To make a charger for a battery bank in a DIY off-grid or backup power system, you do not build a raw power supply; you select and wire a charge controller (for solar) or an AC-DC converter (for grid/generator) matched to your battery chemistry and array output. For a standard 12V 100Ah LiFePO4 bank charged by a 400W solar array, the exact default component you need is a 30A MPPT charge controller, specifically the Victron SmartSolar MPPT 100/30.
This guide walks through the system topology, the exact sizing math including efficiency and Peukert losses, and the hard limits of lithium chemistries so you can build a safe, code-compliant 12V charging system.
The Anatomy of a DIY Battery Charging System
A complete charging system is a chain of energy conversion. The system block flows from source to load as follows:
- Source: Solar PV array or AC Grid/Generator.
- Charge Controller / AC-DC Charger: Regulates voltage/current to match the battery's charging profile (Bulk, Absorption, Float).
- Overcurrent Protection: Fuses or DC breakers on both the PV input and battery output.
- Busbars & BMS: Copper distribution blocks feeding the Battery Management System (BMS).
- Battery Cells: The chemical storage medium.
- Inverter: Converts DC bus voltage to 120V/240V AC for household loads.
Series vs. Parallel: Consequences for Voltage and Capacity
How you wire your cells or pre-packaged batteries dictates your system voltage and amp-hour (Ah) capacity. The physics are strict:
- Series Wiring: Voltage adds, Ah remains constant. Wiring four 12V 100Ah batteries in series yields a 48V 100Ah bank (5,120Wh total). This is ideal for high-power inverters (3000W+) because it keeps DC current low, allowing thinner wires.
- Parallel Wiring: Ah adds, voltage remains constant. Wiring four 12V 100Ah batteries in parallel yields a 12V 400Ah bank (5,120Wh total). This is common for RVs and small cabins but requires massive cabling to handle the high DC current.
Sizing Math: Loads, Efficiency, and Peukert's Effect
To size your charger and battery bank, start at the load and work backward. Let us size a system for a 1000W continuous AC load (e.g., a microwave or power tools).
Step 1: Inverter Efficiency and DC Draw
Inverters are not 100% efficient. A high-quality pure sine wave inverter operates at roughly 88% efficiency under heavy load.
- Required DC Power: 1000W / 0.88 = 1,136W
- DC Current Draw: 1,136W / 12.8V (nominal LiFePO4 voltage) = 88.7 Amps
Your battery bank and BMS must be rated to discharge at a minimum of 90A continuously.
Step 2: Peukert's Law and Usable Capacity
Peukert's Law dictates that as your discharge current increases, the usable capacity of the battery decreases. The Peukert exponent (k) is roughly 1.3 for Flooded Lead-Acid (FLA) and 1.05 for LiFePO4.
If you pull 88.7A from a 100Ah FLA battery (rated at the 20-hour/5A rate), Peukert losses will slash your usable capacity to roughly 45Ah. You will get about 30 minutes of runtime before the voltage collapses. Conversely, a 100Ah LiFePO4 battery suffers negligible Peukert losses (Battery University), delivering nearly its full rated capacity even at a 1C discharge rate.
Step 3: Depth of Discharge (DoD) Sizing
You cannot drain a battery to absolute zero without destroying it.
- LiFePO4: 80% DoD is standard for maximizing cycle life (3,000+ cycles). A 100Ah battery yields 80 usable Ah (1,024Wh).
- Lead-Acid/AGM: 50% DoD is the maximum recommended limit. A 100Ah battery yields only 50 usable Ah (600Wh).
For our 1000W load, running for just one hour requires 1,136Wh. Therefore, a single 12V 100Ah LiFePO4 battery (1,024Wh usable) is insufficient. You need a minimum of two 12V 100Ah LiFePO4 batteries in parallel (200Ah total, 160Ah usable at 80% DoD = 2,048Wh usable) to run the load for one hour safely.
Charge and Discharge Limits You Cannot Ignore
Every battery chemistry has strict C-rate limits. The C-rate is the speed at which you charge or discharge relative to the battery's total capacity. For a 100Ah battery, 1C = 100A; 0.5C = 50A.
| Parameter | LiFePO4 (Lithium Iron Phosphate) | FLA / AGM (Lead-Acid) |
|---|---|---|
| Max Charge C-Rate | 0.5C (50A per 100Ah) | 0.2C (20A per 100Ah) |
| Max Discharge C-Rate | 1.0C to 2.0C (100A - 200A) | 0.2C to 0.5C (depends on Peukert) |
| Absorption Voltage | 14.2V - 14.6V | 14.4V - 14.8V |
| Float Voltage | 13.5V - 13.8V | 13.2V - 13.8V |
| Low Voltage Disconnect | ~10.0V (BMS controlled) | 10.5V (Inverter controlled) |
Component Selection: The Decision Tree
When figuring out how to make a charger for battery systems, your energy source dictates the hardware. Use this decision path to select the exact charge controller or AC-DC charger for your build.
| Energy Source | Array / Input Size | Technology Required | Concrete Component Pick |
|---|---|---|---|
| Solar PV | < 200W (12V system) | PWM Controller | Renogy Wanderer 30A PWM (~$35) |
| Solar PV | 200W - 450W (12V system) | MPPT Controller | Victron SmartSolar MPPT 100/30 (~$160) |
| Solar PV | > 450W (12V system) | High-Amp MPPT | Victron SmartSolar MPPT 150/45 (~$280) |
| AC Grid / Generator | Standard 120V AC Outlet | Smart AC-DC Charger | Victron Blue Smart IP22 12V/30A (~$140) |
Sizing the MPPT Math: A 400W solar panel with a Vmp (voltage at max power) of 40V and Imp of 10A is wired to the MPPT. The MPPT converts the 40V input down to the 14.4V absorption voltage of the battery. Assuming 95% MPPT efficiency: (400W * 0.95) / 14.4V = 26.3A of charging current. This safely respects the 0.5C (50A) max charge limit of a 100Ah LiFePO4 battery while fully utilizing the 30A controller limit.
Inverter Sizing and Final Wiring Verification
With the charging side solved, the inverter must be sized to handle the 1000W continuous load plus startup surges. Motors and compressors can pull 2x to 3x their rated wattage for a fraction of a second.
- Inverter Pick: A 1500W Pure Sine Wave Inverter (e.g., Victron Phoenix 12/1600 or Renogy 1500W). Running a 1500W inverter at a 1000W load keeps it at 66% capacity, which is the peak efficiency and thermal sweet spot for the internal MOSFETs.
Wire Sizing and Overcurrent Protection
DC wiring requires strict adherence to ampacity tables to prevent voltage drop and fire. For our 88.7A continuous draw calculated earlier:
- Wire Gauge: 2 AWG copper (THHN or fine-strand marine wire). According to the 75°C column of standard ampacity tables, 2 AWG is rated for 115A.
- Fusing: A 100A Class T fuse installed on the positive terminal of the battery bank, within 7 inches of the post. Class T fuses have a high interrupt capacity (10,000A at 125VDC), which is mandatory for lithium banks that can dump massive short-circuit current.
- PV Input Wiring: 10 AWG PV wire from the solar panels to the MPPT, with a 15A DC breaker or inline fuse if the panel's short-circuit current (Isc) exceeds 10A.
For comprehensive physical layout and torque specifications, refer to the Victron Energy Wiring Unlimited guide, which details exact busbar stacking orders and crimping standards.
The Verification Step
Do not consider the build complete until you perform a loaded voltage drop test. Turn on your 1000W AC load. Using a digital multimeter, measure the DC voltage directly at the battery terminals, then measure it at the inverter's DC input terminals.
- Pass Criteria: The voltage drop between the battery and the inverter must be less than 0.5V under full load. If the battery reads 12.8V and the inverter reads 12.1V, your wires are too thin, your crimps are loose, or your busbars are undersized. Fix the connections before leaving the system unattended.
By following this exact topology—sizing the MPPT to the array, respecting the 0.5C lithium charge limit, and fusing the 2 AWG inverter feed with a Class T—you transition from guessing to engineering a reliable, fire-safe power system.
