The Core Definition and System Block: Source to Load

At its most fundamental level, a battery is an electrochemical device that stores energy in chemical bonds and releases it as direct current (DC) electricity. In renewable energy and backup power, it acts as the buffer between unpredictable generation and rigid load demands. To understand what is battery and types of battery configurations, you must first look at the complete system block from source to load.

A standard off-grid or hybrid DC-coupled system follows this power path:

  1. Source: Solar PV array or wind turbine generates raw, variable DC voltage.
  2. Regulation: An MPPT (Maximum Power Point Tracking) charge controller steps the voltage down/up and regulates current to safely charge the bank.
  3. Storage (The Battery Bank): The electrochemical reservoir that absorbs excess energy and supplies deficits. This is your DC bus.
  4. Conversion: An inverter draws DC from the battery bus and synthesizes a clean 120V/240V AC sine wave.
  5. Load: Your main AC subpanel, which distributes power to appliances, lights, and outlets.

If any bottleneck exists in this chain—undersized wires between the battery and inverter, or a charge controller with insufficient amperage—the battery will suffer from chronic undercharging or voltage sag, regardless of its chemistry.

Types of Battery: Chemistry, C-Rates, and Depth of Discharge

When evaluating battery chemistries for a 12V, 24V, or 48V system, the two most critical metrics are Depth of Discharge (DoD) and the C-rate. DoD tells you what percentage of the battery's total capacity you can safely use without degrading it. The C-rate defines how fast you can charge or discharge the battery relative to its total capacity (e.g., a 100Ah battery discharged at 1C is delivering 100A; at 0.5C, it delivers 50A).

Battery Chemistry Specification Comparison
Chemistry Nominal Voltage Usable DoD Max Continuous C-Rate Cycle Life (to 80% SOH) Peukert Exponent (k)
Flooded Lead-Acid (FLA) 2.0V / cell (12V nominal) 50% 0.2C (C/5) 500 - 800 ~1.30
AGM / Gel (VRLA) 2.0V / cell (12V nominal) 50% - 60% 0.3C (C/3) 800 - 1,200 ~1.20
LiFePO4 (Lithium Iron Phosphate) 3.2V / cell (12.8V nominal) 80% - 90% 0.5C to 1.0C 3,000 - 6,000+ ~1.05
Lithium Fire-Safety & Cell Matching Warning: While LiFePO4 is inherently safer than NMC/NCA lithium chemistries and highly resistant to thermal runaway, a failed Battery Management System (BMS) or physical puncture can still result in venting and fire. Never parallel mismatched cells or batteries of different ages, capacities, or chemistries. Internal resistance differences will cause the newer/lower-resistance battery to dump massive current into the older one during a load transient, potentially melting terminals or triggering a thermal event. Always use a high-quality BMS with over-current, short-circuit, and cell-balancing protections.

Choosing the right chemistry depends on your operational constraints. Use the decision matrix below to select your bank:

Application Scenario Recommended Chemistry Why?
Weekend cabin, tight budget, infrequent use Flooded Lead-Acid (FLA) Lowest upfront cost per Ah; sits well if kept on a float charge.
Marine/RV, high vibration, engine starting surges AGM (Absorbent Glass Mat) Spill-proof, handles high C-rate surge currents for starter motors.
Daily off-grid solar, high DoD, space-constrained LiFePO4 Yields 2x to 3x the usable energy of lead-acid for the same physical footprint; zero maintenance.

Sizing Math: Peukert’s Law, Efficiency, and Inverter Matching

Sizing a battery bank is where most DIY builds fail. You cannot simply divide your watt-hours by the battery voltage. You must account for inverter efficiency, Depth of Discharge limits, and Peukert’s Law (for lead-acid).

The Worked Example:
Let's size a bank to run a 1,500W space heater and 500W of lighting (2,000W total AC load) for 3 hours.

  1. Calculate AC Energy: 2,000W × 3h = 6,000Wh.
  2. Account for Inverter Efficiency: Inverters are typically 85% to 90% efficient under heavy load. Assuming 85%: 6,000Wh / 0.85 = 7,058Wh of DC energy required.
  3. Apply Chemistry Constraints (The 48V Advantage):
    If you use a 48V FLA bank (50% DoD): 7,058Wh / 0.50 = 14,116Wh total capacity needed. At 48V, that is 294Ah.
    Enter Peukert’s Law: Drawing 2,000W from a 48V system requires ~42A. If you were doing this on a 12V system, you'd be pulling 168A. Peukert's exponent ($k \approx 1.3$) dictates that at high discharge rates, lead-acid capacity plummets. A 200Ah FLA battery rated at the 20-hour rate (10A draw) will only deliver about 120Ah if you pull 100A. LiFePO4 ($k \approx 1.05$) suffers almost no such penalty, which is why lithium is vastly superior for high-draw inverter applications.
  4. Inverter and Charger Sizing: For a 2,000W continuous load, you need an inverter rated for at least 2,000W continuous and 4,000W surge (to handle motor starting loads if you add a fridge later). Your battery charger (or the inverter's internal charger) must be sized to replenish the bank safely. The golden rule for lead-acid is a charge current of 10% to 20% of the Ah capacity. For our 294Ah FLA bank, you need a minimum 30A to 60A charger. LiFePO4 can accept up to 0.5C, meaning a 100Ah lithium battery can safely accept a 50A charge current.

For a deeper dive into wiring these components safely, the Wiring Unlimited guide by Victron Energy is the industry standard reference for proper busbar sizing, fuse placement, and cable routing in DC systems.

Frequently Asked Questions

What happens to voltage and capacity when wiring batteries in series vs parallel?

Wiring batteries in series increases the system voltage while the Amp-hour (Ah) capacity remains the same. For example, four 12V 100Ah batteries in series create a 48V 100Ah bank (4,800Wh total). Wiring batteries in parallel keeps the voltage the same but increases the Ah capacity. Four 12V 100Ah batteries in parallel create a 12V 400Ah bank (still 4,800Wh total). Best practice: High-power systems (over 2,000W inverter) should always use series wiring to achieve 24V or 48V. This keeps the DC amperage low, allowing you to use smaller, manageable wire gauges (like 2/0 AWG) instead of dangerously thick, unmanageable copper cables required for 12V high-current setups.

What charge and discharge limits apply to different battery types?

Lead-acid batteries (FLA, AGM, Gel) require a three-stage charging profile: Bulk (constant current), Absorption (constant voltage, typically 14.4V to 14.8V for a 12V nominal bank), and Float (13.2V to 13.8V). Discharging below 10.5V (for a 12V bank) will cause severe sulfation and permanent capacity loss. LiFePO4 batteries use a simpler CC/CV (Constant Current / Constant Voltage) profile. They charge up to 14.2V - 14.6V, but do not require and should not be held at a continuous float voltage. Discharging a 12.8V LiFePO4 battery below 10.0V will trigger the BMS low-voltage disconnect to protect the cells from copper dissolution and internal shorting. For comprehensive cell-level voltage limits, Battery University provides excellent baseline chemistry data.

How do I size an inverter and charger for my battery bank?

First, sum the continuous wattage of all loads you expect to run simultaneously, then add 20% for overhead. This is your minimum inverter continuous rating. Ensure the inverter's surge rating covers the Locked Rotor Amperage (LRA) of any compressors or well pumps. Next, size the charger based on the battery's accepted C-rate. For a 200Ah lead-acid bank, a 20A to 40A charger is ideal. For a 200Ah LiFePO4 bank, you can safely use a 60A to 100A charger, drastically reducing your recharge time from solar or a generator. Always place a Class T or ANL fuse on the positive inverter cable, sized 1.25x the inverter's maximum continuous DC draw, positioned within 7 inches of the battery terminal.