To charge a 12V 100Ah battery from 50% to 100% depth of discharge (DoD) using a 10A charger, it takes approximately 5.3 hours for a LiFePO4 (lithium) battery and 6.5 hours for an AGM (lead-acid) battery. The difference stems from charge acceptance curves, absorption phase requirements, and round-trip efficiency losses.
The baseline formula is simple: Time = (Ah needed) / (Charger Amps). However, real-world bench testing requires factoring in battery chemistry, Peukert's law for lead-acid, and the constant-current/constant-voltage (CC/CV) transition point. Below is the exact math, system architecture, and hardware sizing required to build a reliable 12V charging path.
The Core Math: Sizing Your 12V Charger and Charge Time
Before calculating time, map your system block. A standard off-grid or backup charging path flows as follows: AC Source (Grid/Generator) → Automatic Transfer Switch → Inverter/Charger (AC/DC conversion) → DC Busbar (with Class T fuse) → 12V Battery Bank → DC Loads/Inverter. The bottleneck is almost always the inverter/charger's internal AC-to-DC converter rating.
For the table below, we assume a 12V 100Ah battery discharged to 50% (requiring 50Ah to reach full). LiFePO4 batteries maintain a near-constant voltage of ~13.4V until roughly 90% state of charge (SoC), meaning the charger delivers its full rated amperage for almost the entire cycle. AGM batteries hit the absorption voltage (14.4V - 14.7V) much earlier, forcing the charger to taper current while the battery slowly saturates.
| Battery Type (100Ah Nominal) | Charger Output | Ideal Math Time (50Ah) | Real-World Time (w/ Efficiency & Absorption) | Max Recommended Charge Rate |
|---|---|---|---|---|
| LiFePO4 (e.g., Epoch or SOK 100Ah) | 10A | 5.0 hrs | 5.3 hrs (95% efficiency) | 50A (0.5C) |
| LiFePO4 | 20A | 2.5 hrs | 2.7 hrs | 50A (0.5C) |
| AGM Lead-Acid (e.g., Odyssey PC1200) | 10A | 5.0 hrs | 6.5 hrs (Absorption tail + 85% eff.) | 20A (0.2C) |
| AGM Lead-Acid | 20A | 2.5 hrs | 4.0 hrs | 20A (0.2C) |
| Flooded Lead-Acid (e.g., Trojan T-105 6Vx2) | 10A | 5.0 hrs | 7.5 hrs (Equalization + gassing) | 15A (0.15C) |
Factoring in Peukert's Law and Efficiency
Peukert's law dictates that a lead-acid battery's effective capacity drops as the discharge current increases, expressed as $T = H / (I/I_H)^k$, where $k$ is the Peukert exponent (typically 1.1 to 1.3 for lead-acid, and ~1.0 for LiFePO4). While Peukert primarily penalizes discharge, the inverse reality affects charging: lead-acid batteries suffer from internal resistance heating and gassing if pushed beyond their acceptance rate.
If you push 40A into a 100Ah AGM battery, the battery will heat up, internal resistance will rise, and the charger will prematurely drop to float voltage to prevent thermal runaway. You will measure 40A on the meter, but the battery is only storing a fraction of that energy as chemical potential. LiFePO4 cells, with an internal resistance often below 2 milliohms, do not suffer from this penalty, making high-amperage charging highly efficient up to the BMS cutoff limit.
Charge and Discharge Limits: C-Rates, DoD, and Safety
The "C-rate" defines the charge or discharge current relative to the battery's capacity. A 1C rate for a 100Ah battery is 100A. A 0.5C rate is 50A. Pushing beyond the manufacturer's specified C-rate degrades the anode/cathode chemistry and triggers protective hardware lockouts.
- LiFePO4 Limits: Standard cylindrical or prismatic LiFePO4 cells safely accept a 0.5C charge rate (50A for a 100Ah battery) and a 1C discharge rate. Depth of Discharge (DoD) can safely reach 80% to 100% without immediate cycle-life degradation, though stopping at 90% SoC on the top end prolongs calendar life.
- Lead-Acid Limits: AGM and Gel batteries should not be charged faster than 0.2C (20A for 100Ah). Discharge rates should be kept under 0.1C to achieve the rated Ah capacity. DoD should be limited to 50%; discharging an AGM to 80% regularly will cut its cycle life from ~400 cycles down to under 150.
Series vs. Parallel: Scaling Voltage and Capacity
When scaling a 12V system for larger loads, you must choose between series and parallel wiring. The physical wiring topology fundamentally changes your charging requirements and wire sizing.
| Configuration | Math (Using 2x 12V 100Ah Batteries) | Consequence for Charging & Wiring |
|---|---|---|
| Series | Voltage adds (12V + 12V = 24V). Ah stays the same (100Ah). Total Energy: 2560Wh. | Preferred for high power. A 2000W load draws ~83A at 24V, allowing the use of 4 AWG wire. Requires a 24V charger. |
| Parallel | Ah adds (100Ah + 100Ah = 200Ah). Voltage stays the same (12V). Total Energy: 2560Wh. | Current bottleneck. A 2000W load draws ~166A at 12V, requiring massive 2/0 AWG wire and 200A Class T fuses. Charging a 200Ah bank at 0.5C requires a 100A charger. |
For any inverter load exceeding 1000W, wiring in series to create a 24V (or 48V) system is vastly superior. It halves the DC current, drastically reducing $I^2R$ (heat) losses in the cables, allowing for smaller, cheaper wire, and keeping the DC busbar within standard ampacity limits. If you must parallel 12V batteries to maintain a 12V DC bus for legacy RV or marine appliances, use identical batteries bought on the same date, and ensure your parallel busbars are perfectly symmetrical to balance the current draw.
Sizing the Inverter/Charger for Your Actual Load
An inverter/charger (like the Victron MultiPlus or Magnum Energy MMS series) handles both DC-to-AC inversion and AC-to-DC battery charging. Sizing this unit requires calculating the simultaneous DC load and the required charge current.
The Sizing Decision Tree
Assume you are running a 12V system with a 200Ah LiFePO4 bank, powering a 1500W microwave, and you want to recharge the bank from 20% to 80% in roughly 3 hours using a Honda EU2200i generator.
- Calculate Inverter DC Draw: 1500W AC load / 12V DC nominal = 125A. Factor in 85% inverter efficiency: 125A / 0.85 = 147A DC draw. You need an inverter rated for at least 2000W continuous (166A DC).
- Calculate Required Charge Current: You need to replace 120Ah (60% of 200Ah) in 3 hours. 120Ah / 3 hrs = 40A charge current.
- Calculate Total DC Bus Current: If the microwave runs while the generator is charging the batteries, the DC busbar must handle 147A (load) + 40A (charge going into the battery) = 187A. (Note: In a properly wired system, the charger current feeds the inverter directly, and the battery only supplies the deficit. However, your main battery fuse must still be sized for the maximum possible inverter draw, which is 147A+).
- Select the Hardware: Choose a 12V 2000W Inverter/Charger with a built-in AC transfer switch and a minimum 50A internal charger (e.g., Victron MultiPlus 12/2000/80). Set the internal DIP switches or software to limit the AC input current to 16A so you do not overload the Honda generator's 1800W continuous rating.
By explicitly calculating the CC/CV transition times, respecting the C-rate limits of your specific chemistry, and sizing your DC bus wiring for the worst-case simultaneous load, you eliminate the guesswork from your 12V power system. Always verify your final terminal voltages with a calibrated multimeter under load, as voltage drop across undersized cables will cause the BMS to prematurely cut off the charge cycle.
