Battery Protection and Balancing
Battery management systems, cell balancing, over-charge/discharge protection, and safe battery handling in robotics
Battery Protection and Balancing
Batteries are energy-dense and can fail catastrophically if mishandled. Understanding protection and balancing extends battery life and prevents fires.
Battery Hazards
Thermal Runaway
The most dangerous failure mode:
Short Circuit
Direct connection between positive and negative terminals:
Battery (+) ───[Short circuit]─── Battery (−)
↓
Massive current (1000+ A)
↓
Extreme heat → fireOvercharge
Exceeding maximum voltage per cell:
LiPo cell max: 4.2V
Overcharged to: 4.5V → Unstable, fire riskOver-Discharge
Draining below minimum voltage:
LiPo cell min: 3.0V
Overdischarged to: 2.0V → Permanent damage, fire risk on next chargeBattery Management Systems (BMS)
What BMS Does
A BMS is a small circuit protecting the battery:
BMS Protection Functions
| Protection | Triggers at | Action |
|---|---|---|
| Over-charge | > 4.3V/cell | Stops charging |
| Over-discharge | < 2.75V/cell | Disconnects load |
| Over-current | > Max discharge | Cuts power |
| Over-temperature | > 60-70°C | Disconnects |
| Cell balance | > 0.1V difference | Rebalances |
BMS Types
Passive BMS:
- Simple shunt resistors
- Dissipates excess charge as heat
- Cheap, no active components
- Slow balancing
Active BMS:
- Buck/boost converters
- Can move energy between cells
- Expensive, complex
- Fast, efficient balancing
Cell Balancing
Why Balancing Needed
In a multi-cell battery, cells age differently:
Initial state:
Cell 1: 4.2V
Cell 2: 4.2V
Cell 3: 4.2V
After 100 charges:
Cell 1: 4.15V (degraded faster)
Cell 2: 4.2V
Cell 3: 4.25V (charging faster)
Problem: Cell 3 at 4.25V might trigger over-voltage protection!Passive Balancing (Most Common)
Small resistors in parallel with each cell:
[Cell 1]──[Resistor 100kΩ]──┐
[Cell 2]──[Resistor 100kΩ]──┤
[Cell 3]──[Resistor 100kΩ]──┘
When one cell is high voltage:
- Resistor conducts slight current
- Discharges excess charge
- Slow process (over hours)Example: Passive Balancing in Action
Before: Cell 1=4.05V, Cell 2=4.20V, Cell 3=4.15V (imbalanced)
With balancing resistors:
Cell 2 > others → Resistor discharges Cell 2
After 1 hour: Cell 1=4.10V, Cell 2=4.15V, Cell 3=4.14V
After 2 hours: Cell 1=4.12V, Cell 2=4.13V, Cell 3=4.13V (balanced!)Active Balancing (Professional)
Uses switching circuits to transfer charge:
High cell → Switch → Low cell
↓ ↓
(Buck-boost converter)
Transfer charge directly (fast)
More efficient than resistor heatingOvercharge Protection
Charger Cut-off
Smart chargers stop charging at correct voltage:
Target: 4.2V per cell (LiPo)
Charger monitors voltage:
0-90% charge: High current (fast)
90-100% charge: Reduced current (slow)
At 4.2V per cell: STOP (cut-off)Constant Current / Constant Voltage (CC/CV)
The most common charging profile:
Current (A)
│
100%├─────┐ ← Constant Current phase
│ │
50%├ └─────┐ ← Constant Voltage phase
│ │
0%├───────────┴─────
└─────────────────
Time →
CC phase: Draw set current until voltage reaches limit
CV phase: Hold voltage constant while current tapers to zeroExample: 5A charger charging 5000mAh LiPo
CC Phase (0-50 min): 5A constant, voltage rises to 4.2V per cell
CV Phase (50-60 min): Voltage held at 4.2V, current drops from 5A to 0.2A
Charge complete: Current < 0.2AOver-Discharge Protection
Low Voltage Cutoff (LVC)
Circuits detect when battery voltage drops too low:
Target: Stop at 3.0V per cell (LiPo)
Monitor voltage:
4.2V → OK, full power
3.5V → OK, full power
3.2V → OK, full power
3.0V → WARNING, 80% capacity lost
2.9V → CUTOFF, disconnect motor
Below 3.0V: Permanent damage riskBMS Automatic Cutoff
Battery voltage drops → BMS detects
At minimum → Relay clicks off
Motor loses power → Robot stopsPrevents over-discharge automatically.
Over-Current Protection
Current Limiting
Prevents excessive current draw:
Peak current allowed by BMS: 50A
Robot tries to draw: 100A (jamming motor)
BMS detects over-current → Disconnects
Robot stops instead of melting batteryFuses vs Electronic Protection
| Method | Speed | Reliability | Cost |
|---|---|---|---|
| Fuse | Very fast | One-time use | Cheap |
| Circuit breaker | Very fast | Reusable | Moderate |
| BMS electronic | Fast | Usually reliable | Moderate |
| PTC thermistor | Slow | Reusable | Cheap |
Typical design: Fuse + BMS electronic = redundant protection
Temperature Management
Temperature Effects
Temperature Protection
High temp cutoff:
Sensor: Battery temperature
Limit: 60-70°C maximum
Action: Reduce charge/discharge current or disconnectLow temp protection:
Sensor: Battery temperature
Limit: 0°C minimum
Action: Don't charge below 0°C (lithium risk)Thermal Management
Keep battery cool:
- Airflow around battery
- Thermal paste if mounted to metal
- Don't pile components on battery
- Avoid direct sunlight
Example: Robot operated in 50°C heat
- Battery reaches 70°C internal
- Risk of thermal runaway
- Solution: Add cooling, use active BMS
Safe Handling Practices
Lifespan Extension Tips
Maximize Cycle Life
Cycle life = Total discharge cycles battery survives
LiPo: 300-500 cycles typical
To extend:
✓ Avoid full discharge (stop at 3.0V)
✓ Avoid full charge daily (charge to 80% for storage)
✓ Keep cool (20-40°C ideal)
✓ Don't over-stress (follow C-rating)
✓ Use balancing charger
✓ Discharge fully once per monthExample: Racing LiPo vs Storage LiPo
Racing use:
- Charge to 100% (4.2V/cell)
- Fully discharge each use
- Cycle life: 300 cycles
- Duration: ~40-50 hours racing
Storage/casual use:
- Charge to 60% (3.85V/cell)
- Only discharge for flight
- Cycle life: 800+ cycles
- Duration: Years of use
Disposal and Recycling
Why Recycling Matters
Lithium: Toxic, valuable (recovered for new batteries)
Cobalt: Toxic, expensive (recovered for other uses)
Nickel/Metal: Recyclable
Lead: Hazardous, must be recoveredDisposal Process
- Drain if possible: Connect to load until dead
- Tape terminals: Prevent short circuits
- Store safely: Fireproof container
- Find facility: Search "battery recycling near me"
- Drop off: Most electronics stores accept free
Summary
Battery Protection Essentials:
✓ BMS prevents over-charge, over-discharge, over-current ✓ Passive balancing keeps cells within 0.1V ✓ Temperature management prevents thermal runaway ✓ Proper storage and charging extends life ✓ Safe handling prevents fires
Key Rules:
- Don't exceed C-rating
- Don't go below 3.0V per cell
- Don't store fully charged
- Keep cool and dry
- Use fireproof bags
- Have fire extinguisher nearby
- Recycle properly when done
How is this guide?
Batteries and Cells
Battery chemistry, types (Li-ion, LiPo, NiMH, Lead-Acid), characteristics, advantages and disadvantages for robotics
Battery Ratings and Specifications
Understanding voltage, capacity (mAh/Ah), discharge rate (C-rating), and how to select appropriate batteries for robotic applications