The most common question in overland and van build electrical design is simple: how long driving to charge battery banks back to full? The direct answer is that it takes 1.5 to 4 hours of continuous driving to recharge a 100Ah LiFePO4 battery from 20% to 90% State of Charge (SoC) using a 30A to 50A DC-DC charger. However, this assumes your vehicle’s alternator can sustain the output, your DC-DC converter is properly sized for the battery’s C-rate, and you account for absorption tapering and converter efficiency losses.
Guessing your drive-time leads to chronically undercharged banks, sulfated AGM cells, or tripped BMS over-current protections. To get exact drive-time estimates, we have to map the entire source-to-load path, apply Peukert’s law where relevant, and respect the strict charge limits of modern lithium chemistries.
The Source-to-Load System Block: Alternator to House Battery
Before calculating time, you must define the system block. A vehicle charging setup is not just a wire from the alternator to the house battery; it is a regulated pipeline. Here is the standard source-to-load architecture for a 12V system:
- Source (Alternator): Modern vehicles (like the 2025/2026 Ford Transit or Mercedes Sprinter) use 'smart' alternators rated between 180A and 250A. These alternators drop output voltage to ~12.2V once the starter battery is full to save fuel. They will not charge a house battery without intervention.
- Buffer (Starter Battery): The vehicle’s OEM lead-acid or AGM starter battery acts as a voltage buffer.
- Regulation (DC-DC Charger): An isolated DC-DC charger (e.g., Victron Orion-Tr Smart 12/12-30 or Renogy 40A) reads the alternator's voltage, boosts it to the required 14.2V–14.6V for lithium, and limits current to protect the alternator diodes.
- Storage (House Battery): Your LiFePO4 or AGM deep-cycle bank.
- Load (Inverter): Converts 12V DC to 120V AC for appliances.
Inverter and Charger Sizing for the Stated Load
Let’s size this for a real-world load: running a 1200W induction cooktop or coffee maker. At 12V nominal, a 1200W AC load draws 100A DC. Factoring in a 90% inverter efficiency, the DC draw spikes to 111A. Your inverter must be rated for at least 1500W continuous (to handle the surge). More importantly, your DC-DC charger must be sized to handle the battery's maximum charge C-rate. For a 100Ah LiFePO4 battery, the standard max charge rate is 0.5C (50A). Therefore, a 40A or 50A DC-DC charger is the correct spec. If you attempt to run the 1200W inverter load while driving, the house battery buffers the difference between the 40A alternator input and the 111A inverter draw.
Drive-Time Sizing Math: C-Rates, DoD, and Efficiency
To calculate exact drive times, we must account for Depth of Discharge (DoD), the battery's C-rate (charge acceptance rate), and DC-DC converter efficiency (typically 93%–96%). Furthermore, we must apply Peukert’s Law, which dictates that a battery's effective capacity decreases as the discharge/charge current increases.
For Lead-Acid/AGM, the Peukert exponent ($k$) is typically 1.15 to 1.30, meaning high-current charging is highly inefficient and generates excess heat. For LiFePO4, $k$ is nearly 1.0 (highly linear), but efficiency still drops at the top-end of the charge cycle due to the Constant Voltage (CV) absorption taper. A LiFePO4 battery will accept bulk current (Constant Current, or CC phase) up to about 85% SoC, after which the current tapers exponentially.
| Battery Chemistry | Nominal Capacity | Usable DoD Limit | DC-DC Charger Rating | Theoretical Bulk Time | Real Drive Time (w/ Taper & 95% Eff.) |
|---|---|---|---|---|---|
| LiFePO4 | 100Ah | 80% (80Ah used) | 30A | 2.66 hours | 3.2 hours |
| LiFePO4 | 100Ah | 80% (80Ah used) | 50A (0.5C) | 1.60 hours | 2.1 hours |
| LiFePO4 | 200Ah | 80% (160Ah used) | 60A | 2.66 hours | 3.4 hours |
| AGM (Deep Cycle) | 100Ah | 50% (50Ah used) | 30A | 1.66 hours | 3.5 hours (Peukert + long absorption) |
| AGM (Deep Cycle) | 200Ah | 50% (100Ah used) | 40A | 2.50 hours | 5.5+ hours (Peukert + absorption) |
Note: 'Real Drive Time' includes the 95% DC-DC conversion efficiency loss and the CV absorption taper phase required to push the battery from 85% to 95%+ SoC. AGM batteries require significantly longer drive times due to Peukert losses and the necessity of a prolonged absorption phase to prevent sulfation.
The Smart Alternator Gotcha: If your DC-DC charger lacks an 'Engine Running' detection circuit or ignition-sense wire, a smart alternator will see the house battery as a parasitic drain and drop voltage to 12.4V. At 12.4V, your DC-DC charger will output zero amps to a 12V LiFePO4 bank. Always use a DC-DC charger with automatic engine-shutdown and smart-alternator compatibility modes.
Series vs. Parallel: Voltage, Ah, and Charge Limits
When scaling your system to 24V or 48V, or increasing capacity at 12V, you must wire batteries in series or parallel. The consequences for voltage, Amp-hours (Ah), and charge limits are absolute.
Series Wiring (Voltage Adds, Ah Stays Constant)
Wiring two 12V 100Ah batteries in series yields 24V at 100Ah. The total energy (Wh) remains 2400Wh.
Charge Limits: Your DC-DC charger or inverter/charger must now output 28.4V for bulk charging. The BMS Over-Voltage Protection (OVP) limit is cumulative; if one cell group drifts high, the entire series string will trigger a high-voltage disconnect. You must use a battery balancer or a BMS with active cell balancing for series strings.
Parallel Wiring (Ah Adds, Voltage Stays Constant)
Wiring two 12V 100Ah batteries in parallel yields 12V at 200Ah.
Charge Limits: The charge current limit is the sum of the individual BMS limits. Two batteries with 100A charge limits can theoretically accept 200A. However, wiring resistance means the battery closest to the charger will accept more current, potentially tripping its individual BMS.
Do not parallel batteries of different chemistries, ages, or internal resistances. If you parallel a new 100Ah LiFePO4 with an older one that has higher internal resistance, the new battery will 'hog' the charge current, exceeding its C-rate and triggering thermal runaway. Always parallel identical batteries from the same manufacturing batch, and use symmetrical busbar wiring (diagonal wiring) to equalize resistance.
Lithium Fire-Safety and BMS Charge Limits
LiFePO4 (Lithium Iron Phosphate) is the safest lithium chemistry available, but it is not immune to thermal events. According to NFPA fire research and Battery University guidelines, lithium fires are driven by internal short circuits, overcharging, or low-temperature lithium plating.
Strict Charge and Discharge Limits
- Max Charge Voltage: 14.2V to 14.6V. Never use a standard automotive alternator directly, as voltage spikes during load dumps can exceed 15V, destroying the BMS MOSFETs.
- Low Voltage Disconnect (LVD): 10.0V to 11.0V. Discharging below this threshold causes irreversible copper shunt dissolution inside the cell.
- Max Charge C-Rate: Typically 0.5C (50A for a 100Ah battery). Pushing 1.0C generates excessive internal heat.
The Low-Temperature Charging Hazard
The most common cause of LiFePO4 degradation in vehicle builds is charging below freezing. If you force current into a lithium cell when the internal temperature is below 0°C (32°F), lithium ions cannot intercalate into the anode fast enough. Instead, they plate onto the surface as metallic lithium. This 'lithium plating' creates dendrites that eventually pierce the separator, causing an internal short circuit and a catastrophic fire.
The Fix: Your BMS must have a Low-Temperature Charge Cutoff (LTCC). Furthermore, in cold climates, you must install a battery heating pad (like those found on Redway or Epoch heated batteries) wired to a thermostat that draws from the alternator only when the battery core temp is above 2°C. Never rely on ambient air temperature sensors; the sensor must be strapped directly to the cell busbars.
By matching your DC-DC charger amperage to your battery's C-rate, respecting the Peukert losses in AGM banks, and enforcing strict BMS temperature limits, you can accurately predict your drive-time and ensure your power storage system survives the rigors of the road.
