Charging and Safety

Introduction

Charging system design and safety protection are critical aspects of robot power engineering. Improper charging can damage batteries or even cause fires, while thorough safety design ensures the robot operates safely under all conditions.

Lithium Battery Charging Principles

CC-CV Charging Method

The standard charging method for lithium batteries (Li-ion/LiPo/LiFePO4) is Constant Current - Constant Voltage (CC-CV):

graph LR
    subgraph CC Phase [Constant Current Phase CC]
        A[Charge at set current] --> B[Voltage gradually rises]
    end
    subgraph CV Phase [Constant Voltage Phase CV]
        C[Voltage held constant] --> D[Current gradually decreases]
    end
    subgraph Termination [Charge Complete]
        E[Current drops to cutoff] --> F[Charging complete]
    end
    B --> C
    D --> E

CC Phase:

Charges at constant current (typically 0.5C–1C)
Battery voltage gradually rises from low
Approximately 80% of capacity is charged during this phase

CV Phase:

Voltage is held constant after reaching full charge voltage (4.2V/cell)
Charging current decays exponentially as the battery fills
Charging ends when current drops to the cutoff current (typically C/10 or C/20)

Charging Parameters

Battery Type	CC Current	CV Voltage/Cell	Cutoff Current	Charge Time
Li-ion/LiPo	0.5–1C	4.20V	C/10	2–3 h
LiFePO4	0.5–1C	3.65V	C/10	2–3 h
Fast-charge Li-ion	2–3C	4.20V	C/10	0.5–1 h

Charging Power Calculation

\[P_{charge} = V_{charge} \times I_{charge}\]

For example, a 4S Li-ion battery (16.8V full charge) at 2A:

\[P = 16.8V \times 2A = 33.6W\]

Charging IC Selection

Single-Cell Charging ICs

Model	Input	Charge Voltage	Max Current	Features	Cost
TP4056	4.5–8V (USB)	4.2V	1A	With protection, minimal	$0.3
MCP73831	3.75–6V	4.2V	500 mA	Microchip classic	$0.5
BQ24074 (TI)	4.35–6.4V	4.2V	1.5A	Supports charge-while-use	$2
LTC4054	4.25–6.5V	4.2V	800 mA	Thermal regulation	$1

The TP4056 module is the most common single-cell charging solution:

Micro-USB or Type-C input
Built-in CC-CV charging curve
Includes DW01A + FS8205 protection IC (overcharge/overdischarge/overcurrent/short circuit)
Suitable for simple 3.7V single-cell lithium battery projects

Multi-Cell Charging ICs

Model	Cell Count	Max Current	Input	Features
BQ25700A (TI)	1–4S	6.35A	3.5–24V	USB PD support
BQ25713 (TI)	1–4S	6A	3.5–24V	NVDC architecture
LTC4020 (ADI)	Multi-cell	Adjustable	4.5–55V	Buck charger
MP2639A	1S	2A	4.5–12V	Integrated boost + charging

High-Power Charging Solutions

For large battery packs (e.g., 8S 12Ah), dedicated chargers are typically used rather than onboard charging ICs:

RC balance chargers: ISDT Q6, SkyRC B6 (1–6S, 50–300W)
Custom PSU + BMS charging: Charge directly through the BMS charge port
Charging dock solutions: See below

Charging Dock Design

An automatic charging dock enables a robot to autonomously return for charging, an important feature for service and household robots.

Docking Methods

Method	Description	Pros	Cons
Spring contacts	Metal spring pins make contact	Simple, reliable, low cost	Requires precise alignment
Magnetic connector	Magnets + Pogo pins	Higher alignment tolerance	Slightly higher cost
Wireless charging	Qi standard coils	No physical wear	Lower efficiency (70–80%), limited power

Alignment Guidance

Infrared Guidance (robot vacuum approach):

Charging dock emits infrared coded signals (narrow beam forward + wide beams on sides)
Robot uses infrared receivers to determine direction and distance
Gradually aligns and slowly drives into the dock

Visual Guidance:

Charging dock has ArUco markers or specific patterns
Robot camera recognizes markers and computes pose
Precise docking through visual servoing

Combined Approach:

Long range: Infrared or UWB coarse positioning
Close range: Visual precise alignment
Final docking: Mechanical guide channel + spring contacts

Charging Dock Circuit

graph TD
    AC[AC 220V] --> PSU[Switching Power Supply<br/>24V/5A]
    PSU --> DOCK[Charging Dock PCB]
    DOCK --> CONTACT[Spring Contacts<br/>Pogo Pins]
    DOCK --> IR[IR Transmitters<br/>IR LEDs]
    DOCK --> LED_IND[Status LED Indicators]

    CONTACT --> |Charging current| ROBOT[Robot Battery]

    ROBOT --> |Status feedback| DOCK

Safety Protection Design

Overcurrent Protection

Fuse Selection:

\[I_{fuse} = 1.25 \times I_{max\_normal}\]

Type	Response Time	Resettable	Suitable For
Glass tube fuse	Slow (ms–s)	No	Main power protection
SMD fuse	Slow (ms–s)	No	Onboard PCB
PTC self-resettable	Slow (s)	Yes	Low-power branches
eFuse IC	Fast (us)	Yes	Precise protection

Overvoltage / Undervoltage Protection

BMS provides cell-level protection: Per-cell monitoring
Bus-level undervoltage lockout (UVLO): Prevents battery overdischarge
TVS diodes: Transient overvoltage suppression

Temperature Monitoring

Temperature is a critical battery safety indicator. The BMS should place NTC thermistors at the following locations:

Battery pack surface: Monitor cell temperature
BMS PCB: Monitor MOSFET temperature
Charging port: Monitor contact resistance heating

Temperature threshold settings:

State	Temperature Range	Action
Normal charging	0–45°C	Normal charging
Low-temperature charging	-10–0°C	Reduce charging current (0.1C)
High-temperature warning	45–55°C	Reduce charge/discharge current
High-temperature protection	>55°C	Disconnect charge/discharge
Danger	>70°C	Emergency power cutoff, alarm

Short Circuit Protection

Hardware protection: BMS SCP function, microsecond-level response
Fuse backup: Last line of defense when electronic protection fails
Physical isolation: Adequate separation between battery positive and negative traces

Lithium Battery Fire Safety

Thermal Runaway Mechanism

Lithium battery thermal runaway is the most serious safety incident:

Internal short circuit or external heating causes temperature rise
SEI film decomposition (~90°C)
Anode reacts with electrolyte (~120°C)
Separator melts causing large-area short circuit (~130°C)
Cathode decomposes releasing oxygen (~180°C)
Vigorous exothermic reaction, fire or explosion

Preventive Measures

Design Level:

Use high-quality cells (branded genuine products)
Complete and tested BMS protection functions
Allow expansion space in the battery pack
Wrap with flame-retardant materials (silicone sleeves, flame-retardant tape)
Physically isolate battery compartment from main circuit board

Usage Level:

Do not use swollen or deformed batteries
Monitor charging (or charge in a fireproof bag)
Avoid charging in extreme temperatures
Do not use non-original chargers
Regularly inspect battery condition

Emergency Level:

Have lithium battery-specific fire extinguishers available (dry chemical or CO2)
LiPo Safe Bag for storage and charging
Keep charging area away from combustible materials
Install smoke alarms

Storage Guidelines

Condition	Recommendation
Storage voltage	3.7–3.85V/cell (~50% charge)
Storage temperature	15–25°C
Storage humidity	<65% RH
Long-term idle	Check and top off every 3 months
Storage container	LiPo Safe Bag or metal box

Charging System Design Checklist

Design Verification Items

[ ] CC-CV charging curve verified correct
[ ] Charge termination voltage accuracy within ±1%
[ ] Overcharge protection triggers before 4.25V/cell
[ ] Overdischarge protection triggers before 3.0V/cell
[ ] Charging temperature limits functional
[ ] Short circuit protection response time <100 us
[ ] Reverse polarity protection functional
[ ] Charging dock reliability (1000-cycle test)
[ ] Thermal design meets maximum charging current requirements

Certifications and Standards

Standard	Scope	Key Points
UN38.3	Lithium battery transport	Safety testing
IEC 62133	Portable device batteries	Safety requirements
UL 2054	Household/commercial batteries	US safety certification
GB 31241	China portable device batteries	National standard safety requirements

References

Battery University: Charging Lithium-ion
TI: Battery Charger Design Guide
iRobot/Roborock charging dock teardown analysis
NFPA 855: Standard for the Installation of Stationary Energy Storage Systems
"Lithium-Ion Battery Safety"