Batteries, Machines and Electricity Systems
Lead-Acid Batteries, Capacity and Starting Power
Connect individual cells, series voltage and the meaning of capacity ratings.
You are connecting the subsystems described in “Lead-Acid Batteries, Capacity and Starting Power” and need their mechanical, electrical and control interfaces to work together.
Understand lead-acid discharge and recharge, the six-cell 12 V architecture, and why ampere-hour capacity and cold-cranking ratings answer different questions under different tests.
- Select or check first
- Rechargeability does not mean unlimited reversibility. Sulfation, corrosion, active-material shedding, water loss, temperature and time can reduce capacity and life. Deep discharge, undercharge, overcharge and unsuitable charge settings accelerate different failure mechanisms.
- Controlling assumption
- Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
- Top risk
- Lead-acid batteries combine electrical and chemical hazards. A short circuit can produce destructive current; charging may release hydrogen; electrolyte is corrosive; and a battery remains energized when equipment is off.
- Proof
- Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Start with the basics Optional foundation before “Lead-Acid Batteries, Capacity and Starting Power”
Electrical-engineering basics
Electrical behaviour becomes easier to reason about when every voltage has a reference and every current has a complete return path.
- Voltage
- Voltage is potential difference between two points. A voltage statement is incomplete unless both points or its reference are known.
- Current
- Current is charge flow through a path. It does not disappear after a component; in a complete circuit it returns to the source.
- Resistance and impedance
- Resistance opposes steady current. Impedance extends that idea to changing signals and can depend on frequency through capacitance and inductance.
- Power and heat
- Electrical power is the rate of energy transfer. Some becomes useful output and some becomes heat that must stay within component and enclosure limits.
- Rating versus operating point
- A rating is a limit or qualified condition, not a target. The operating point must remain inside all relevant voltage, current, power, temperature and timing limits.
How to use this article: Draw the source, load, reference and return path before calculating. Treat exposed or stored electrical energy as hazardous until isolation is proved.
On this page
A lead-acid battery can deliver high current, accept recharge and serve starting, standby or cyclic duties—but those uses do not share one performance number. Chemistry establishes the reversible reaction; series-connected cells establish voltage; capacity tests describe charge delivered at a stated rate and cutoff; cranking tests describe short, cold, high-current performance under a named standard. A responsible selection keeps those layers separate.
Discharge converts both active plates toward lead sulfate
Diagram description
The plate depicts the discharge direction and a flooded construction; AGM and gel batteries retain electrolyte differently but use lead-acid electrochemistry.
Simplified overall discharge reaction
Pb + PbO₂ + 2H₂SO₄ ⇌ 2PbSO₄ + 2H₂O
The rightward direction represents discharge and the leftward direction represents charging as a net chemical statement. It does not prescribe charge voltage, current, time or temperature compensation; those come from the exact battery maker.
- Pb
- lead active material at the negative plate
- PbO₂
- lead dioxide at the positive plate
- H₂SO₄
- sulfuric acid electrolyte
- PbSO₄
- lead sulfate formed during discharge
- H₂O
- water
A nominal 12 V battery normally uses six cells in series
Diagram description
The diagram shows six electrochemical cells and distinguishes nominal voltage from actual voltage under charge, rest or load.
Open-circuit voltage depends on state of charge, temperature and rest. Loaded voltage also includes internal drop; charging voltage is deliberately higher and depends on product type and regime. Therefore “12 V” is a system class, not a fixed meter reading. One weak cell can limit the series string even when the summed open-circuit voltage initially appears plausible.
Ampere-hours and cold-cranking amps answer different tests
Diagram description
The plate cites a 170 A CCA product example and a 7.0 Ah 20-hour-rate product example, each tied to its published conditions.
| Rating | Question answered | Conditions that must accompany it |
|---|---|---|
| Ampere-hours (Ah) | How much charge was delivered? | Discharge rate, cutoff voltage, temperature and battery condition |
| Watt-hours (Wh) | How much electrical energy was delivered? | Voltage throughout the discharge as well as current and time |
| CCA | Can the battery sustain a specified cold starting test? | Named standard, temperature, time and terminal-voltage criterion |
| Reserve capacity | How long can a specified current be delivered? | Test current, temperature and cutoff voltage |
| Cycle life | How many cycles under a stated regime? | Depth of discharge, charge method, temperature and end-of-life threshold |
Charge capacity at a stated discharge
Q_Ah = ∫ I(t) dt / 3600
For constant current, Ah is amperes times hours until the specified cutoff. It does not state voltage, instantaneous starting ability or safe charge current. Lead-acid capacity usually falls as discharge rate rises, so do not scale a 20-hour rating linearly into a cranking prediction.
- Q_Ah
- delivered charge capacity, Ah
- I(t)
- discharge current, A
- t
- elapsed time, s
Select chemistry, construction and duty together
| Question | Why it changes the choice |
|---|---|
| Starting, standby or repeated cycling? | Plate design and recommended depth of discharge differ |
| Flooded, AGM or gel construction? | Ventilation, mounting and charge limits differ |
| What temperature range? | Capacity, cranking and charge voltage are temperature-sensitive |
| What charger profile? | Float and cyclic limits are product-specific |
| What fault current is possible? | Protection, conductor size, connectors and tools must handle high prospective current |
| What end-of-life criterion? | Starting margin, runtime or capacity retention may govern replacement |
Use How a Battery Cell Converts Chemical Energy for electron, ion and voltage-sag foundations. For the difference between generation time and a storage requirement, continue with Solar Generation During Day, Night and Bad Weather.
Selection boundary and evidence to keep with the decision
Treat “Lead-Acid Batteries, Capacity and Starting Power” as a selection problem, not only a definition. Record the required result, alternatives considered, the controlling assumption and the condition that would change the choice.
Keep evidence proportional to consequence: Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test. Revisit the decision when the load, environment, duty, interface, regulation or maintenance plan changes.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Lead-Acid Batteries, Capacity and Starting Power?
Rechargeability does not mean unlimited reversibility. Sulfation, corrosion, active-material shedding, water loss, temperature and time can reduce capacity and life. Deep discharge, undercharge, overcharge and unsuitable charge settings accelerate different failure mechanisms.
Which assumption can change the conclusion?
Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
What can go wrong if I copy the example directly?
Lead-acid batteries combine electrical and chemical hazards. A short circuit can produce destructive current; charging may release hydrogen; electrolyte is corrosive; and a battery remains energized when equipment is off.
How do I verify the result in the real system?
Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Sources and further reading
- PS-1270 General Purpose VRLA Battery DatasheetPower-Sonic Corporation · 2026-10-13Current manufacturer sheet reviewed for nominal voltage, six-cell construction, capacity by rate, cutoff voltages, charging limits and short-circuit context.
- ODS-AGM16L (PC680) Product SpecificationsODYSSEY Battery · 2026-10-13Manufacturer source reviewed for the documented 170 A CCA and 16 Ah 20-hour-rate example.
- ODYSSEY Battery Product GuideEnerSys / ODYSSEY Battery · 2026-10-13Reviewed for product ratings and the guide’s statement that CCA is determined in accordance with SAE J537.