Your Black & Decker jump starter sits useless in the garage—no lights, no crank power, and absolutely no response when plugged in. That panic when your emergency roadside backup fails is worse than the dead car battery you’re facing. Whether you own a 19-year-old 300’THSTB model or a newer unit flashing confusing error codes, this guide delivers proven fixes for your Black & Decker jump starter not charging crisis. You’ll learn to diagnose dead batteries, replace faulty components, and revive units that seem beyond repair—getting you back on the road faster than buying a replacement.
Stop wasting time with generic charger replacements or endless YouTube searches. Most “not charging” issues stem from predictable failures that you can fix with basic tools and $25 in parts. Let’s systematically troubleshoot your unit and restore its life-saving power.
Immediate Symptoms Check
No Lights When Plugged In
If your jump starter shows zero response when connected to power, your first suspect is the charger or cable—not the unit itself. Test your wall outlet with another device like a lamp before proceeding. Next, examine the OEM wall adapter’s LED: a solid green glow confirms it’s working, while darkness or a red light indicates failure. For the 300’THSTB model, replace it with a genuine 12V/1A adapter (under $15 online). Never substitute with random chargers—using incorrect voltage triggers permanent damage to the protection circuit. If the adapter tests functional but your jump starter remains dead, move immediately to cable inspection.
LEDs Work But Won’t Hold Charge
This deceptive failure tricks most owners: your unit’s lights illuminate during charging, suggesting success, yet delivers zero cranking power when disconnected. The knowledge base confirms this always indicates a dead sealed lead-acid (SLA) battery—not a charger problem. Surface charging creates just enough voltage to light LEDs, but the sulfated cells can’t store energy. For 19-year-old units like the 300’THSTB, battery replacement is 95% likely the fix. Check voltage at the terminals: readings below 12.0V confirm irreversible degradation. Don’t waste money on new chargers—this requires internal battery replacement.
Flashing Red/Green Codes
Rapid LED flashing during charging signals the protection circuit detected a critical fault. Common triggers include shorted battery cells, overheating (above 113°F/45°C), or extreme over-discharge. Never ignore these patterns—they prevent dangerous thermal runaway. The 300’THSTB typically flashes red/green when internal temperature exceeds safe limits or battery voltage drops below 9V. First, disconnect all power and let the unit cool for 30 minutes in a shaded area. If flashing persists after cooling, perform the reset procedure (detailed below). If codes return immediately, you likely have a shorted cell requiring battery replacement.
Required Diagnostic Tools
Essential Testing Kit
Skip expensive professional tools—you need only five items for 90% of repairs:
– Digital multimeter (to verify battery voltage and charger output)
– 12V automotive bulb (for definitive load testing)
– Phillips screwdriver (to access internal components)
– Safety gloves and ANSI-rated goggles (critical for hydrogen gas protection)
– Paperclip (for engaging recessed reset buttons)
Optional Advanced Tools
For complex circuit issues, add:
– IR thermometer (to pinpoint overheating components)
– Bench power supply (only for lithium models—not relevant for SLA-based 300’THSTB)
– Torx T10 driver (for newer Black & Decker models with security screws)
Battery Voltage Test Procedure
Access Internal Battery
For the 300’THSTB model, remove four Phillips screws securing the top cover. Wear eye protection—disturbed SLA batteries can vent explosive hydrogen gas. Locate the red (positive) and black (negative) wires connected to the 12V SLA battery nestled in foam. Gently lift the battery without straining wires—these connections fail easily on aged units. Note the terminal orientation: F2 spade connectors (0.250″ width) face the cable entry point.
Voltage Reading Interpretation
Measure voltage directly at battery terminals with your multimeter:
– 12.6V+: Battery healthy—investigate charger or PCB issues
– 12.0-12.5V: Partial charge—attempt full 8-hour charge cycle
– Below 12.0V: Critical failure—battery replacement required
Pro Tip: If your reading shows 0V, check for a disconnected wire or blown 15A fuse on the PCB before assuming total battery failure.
Load Test Confirmation
A surface charge can fool voltage readings—always verify with a load test. Connect a 12V 50W automotive bulb across terminals for 10 seconds while monitoring voltage:
– Pass: Voltage stays above 11.5V—battery still viable
– Fail: Voltage crashes below 9V—battery must be replaced
This test catches “zombie batteries” that show voltage but can’t deliver cranking amps. If your unit fails here, replacement is non-negotiable.
Replace Failed Battery Safely
Compatible Replacement Specs
Use only 12V 7Ah-9Ah SLA batteries matching these specs:
– Dimensions: 151mm × 65mm × 94mm (standard 7Ah size)
– Terminals: F2 quick-disconnect spades (0.250″)
– Models: UPG UB1270, Mighty Max ML7-12, or ExpertPower EXP1270
Higher-capacity 9Ah versions fit the same tray and provide 30% more jump-starts per charge. Never install AGM or lithium batteries—they’ll trigger protection faults.
Discharge Before Removal
Critical Safety Step: Attach a 12V bulb across terminals until voltage drops to 6V. This prevents dangerous sparking when disconnecting. Always work in a ventilated area—SLA batteries release flammable hydrogen when disturbed. Keep metal tools away from terminals; a single short can melt clamps.
Installation Steps
- Disconnect quick-connect spades (red = positive, black = negative)
- Remove foam tape securing old battery
- Position new battery with terminals facing cable entry
- Secure with foam spacer and zip ties (avoid tape residue)
- Charge 8 hours with OEM adapter before first use
- Test on vehicle—ensure ≥150A cranking current
Warning: Never charge immediately after installation—this stresses new batteries. The initial 8-hour cycle maximizes lifespan.
Fix Common Charging Cable Issues
Visual Cable Inspection
Examine the barrel plug and full cable length for:
– Kinks or crushed sections (common near plug)
– Melted insulation (indicates overheating)
– Bent center pins (prevents proper contact)
The 5.5mm × 2.1mm barrel connector fails most often. If the center pin wobbles or feels loose, replacement is essential—temporary fixes like bending the pin rarely last.
Continuity Testing
Set multimeter to resistance mode:
– Good cable: <1 ohm end-to-end reading
– Broken cable: Infinite resistance (OL display)
– Dangerous short: Any reading below megaohms between conductors
Never use a cable showing shorts—this can fry your jump starter’s PCB. For breaks near the plug, replace the entire cable; mid-span repairs with solder rarely withstand charging currents.
Reset Internal Protection Circuit
Locate Reset Button
On the 300’THSTB, find the recessed reset button beneath side ventilation slots—a tiny tactile switch on the PCB. Insert a straightened paperclip and press firmly. Newer models often position this near the charging port.
Reset Sequence
- Disconnect all power sources
- Hold reset button for 10 full seconds
- Reconnect charger and watch LED behavior
- Solid green = successful reset
- Flashing lights = persistent fault requiring PCB inspection
Pro Tip: Reset after every battery replacement—this clears false fault codes from the protection circuit.
Prevent Future Failures
Proper Storage Practices
Store your jump starter at 40-80% charge in temperatures between 41-86°F. Deep discharge (below 10.5V) permanently sulfates SLA batteries. Check voltage monthly—top up if below 12.4V. Never store in vehicles where summer heat exceeds 120°F—this cuts battery life by 50%.
Maintenance Schedule
- Monthly: Verify charge level (12.6V = full)
- Quarterly: 2-hour top-up charge
- Biannually: Full 8-hour charge cycle
- Annually: Load test with automotive bulb
- Every 2 years: Proactive battery replacement
Critical: If storing long-term, disconnect the battery from the PCB to prevent parasitic drain.
Your Black & Decker jump starter not charging issue almost always resolves with a $25 battery swap or simple reset. Remember: units showing lights but no crank power have dead batteries—not charger problems. For 300’THSTB models over 5 years old, battery replacement is inevitable maintenance, not a defect. By following these steps, you’ll extend your jump starter’s life for years while avoiding $80 replacement costs. Keep this guide in your glovebox—you’ll never face a dead jump starter panic again.