order? If you connect them in the wrong order, you can fry the BMS. This specific order is safe.

After connecting:

1. Your BMS should light up
2. Use your multimeter to check that each group has the right voltage (around 4.2V each if fully charged)

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What: This is like adding a charging port to your device

Steps:

1. Take an XT60 connector (it looks like a 2-part puzzle)
2. Red wire goes to P+ (positive output from BMS)
3. Black wire goes to P- (negative output from BMS)
4. Solder them securely
5. Wrap with heat shrink tubing (like putting a safety coat on)

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What's a Buck-Boost Converter? A magic box that changes voltage:

1. Input: 11.1V from your battery pack
2. Output: Whatever voltage you set (5V, 6V, 9V, 12V, 14V, etc.)

It has two knobs (potentiometers) - like volume controls:

1. Left knob: Controls the voltage (how much electrical push)
2. Right knob: Controls the current limit (how much electrical flow)

Installation:

1. Connect your battery pack to the green LED side (input)
2. Leave the red LED side (output) for later
3. Use 2 wires for safety

Simple Test:

1. Turn it on
2. You should see a number on the display (like 12V or so)
3. Green light should turn on at the input
4. If something smells bad or sparks, turn it off immediately!

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Setting Voltage for Different Devices:

Device Voltage - Why

Car Battery

14V

Car batteries are 12V, but need 14V to charge

Motorcycle Battery (old 6V)

7V

Old bikes use 6V, need 7V to charge

Phone/USB

5V

Phone chargers use 5V

Effects Pedal

9V

Guitar effects boxes need 9V

Laptop

19V

Laptop chargers typically use 19V

How to Set:

1. Connect multimeter to the output terminals
2. Turn the voltage knob slowly (like adjusting a shower temperature)
3. Clockwise = higher voltage
4. Counter-clockwise = lower voltage
5. Stop when multimeter shows your target voltage
6. Mark it with a pen so you remember

Setting Current Limit (Protection):

What it does: Limits how fast electricity flows (like a water tap - prevents flooding)

Why it matters: Protects both:

1. Your battery pack (prevents overheating)
2. The device you're charging (prevents damage)

How to Set:

1. Connect a 40W light bulb to the output as a test load
2. Use clamp meter to measure current
3. Turn the current knob to reduce current
4. Recommended: Set to 1.5A or 2A for charging (slow and safe)

Test it:

1. Light bulb should glow
2. Current meter should show around 1-2A
3. Nothing should get hot

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What: The power converter gets warm. We need to protect your batteries.

Why: If converter heat transfers to batteries, they can get damaged or catch fire.

How to Fix:

1. Cut a piece of plastic, rubber, or cardboard
2. Place it like a blanket between converter and battery pack
3. This stops heat transfer - like wearing an oven mitt

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Battery Bundle:

1. Place battery pack and converter with the heat barrier between them
2. Secure with zip ties (like a hug to keep things together)
3. Make sure no wires are bent sharply or pinched

Add On/Off Switch:

1. Hot glue a toggle switch to the converter
2. Wire it so turning it on/off controls the power
3. This lets you save battery when not using it

Add Charging Cables:

1. Take two charging cables with alligator clips
2. Red cable → positive output terminal
3. Black cable → negative output terminal
4. Cover everything with heat shrink tubing (like putting on a safety suit)

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Before Plugging Into Anything:

1. Make sure the alligator clips are not touching each other (this would cause a short circuit - like spilling water on electronics)
2. Turn on the power switch
3. Check that the display shows your set voltage (like 14V)
4. Listen and smell - if you hear anything weird or smell burning plastic, turn it off!

Load Test:

1. Connect a 40W light bulb to the output
2. Bulb should light up nicely
3. Check voltage and current on display
4. Everything should stay cool to touch

Using Your Charger

Charging a Dead Car Battery

Dead battery symptoms:

1. Won't start the engine
2. Opens at like 11V (should be 12-13V)

Steps:

1. Set output to 14V
2. Clamp red alligator clip to positive terminal (usually marked with +)
3. Clamp black clip to negative terminal (usually marked with -)
4. Turn on switch
5. Watch the multimeter voltage slowly climb
6. After 4-6 hours, battery should be at 13V (revived!)
7. Try starting the car

Result: Your dead battery works again!

Charging Old Motorcycle Batteries (6V)

Before charging:

1. Open battery case
2. Check water level inside (should cover the plates)
3. Add distilled water if needed (not tap water!)
4. Clean any green corrosion from terminals with a wire brush

Charging:

1. Set output to 7V (one turn counter-clockwise from 14V setting)
2. Connect clips - red to +, black to -
3. Charge for 2-4 hours
4. Should go from 2-3V to 6V

Using as a Bench Power Supply

Different Devices:

1. USB Phone Charger: Set to 5V
2. Guitar Effect Pedal: Set to 9V
3. Car Accessory: Set to 12V
4. Laptop: Set to 19V (if converter supports it)

Just adjust the knob and connect your device!

Safety Rules (SUPER IMPORTANT!)

Lithium Battery Safety

NEVER:

1. ❌ Short circuit the terminals (don't let wires touch wrong way)
2. ❌ Charge above 12.6V (your battery will catch fire)
3. ❌ Discharge below 9V (damages battery permanently)
4. ❌ Drop or bend the batteries (can puncture them)
5. ❌ Leave near heat or in hot car
6. ❌ Ignore the BMS (it's your safety net)

DO THIS:

1. ✅ Stop immediately if battery feels hot (>60°C = hotter than hot water)
2. ✅ Store in cool, dry place
3. ✅ Recharge every 3 months even if not used
4. ✅ Keep fire extinguisher nearby (especially Class D for lithium fires)

When Charging

NEVER:

1. ❌ Leave charging batteries alone - stay nearby
2. ❌ Charge in bedroom or inside a tent
3. ❌ Charge under blanket or pillow
4. ❌ Charge without current limiting

DO THIS:

1. ✅ Charge in well-ventilated area (like garage with door open)
2. ✅ On a non-flammable surface (like tile, not wood)
3. ✅ Where you can see it
4. ✅ Use current limiting (never more than 2A for beginners)

Electrical Safety

ALWAYS:

1. ✅ Wear safety glasses
2. ✅ Work in bright, clean area
3. ✅ Check polarity (+ and -) twice before soldering
4. ✅ Let soldering iron cool before putting it down
5. ✅ Cover all exposed wires with heat shrink

NEVER:

1. ❌ Work tired or distracted
2. ❌ Solder near flammable materials
3. ❌ Touch the hot soldering iron
4. ❌ Leave iron plugged in when done

Troubleshooting

BMS Not Lighting Up

Possible causes:

1. Wrong wire connected to wrong spot
2. Bad solder joint (connection is loose)
3. Battery cells are completely dead

How to fix:

1. Check BMS wiring diagram again - follow the order we listed
2. Heat up solder joints again - sometimes heat helps
3. Test each battery with multimeter - recharge any below 3V
4. Try plugging in and waiting 5 minutes - BMS might wake up

No Power Coming Out

Possible causes:

1. Switch is OFF
2. Loose wire somewhere
3. Battery pack completely empty
4. BMS protecting itself (shut down mode)

How to fix:

1. Make sure switch is in ON position
2. Gently wiggle all wire connections while watching for LED lights
3. Recharge entire battery pack from wall outlet
4. Try unplugging battery from BMS for 30 seconds, then reconnect

Wrong Voltage Coming Out

Possible causes:

1. Potentiometer (knob) is in wrong position
2. Converter broken

How to fix:

1. Turn voltage knob while watching multimeter display
2. Find the right number you want
3. If knob doesn't change voltage, converter might be broken - test with different battery

Current Too High or Too Low

Possible causes:

1. Current limiting knob not set right
2. Not using proper test load

How to fix:

1. Turn current limiting knob slowly
2. Watch clamp meter for changes
3. Set to 1-2A range
4. If still wrong, use multimeter to measure current instead

Simple Performance Info

Battery Pack Specs:

1. Voltage: 11.1V normally (12.6V when full)
2. Can discharge: 3-6A safely
3. Capacity: 6000-9000mAh (how long it lasts)

How Long It Runs:

1. At 1A (slow): 6-9 hours
2. At 2A (faster): 3-4.5 hours
3. Charging a dead 35Ah car battery: 4-6 hours

Converter Efficiency: 85-92% (some energy becomes heat, which is normal)

Key Takeaways

✅ Most important: Always use the BMS - it's your safety net

✅ Always use current limiting - prevents damage and fires

✅ Check polarity twice - wrong connection = disaster

✅ Start with slow charging - faster = more danger

✅ Keep fire extinguisher nearby - just in case

✅ Never leave it charging unattended - stay nearby

Final Checklist Before Using

1. All solder joints are solid (no loose wires)
2. BMS LEDs light up when connected
3. Voltage can be adjusted with potentiometer
4. Current limiting set to 1-2A
5. Thermal barrier is in place between converter and battery
6. All wires are covered with heat shrink
7. Switch works (on/off)
8. No loose parts or wires
9. Tested with 40W light bulb - worked fine
10. Fire extinguisher nearby
11. You've read the safety section

Once all checks pass - you're ready to charge!

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