A lead-acid battery works by converting chemical energy into electrical energy through a reversible reaction between lead dioxide (positive), sponge lead (negative), and a sulfuric acid electrolyte. When you connect a load, electrons flow through the external circuit while sulfate ions bind to both plates, forming lead sulfate and releasing water into the electrolyte, which drops the specific gravity. This fundamental electrochemical exchange dictates everything from your off-grid solar bank's usable capacity to the cranking amps available to start your diesel generator.
The Core Chemistry and Discharge Cycle
To understand how lead acid batteries work at the bench level, you have to look at the physical state of the plates and the electrolyte. A fully charged 12V lead-acid battery consists of six cells in series, each producing a nominal 2.1V. Inside each cell, the positive plate is coated in lead dioxide (PbO2), and the negative plate is made of porous sponge lead (Pb). The electrolyte is a solution of sulfuric acid (H2SO4) and water.
During discharge, the sulfuric acid is consumed. The sulfate ions (SO4) combine with the lead on both plates to form lead sulfate (PbSO4), while the remaining hydrogen and oxygen form water (H2O). This is why the electrolyte's specific gravity drops from roughly 1.265 (fully charged) to 1.120 (fully discharged). You can literally measure the state of charge in a flooded battery using a hydrometer to read this density change.
When you apply a charging voltage, the reaction reverses. The electrical energy forces the sulfate ions off the plates and back into the solution, restoring the sulfuric acid concentration. However, if a lead-acid battery sits in a partially discharged state, the lead sulfate crystallizes into a hard, non-conductive layer. This is known as hard sulfation, and it permanently reduces the battery's amp-hour capacity by physically blocking the electrolyte from reaching the active lead material.
Lead-Acid Battery Variants: Specs and Performance
While the core chemistry remains identical across all lead-acid batteries, the physical construction drastically changes how they handle charge voltages, depth of discharge, and internal resistance. Below is a specification matrix comparing the four most common variants you will encounter in 12V/24V/48V solar and UPS systems.
| Battery Variant | Absorption Charge Voltage (12V Nominal) | Max Recommended Depth of Discharge (DoD) | Typical Internal Resistance (100Ah Cell) | Estimated Cycle Life at Max DoD |
|---|---|---|---|---|
| Flooded (FLA) | 14.4V - 14.8V | 50% | 4.5 mΩ | 500 - 800 cycles |
| Standard AGM | 14.2V - 14.4V | 50% - 60% | 3.2 mΩ | 400 - 600 cycles |
| Gel (e.g., Sonnenschein) | 13.8V - 14.0V | 60% | 3.8 mΩ | 600 - 900 cycles |
| Carbon-Enhanced AGM | 14.4V | 80% | 2.1 mΩ | 800 - 1200 cycles |
Data compiled from Trojan Battery Charging Guidelines and manufacturer spec sheets. Always verify exact voltages with your specific battery's datasheet, as grid alloys (lead-calcium vs. lead-antimony) shift these targets by up to 0.3V.
The most critical takeaway from this table is the internal resistance (IR). AGM and Carbon-Enhanced batteries pack the glass mat or carbon additive tightly against the plates, lowering the IR. This allows them to deliver higher surge currents for inverter startup without severe voltage depression, a crucial factor when sizing battery banks for heavy inductive loads like well pumps or compressors.
Where You Meet This In Practice: Sizing and Voltage Sag
In a real circuit, the battery's internal resistance and chemical depletion rate dictate the voltage sag under load, which directly changes the trip threshold of your inverter's Low Voltage Disconnect (LVD) and the wire gauge required to maintain acceptable voltage drop. You cannot simply size a lead-acid bank based on its printed Amp-hour (Ah) rating; you must calculate the dynamic voltage under peak current draw.
Worked Numeric Example: Calculating Voltage Sag
Let's say you have a 12V 100Ah Standard AGM battery powering a 1000W microwave through an inverter. The microwave and inverter inefficiencies combine to pull a steady 50A DC from the battery. According to our table, a standard 100Ah AGM has an internal resistance of roughly 3.2 mΩ (0.0032 Ω).
First, we calculate the voltage drop per cell using Ohm's Law (V = I × R):
- Current (I): 50A
- Resistance (R): 0.0032 Ω
- Voltage Drop per Cell: 50A × 0.0032 Ω = 0.16V
Since a 12V battery contains 6 cells in series, we multiply the drop by 6:
- Total Voltage Sag: 0.16V × 6 = 0.96V
If your inverter's Low Voltage Disconnect (LVD) is factory-set to 11.5V, you are operating with only a 0.14V margin. As the battery chemically depletes and its resting voltage drops to 12.4V (roughly 70% SoC), that same 50A load will pull the terminal voltage down to 11.44V, tripping the inverter offline even though the battery still has significant chemical energy left. This is why off-grid solar banks are typically wired in 24V or 48V configurations: doubling the voltage halves the current draw for the same wattage, quartering the I²R voltage sag losses.
Furthermore, you must account for Peukert's Law. If you draw 50A from a 100Ah battery (a C/2 rate), you will not get 2 hours of runtime. Due to the chemical inefficiencies at high discharge rates, a lead-acid battery with a Peukert exponent of 1.25 will yield only about 65 minutes of runtime before hitting the 10.5V cutoff. For deep, sustained loads, lithium iron phosphate (LiFePO4) is vastly superior as its Peukert exponent is effectively 1.0.
Common Confusions and Installation Mistakes
When integrating lead-acid into solar charge paths or UPS systems, DIYers and junior technicians frequently make three specific errors that destroy equipment or leave systems underpowered.
Confusion 1: Amp-Hours (Ah) vs. Usable Watt-Hours (Wh)
People commonly confuse a battery's total Ah capacity with its usable energy. A 100Ah lead-acid battery is not equivalent to a 100Ah LiFePO4 battery. Because discharging a standard lead-acid battery below 50% DoD drastically accelerates grid corrosion and sulfation, a 100Ah AGM only provides 50Ah of usable, daily-cycling capacity. A 100Ah LiFePO4 can safely deliver 80Ah to 90Ah daily. If you are replacing a lithium bank with lead-acid, you must physically double the lead-acid Ah rating to achieve the same usable runtime.
Confusion 2: Mixing Up Charge Profiles
Applying a flooded lead-acid charge profile to an AGM or Gel battery is a fast track to bricking the unit. Flooded batteries require higher absorption voltages (up to 14.8V) and rely on the gassing phase to mix the electrolyte. AGM and Gel batteries are sealed (Valve Regulated Lead-Acid, or VRLA). If you push 14.8V into an AGM designed for 14.4V, the excess energy splits the water into hydrogen and oxygen gas faster than the internal recombination catalyst can handle. The pressure relief valve vents the gas, the glass mat dries out, and the internal resistance spikes permanently.
Confusion 3: Ignoring Temperature Compensation
Lead-acid chemistry is highly sensitive to ambient temperature. The standard charge voltages listed in the table above assume a baseline of 25°C (77°F). For every 1°C above 25°C, you must drop the charge voltage by roughly 0.03V per cell (0.18V for a 12V battery). Conversely, in freezing environments, the voltage must be increased. Failing to enable temperature compensation on your MPPT charge controller will result in chronic undercharging in the winter (leading to sulfation) and severe overcharging in the summer (leading to dry-out and grid corrosion). For authoritative sizing and temperature derating formulas, refer to the Victron Energy Whitepapers on battery bank design.
Understanding how lead acid batteries work goes far beyond memorizing the chemical equation. It requires respecting the physical limits of internal resistance, strictly adhering to variant-specific charge profiles, and sizing the bank to absorb the voltage sag of your heaviest loads without triggering protective disconnects.
