RV Battery Charging: System Design & Protocol Analysis

Charging System Architecture

Modern RV electrical systems typically employ multi-source charging topology: alternator (vehicle running), shore power converter/charger, and solar MPPT controller. Understanding charge source characteristics and battery chemistry requirements is critical for system longevity.

LiFePO₄ Charge Profile Requirements

CC-CV Charging Protocol

Constant Current Phase: 0.5C recommended (0.3C for extended cycle life). Charge until cell voltage reaches 3.65V ± 0.05V.

Constant Voltage Phase: Hold at 14.6V (4S configuration) until current tapers to C/20 (typically 50-100mA for 100Ah battery). Total charge time: 2-3 hours from 20% SoC.

Critical Parameters

Charging Source Analysis

1. Alternator Charging

Challenge: Standard alternator regulators target 13.8-14.2V (lead-acid profile), insufficient for LiFePO₄ bulk charging.

Solution: DC-DC charger with programmable voltage output. Recommended: 30-60A DC-DC converter with LiFePO₄ preset. Isolates alternator from battery, prevents voltage sag during high-current draw.

Wiring: Minimum 6 AWG for 30A, 4 AWG for 60A. Keep cable runs <3m to minimize voltage drop. Target: <0.2V drop at max current.

2. Shore Power Converter/Charger

Specification: Multi-stage charger with LiFePO₄ profile. Power factor corrected (PFC) input stage recommended for EU installations.

Sizing: Charger output = (Battery capacity × desired charge rate) + DC loads. Example: 200Ah battery, 0.5C charge = 100A + 20A loads = 120A charger minimum.

AC input: 230V/50Hz (EU), 120V/60Hz (US). Verify input current rating vs. campground breaker capacity (typically 10-16A EU, 30-50A US).

3. Solar MPPT Controller

MPPT vs PWM: MPPT required for >200W arrays. Efficiency gain: 20-30% vs PWM, especially in cold conditions or partial shading.

Voltage rating: Controller Voc rating must exceed panel Voc × 1.25 safety factor × temperature coefficient. Example: 4×100W panels (Voc 22V each) in series = 88V × 1.25 = 110V minimum controller rating.

Current sizing: Controller current rating ≥ (Total panel wattage ÷ battery voltage) × 1.25. Example: 400W ÷ 12V × 1.25 = 42A controller minimum.

BMS Integration Considerations

Battery Management System must communicate charge termination to all sources. Two common protocols:

Relay-Based Cutoff

BMS opens relay on overvoltage (>3.75V/cell), overcurrent, or temperature fault. Charger must detect open circuit and cease output. Suitable for simple systems.

CAN Bus Communication

BMS transmits SoC, voltage, current, temperature via CAN bus (250 kbps, 120Ω termination). Charger adjusts output dynamically. Protocols: REC BMS, Victron VE.Can, SMA CAN. Recommended for systems >5kWh.

Temperature-Dependent Charging

Charge cutoff temperatures:

• Below 0°C: Disable charging (lithium plating risk)
• 0-5°C: Reduce charge current to 0.1C
• 5-45°C: Normal charging permitted
• Above 45°C: Disable charging (accelerated degradation)

Cold weather solution: Battery heating pad (50-100W) activated below 5°C. Power from shore/alternator, not battery. Target: bring cells to >5°C before charge initiation.

Multi-Source Priority Logic

When multiple charge sources available simultaneously:

1. Solar (highest priority): Free energy, no generator runtime
2. Shore power: Unlimited capacity, use for high-current bulk charging
3. Alternator: Lowest priority, limits driving range (fuel consumption)

Implement diode isolation or MOSFET-based ideal diode controller to prevent backfeed between sources. Voltage drop: <0.3V per diode.

Charge Efficiency & Losses

Round-trip efficiency: LiFePO₄ cell: 95-98%. System losses:

• Charger conversion: 85-95% (depending on topology)
• Wiring resistance: 1-3% (proper sizing)
• BMS quiescent draw: <1W

Total system efficiency: 80-90%. Account for this in solar array sizing and generator runtime calculations.

Monitoring & Diagnostics

Essential telemetry for charge system validation:

• Battery voltage (±0.01V accuracy)
• Charge current (±1% accuracy)
• Individual cell voltages (if BMS supports)
• Battery temperature (±1°C)
• Charge source status (active/inactive)
• Cumulative Ah charged (coulomb counting)

Bluetooth or WiFi-enabled monitoring recommended for remote diagnostics. Data logging interval: 1-5 minutes for trend analysis.

Common Integration Errors

Incorrect voltage setpoint: Using 14.4V (lead-acid) instead of 14.6V results in chronic undercharging, reduced usable capacity.

Inadequate wire gauge: Voltage drop >0.5V causes charger to prematurely terminate, incomplete charge cycles.

Missing temperature sensor: Charging below 0°C causes permanent capacity loss via lithium plating.

No cell balancing: Voltage drift >100mV between cells reduces pack capacity, triggers premature BMS cutoff.

System Commissioning Checklist

1. Verify all charge sources configured for LiFePO₄ profile (14.6V bulk, 13.6V float)
2. Measure voltage at battery terminals under charge - confirm <0.3V drop from charger output
3. Test BMS cutoff: manually trigger overvoltage condition, verify charger cessation
4. Confirm temperature sensor placement: on cell surface or terminal, not ambient air
5. Load test: verify system can deliver rated current without voltage sag >0.5V
6. Monitor first 3 charge cycles: confirm cell balance <50mV at full charge

Technical Support

For application-specific charging system design, contact our engineering team. Provide: battery capacity, charge sources (alternator/shore/solar ratings), typical daily consumption, and ambient temperature range.