The Core Chemistry: How a Lead Acid Battery Actually Works

At the bench level, understanding a lead acid battery, how does it work comes down to a reversible electrochemical reaction between three materials: lead dioxide (the positive plate), sponge lead (the negative plate), and a sulfuric acid electrolyte. When you connect a load, the sulfuric acid (H₂SO₄) splits. The sulfate ions bond with both plates to form lead sulfate (PbSO₄), while the hydrogen and oxygen form water (H₂O). This electron flow through your external circuit is your current.

As the battery discharges, the electrolyte literally turns into water. This is why we measure state-of-charge (SoC) in flooded lead acid (FLA) batteries using a hydrometer to check specific gravity. A fully charged cell reads 1.265 specific gravity; a dead cell drops to 1.120. When you apply a charging voltage, the reaction reverses, driving the sulfate off the plates and back into the solution, restoring the acid concentration.

Bench Tip: Never trust a resting voltage reading immediately after charging or discharging. Surface charge can make a 12V FLA battery read 13.8V even if it is half-dead. Apply a 10A dummy load for 3 minutes, let it rest for an hour, and measure again. A true 50% SoC reads 12.05V at rest.

Series vs. Parallel: Building the Bank

Single lead acid cells produce a nominal 2.1V. A standard "12V" battery is actually six cells in series (12.6V nominal). When building larger banks for off-grid or UPS systems, you must wire multiple batteries together. The rules for series and parallel are absolute:

  • Series Wiring: Connects the positive of one battery to the negative of the next. Consequence: Voltage adds up, Amp-hours (Ah) remain the same. Two 6V, 225Ah batteries in series yield 12V at 225Ah.
  • Parallel Wiring: Connects positives to positives, negatives to negatives. Consequence: Amp-hours add up, Voltage remains the same. Two 12V, 100Ah batteries in parallel yield 12V at 200Ah.
CRITICAL WARNING: Never parallel mismatched cells. Paralleling batteries of different ages, capacities, or internal resistances causes the stronger battery to continuously dump current into the weaker one, leading to thermal runaway, boiled electrolyte, and melted terminal lugs. If you must parallel, use identical batteries bought on the same day, and keep parallel strings to a maximum of three.

System Block and Sizing Math: Peukert, DoD, and Inverters

A complete DC-to-AC power system follows a strict source-to-load block path: Source (Solar/Grid) → Charge Controller/Inverter-Charger → Battery Bank → Inverter → AC Load. Sizing the battery bank requires accounting for inverter efficiency, Depth of Discharge (DoD), and Peukert's Law.

The Sizing Example

Let’s size a bank to run a 1,200W microwave and a 300W refrigerator compressor (1,500W total) for 4 hours a day.

  1. Base Load: 1,500W × 4h = 6,000Wh.
  2. Inverter Efficiency: Low-frequency pure sine wave inverters are roughly 85% efficient at partial load. 6,000Wh / 0.85 = 7,058Wh DC required.
  3. Depth of Discharge (DoD): FLA batteries must not be discharged past 50% DoD, or the lead sulfate crystallizes permanently (sulfation). 7,058Wh / 0.50 = 14,116Wh total bank capacity.
  4. Amp-Hours at 12V: 14,116Wh / 12V = 1,176Ah.

The Peukert Penalty

If you pull 1,176Ah from a bank rated at the 20-hour rate (C/20), but your microwave pulls that energy in just 4 hours (C/4), Peukert’s Law dictates your usable capacity shrinks. For FLA, the Peukert exponent (k) is typically 1.25 to 1.30. Drawing current at C/4 instead of C/20 will reduce your effective capacity by roughly 25%. To compensate, you must oversize the bank by another 25%, bringing the true requirement to ~1,470Ah at 12V. This is why high-load systems move to 24V or 48V—to keep the DC current draw low and minimize the Peukert penalty.

Inverter and Charger Sizing

For a 1,500W continuous load with a 3,000W surge (fridge compressor startup), you need a 3,000W Inverter. At 12V, a 3,000W inverter pulling max power will draw 250A DC (3000W / 12V). This requires 2/0 AWG copper wire and a 300A Class T fuse on the positive terminal.

Your AC-to-DC battery charger must be sized at 10% to 15% of the bank's Ah capacity to prevent overheating the plates. For a 1,470Ah bank, you need a 150A to 220A multi-stage charger.

Charge Profiles and Discharge Limits

Lead acid batteries require strict multi-stage charging. Failing to program your MPPT or inverter-charger to these exact setpoints will either boil the water out of FLA cells or leave them chronically undercharged.

Standard 12V Lead Acid Charge Setpoints (at 25°C / 77°F)
StageFlooded (FLA)AGMGelPurpose
BulkConstant CurrentConstant CurrentConstant CurrentDelivers 80% of charge as fast as the battery will accept.
Absorption14.4V - 14.8V14.2V - 14.6V13.8V - 14.2VHolds voltage constant while current tapers; dissolves hard sulfate.
Float13.2V - 13.8V13.2V - 13.8V13.2V - 13.6VMaintains 100% SoC without gassing the electrolyte.
Equalization15.5V (Monthly)NEVERNEVERControlled overcharge to stir electrolyte and balance cell voltages.

C-Rate Limits: The maximum continuous discharge rate for a healthy FLA battery is C/5 (e.g., a 200Ah battery can safely deliver 40A continuously). Exceeding this warps the lead plates and accelerates shedding of the active material into the bottom of the battery case, eventually causing an internal short circuit.

Decision Tree: Which Battery Should You Buy?

Stop guessing. Use this decision matrix to select the exact chemistry and form factor for your build.

Your ScenarioRequired TraitsRecommended ChemistryConcrete Pick (Part Number)
Weekend cabin, <2000Wh/day, no maintenance access Sealed, mountable on side, low self-discharge AGM (Absorbent Glass Mat) Renogy 12V 100Ah AGM (RBT100LFP12S is Li, use RBT100AGM12)
Marine/RV starting & deep cycle hybrid High CCA, vibration resistance, moderate DoD AGM Dual Purpose Odyssey 34M-PC1500T
Daily off-grid homestead, >5000Wh/day, ventilated shed High Ah per dollar, 10+ year life, user-serviceable Flooded Lead Acid (Golf Cart style) Trojan T-105-RE 6V 225Ah

The Default Recommendation

If you are building a daily-cycled 12V, 24V, or 48V off-grid power system on a budget and have a ventilated battery shed, the Trojan T-105-RE (6V, 225Ah) is the undisputed industry benchmark. By wiring two in series for 12V, or eight in a series-parallel configuration for a 12V 450Ah bank, you get heavy-duty plates designed for daily 50% DoD cycling. Expect to pay roughly $210–$240 per unit in 2026. Budget for a 24-month lifespan if you abuse them, or a 7-to-10-year lifespan if you strictly adhere to the 50% DoD limit and perform monthly equalization charges.

Lithium Fire-Safety & Upgrade Callout: If you decide to bypass lead acid and upgrade this system to LiFePO4 (Lithium Iron Phosphate) to gain an 80% DoD and eliminate Peukert losses, you MUST use cells with an integrated, high-quality BMS (Battery Management System). Never parallel raw, unmatched lithium cells. LiFePO4 does not suffer from thermal runaway as easily as NMC lithium-ion, but charging LiFePO4 below 0°C (32°F) without internal heating elements causes lithium plating on the anode, which will pierce the separator and cause a catastrophic internal short and fire. Always program your MPPT to disable charging via temperature sensors when ambient temps drop below freezing.

For deeper technical reference on charge algorithms, consult the Victron Energy Whitepapers on battery sizing, and review the Trojan T-105-RE official datasheet for exact torque specs (95-115 in-lbs) and discharge curves. Proper terminal torque prevents high-resistance connections that melt lugs under heavy inverter loads.