The chemical reaction in a lead acid battery is a reversible electrochemical process where lead (Pb), lead dioxide (PbO2), and sulfuric acid (H2SO4) convert into lead sulfate (PbSO4) and water (H2O) during discharge. When you apply a charging current, this reaction runs in reverse, restoring the active materials and the acid concentration. Understanding this exact mechanism is the only way to properly size a battery bank, prevent fatal sulfation, and match your inverter to the bank's true C-rate limits without tripping breakers or melting lugs.
The Reversible Chemical Reaction and Specific Gravity
Unlike lithium-ion cells where lithium ions physically shuttle between an anode and cathode (intercalation), lead-acid batteries rely on a double-replacement reaction that fundamentally alters the chemistry of the liquid electrolyte.
During discharge, the sulfuric acid in the electrolyte is consumed to form lead sulfate on both the positive and negative plates, releasing water into the solution. The governing equation is:
Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O
During charge, the external electrical energy forces the reaction backward, converting the lead sulfate back into lead, lead dioxide, and sulfuric acid:
2PbSO4 + 2H2O → Pb + PbO2 + 2H2SO4
Because sulfuric acid is significantly denser than water (specific gravity of ~1.84 vs 1.00), the overall density of the electrolyte drops as the battery discharges. This is why a hydrometer is the most accurate tool for measuring the true state of charge (SoC) in a flooded lead-acid cell. If a battery sits in a partially discharged state, the lead sulfate crystallizes into a hard, non-conductive layer—a failure mode known as sulfation, which permanently destroys capacity.
| State of Charge (SoC) | Specific Gravity (SG) | Resting Voltage (12V Nominal) | Sulfation Risk Level | Required Action |
|---|---|---|---|---|
| 100% | 1.265 - 1.275 | 12.70V - 12.80V | None | Float charge at 13.5V |
| 75% | 1.225 - 1.235 | 12.40V - 12.50V | Low (if < 24 hours) | Recharge within 24 hours |
| 50% | 1.190 - 1.200 | 12.10V - 12.20V | Moderate (Active DoD limit) | Immediate bulk charge required |
| 25% | 1.155 - 1.165 | 11.80V - 11.90V | High (Rapid crystal growth) | Emergency equalization needed |
| 0% (Deep Discharge) | 1.100 - 1.120 | < 11.50V | Severe (Permanent damage) | Desulfation or recycle cell |
Source data adapted from Battery University and standard flooded cell manufacturer spec sheets. Temperature compensation of -0.003 SG per 10°F above 77°F is required for accurate hydrometer readings.
System Architecture and Inverter/Charger Sizing
To apply this chemistry to a real-world off-grid or backup power setup, we must map the power flow from source to load. A standard DC-coupled solar architecture follows this block sequence:
Solar Array / Grid Charger → MPPT Charge Controller → DC Disconnect & Class T Fuse → Battery Bank → Inverter DC Input → AC Load Panel.
Let's size the inverter and charger for a stated continuous load of 1500W (e.g., running a microwave, a refrigerator compressor, and LED lighting simultaneously).
Inverter Sizing: Inverters must handle continuous loads plus a 25% safety margin for thermal headroom and transient spikes.
1500W × 1.25 = 1875W.
Decision: Select a 2000W Pure Sine Wave Inverter. Modified sine wave inverters will cause the microwave transformer to overheat and the fridge compressor to run inefficiently.
Inverter/Charger Sizing: If you are using a combined inverter/charger unit (like a Victron MultiPlus or Magnum MMS), the internal charger must be able to run the AC loads while simultaneously pushing bulk current into the battery bank. At a 24V nominal battery bank voltage, 1500W of AC load draws roughly 70A of DC current from the batteries (factoring in 90% inverter efficiency). To charge the bank at a standard C/10 rate while supporting the load, the charger must output at least 70A (load) + 40A (charge) = 110A DC.
Decision: Specify an inverter/charger with a minimum 120A internal battery charger rating.
Bank Sizing Math: Peukert’s Law and Depth of Discharge
Sizing a lead-acid bank is where most DIY builds fail because they ignore two critical factors: Depth of Discharge (DoD) limits and Peukert’s Law. You cannot use the full Amp-hour (Ah) rating printed on the battery case.
Step 1: Calculate Raw Energy Requirement
Target runtime: 4 hours at 1500W continuous.
1500W × 4 hours = 6000 Watt-hours (Wh).
Step 2: Factor in Inverter Efficiency
Inverters are not 100% efficient; typical pure sine wave units operate at 90% to 93% efficiency under heavy load.
6000Wh / 0.90 = 6666Wh required from the DC battery bank.
Step 3: Apply Depth of Discharge (DoD) Limits
To achieve a reasonable cycle life (e.g., 1000+ cycles), deep-cycle flooded or AGM lead-acid batteries should not be discharged below 50% SoC. Discharging to 80% DoD will cut the battery's lifespan by more than half.
6666Wh × 2 (for 50% DoD) = 13,332Wh of total rated bank capacity.
Step 4: Apply Peukert’s Law
Peukert's Law states that the faster you draw current from a lead-acid battery, the less total capacity it will deliver. A 200Ah battery rated at the 20-hour rate (C/20) will only deliver roughly 140Ah if you drain it at the 4-hour rate (C/4). The Peukert exponent (k) for flooded lead-acid is typically 1.3.
Because our 4-hour discharge window hits the bank hard, we must apply a Peukert derating factor of roughly 0.75 to our usable capacity calculations to ensure we don't accidentally drag the bank below 11.8V.
13,332Wh / 0.75 = 17,776Wh of nameplate capacity required.
Final Bank Configuration:
At 24V nominal, 17,776Wh / 24V = 740Ah.
Parts List: Eight 6V, 390Ah golf cart batteries (e.g., Trojan T-105 or Crown CR-430) wired in four parallel strings of two series batteries to yield 24V at 780Ah.
Series vs. Parallel Wiring and Charge Limits
When wiring multiple batteries to achieve that 740Ah target, you must understand the exact electrical consequences of series and parallel topologies.
- Series Wiring: Voltages add, Amp-hours remain the same. Wiring two 12V 200Ah batteries in series yields 24V at 200Ah. The total energy (Wh) is identical, but the higher voltage halves the DC current for a given wattage, reducing I²R heat losses in the cables.
- Parallel Wiring: Amp-hours add, voltage remains the same. Wiring two 12V 200Ah batteries in parallel yields 12V at 400Ah.
Never wire batteries of different ages, chemistries, or capacities in parallel. A newer battery with lower internal resistance will constantly push current into an older, higher-resistance battery, causing parasitic draining, localized overheating, and accelerated grid corrosion. Always keep parallel strings identical in brand, model, and purchase date.
Charge and Discharge C-Rate Limits:
Lead-acid chemistry physically cannot accept or deliver energy as fast as lithium.
Discharge Limit: The maximum continuous discharge rate for deep-cycle lead-acid is generally C/5 (20% of the Ah capacity). For our 780Ah bank, the max continuous draw is 156A. (Starting surge currents for motors can briefly exceed this, but sustained draws will cause severe voltage sag and plate warping).
Charge Limit: The optimal bulk charge rate is between C/10 and C/5. For a 780Ah bank, your MPPT charge controller should be configured to deliver between 78A and 156A during the bulk phase. Pushing more current than C/5 simply boils the electrolyte (gassing) and heats the plates without storing extra energy.
If you decide to abandon lead-acid and upgrade this 24V system to LiFePO4 (Lithium Iron Phosphate) to escape the 50% DoD penalty and Peukert losses, you must change your safety protocols. Lithium cells do not off-gas hydrogen like lead-acid, but they are susceptible to thermal runaway if a cell shorts internally or if charged below 0°C (32°F) without internal heating elements, which causes lithium plating and internal dendrite shorts.
Mandatory Requirements for Lithium: Never wire raw lithium cells in parallel without a dedicated, properly rated Battery Management System (BMS) on every single parallel string to prevent cascading overcurrent failures. Ensure your inverter/charger has a specific lithium charge profile (no high-voltage equalization phase) and install a Class T fuse within 6 inches of the positive terminal to clear catastrophic short-circuit faults.
Mastering the chemical reaction in a lead acid battery means respecting its physical limits. By tracking specific gravity, respecting the 50% DoD boundary, and applying Peukert's math to your sizing, you will extract a decade of reliable service from flooded or AGM cells before needing to consider a lithium upgrade.
