How deep can I safely discharge a lead-acid battery?

The recommended maximum is 50% DoD for regular use. This gives you 500-1,000 cycles depending on the battery quality. Going to 80% DoD cuts cycle life roughly in half — you might get only 200-400 cycles. Discharging below 80% risks permanent damage from hard sulfation, where lead sulfate crystals harden and can no longer be reversed by charging. For the best lifespan, keep discharge above 50% SOC (i.e., use only 50% of the rated capacity).

Why does my lead-acid battery lose capacity over time?

Lead-acid batteries degrade through sulfation, plate shedding, and electrolyte stratification . Sulfation occurs whenever the battery sits partially discharged — lead sulfate crystals grow on the plates and reduce active material. Plate shedding happens during deep discharge cycles — active material literally falls off the plates and collects at the bottom of the case. Electrolyte stratification causes the acid to concentrate at the bottom, corroding the lower plates faster. Proper maintenance (regular full charges, equalization, water levels) slows all three processes.

Is it worth replacing lead-acid with LiFePO4?

For cycling applications (solar storage, RV, boating), LiFePO4 almost always saves money long-term. A $400 LiFePO4 100Ah battery delivering 80Ah usable for 3,000 cycles equals $0.0017 per Wh cycled. A $150 lead-acid 100Ah battery delivering 50Ah usable for 500 cycles equals $0.0025 per Wh cycled — 47% more expensive per unit of energy used. For standby-only applications (UPS backup, emergency lighting) where the battery rarely cycles, lead-acid remains cost-effective because cycle life is irrelevant.

Lead-Acid Battery Runtime Calc

4 min read

Lead-acid batteries remain the most affordable option for stationary storage, backup power, and vehicle starting. But their runtime calculations require more nuance than lithium — the Peukert effect, sulfation risk, and low DoD limits mean the number on the label tells only part of the story. This calculator gives you a baseline estimate with lead-acid-appropriate defaults. If you are sizing a lead-acid bank for solar storage, the solar battery bank calculator factors in the lower DoD limits that lead-acid requires.

Lead-acid battery voltage thresholds from 12.7V charged to 10.5V discharged at 50 percent DoD.

The Peukert Effect in Lead-Acid Batteries

A 100Ah lead-acid battery rated at the 20-hour rate (C20) delivers 5A for 20 hours. Simple math says it should also deliver 50A for 2 hours or 100A for 1 hour. It does not. At 50A, you might get only 1.5 hours (75Ah delivered). At 100A, maybe 40 minutes (67Ah delivered).

The Peukert equation quantifies this: t = C / I^k, where t is time, C is the rated capacity at C20, I is the discharge current, and k is the Peukert exponent. For lead-acid batteries, k ranges from 1.1 (premium AGM) to 1.4 (cheap flooded). Higher k means more capacity loss at high discharge rates.

What causes this? At high currents, lead sulfate crystals form faster than they can dissolve back into the electrolyte. These crystals block the plate surface, preventing further chemical reaction. The energy is not lost — it is trapped in the plates and becomes available when the battery rests or is recharged. But in practice, if you need 100A right now, the Peukert effect means your battery delivers less total energy than the label suggests.

The runtime calculator above uses the simplified linear formula. For loads below C/5 (20A for a 100Ah battery), it is reasonably accurate. For higher loads, reduce the calculated runtime by 10-25% to account for Peukert losses.

Lead-acid voltage curve showing gradual decline from 12.7V to 10.5V during discharge. — Unlike LiFePO4, lead-acid voltage drops steadily throughout discharge — the 50% DoD cutoff at 11.9V protects longevity.

Flooded Lead-Acid vs Sealed Lead-Acid

Feature	Flooded (FLA)	Sealed (SLA/VRLA)
Maintenance	Add distilled water every 1-3 months	Maintenance-free
Gassing	Produces hydrogen when charging — needs ventilation	Minimal gassing (recombination)
Mounting	Must be upright — acid can spill	Any orientation (no free liquid)
Cost per Ah	Lowest ($0.40-0.80/Ah)	Higher ($1.50-3.00/Ah for AGM)
Cycle Life at 50% DoD	600-1,000 cycles	400-800 cycles (AGM), 700-1,200 (Gel)
Self-Discharge	5-15% per month	3-5% per month
Charge Acceptance	Moderate	Good (AGM can accept higher charge rates)

Flooded batteries win on cost and cycle life but require maintenance and ventilation. Sealed (AGM or Gel) batteries trade cycle life for convenience. For off-grid cabins where you can maintain the batteries, flooded is the value play. For RVs, boats, and enclosed spaces, sealed is safer and more practical.

Worked Examples

Why Lead-Acid Runtime Drops at High Discharge Rates

Context

A 200Ah 12V flooded battery is rated at the 20-hour rate (10A draw). You need to run a 500W load (42A draw). The Peukert exponent for this battery is 1.25.

Calculation

At 20-hour rate: 200Ah available = 2,400 Wh

At 42A draw with Peukert: effective capacity ≈ 200 x (10/42)^(1.25-1) = 200 x 0.238^0.25 = 200 x 0.699 = 139.8 Ah

Usable at 50% DoD: 139.8 x 0.50 x 12 x 0.85 = 712.8 Wh

Runtime: 712.8 / 500 = 1.43 hours

Interpretation

Peukert cuts the effective capacity from 200Ah to about 140Ah — a 30% reduction just from the higher discharge rate. This is why lead-acid batteries consistently underperform their rated Ah at real-world loads.

Takeaway

LiFePO4 batteries are nearly immune to the Peukert effect. For a deeper look at how DoD limits affect long-term battery life, see our depth of discharge guide.

Sizing a Flooded Lead-Acid Bank for Solar Storage

Context

Your off-grid cabin uses 2,000 Wh per day. You want 2 days of autonomy from a 12V flooded lead-acid bank at 50% DoD with 85% efficiency.

Calculation

Energy needed: 2,000 x 2 = 4,000 Wh

Battery capacity: 4,000 / (12 x 0.50 x 0.85) = 784 Ah

Use 4 x 200Ah batteries in parallel (800Ah)

Interpretation

800Ah at 12V is a large bank — weighing about 240 lbs for flooded batteries. The same energy from LiFePO4 at 80% DoD needs only 490Ah and weighs half as much.

Takeaway

For the solar panel sizing to recharge this bank, use our solar panel and battery sizing calculator — it handles the complete sizing calculation.

Frequently Asked Questions

Glossary

Peukert Effect

The phenomenon where lead-acid batteries deliver less total energy at higher discharge rates. A battery rated 200Ah at the 20-hour rate (10A) might only deliver 140Ah at a 40A rate. The Peukert exponent (typically 1.1-1.3) quantifies this loss.

Flooded Lead-Acid

A battery with liquid sulfuric acid electrolyte that must be periodically topped up with distilled water. Cheapest per Ah but requires maintenance, proper ventilation (hydrogen gas release), and upright mounting.

20-Hour Rate

The standard discharge rate used to define a battery's Ah capacity. A 200Ah battery at the 20-hour rate delivers 10A for 20 hours. Drawing more than 10A reduces the effective capacity due to the Peukert effect.

Planning solar charging for your lead-acid bank? The <a href="/solar/solar-battery-charge-time-calculator">solar charge time calculator</a> accounts for the slow absorption phase that lead-acid requires.

Lead-acid batteries are proven, affordable, and widely available. They are also heavy, maintenance-intensive, and limited to 50% usable capacity. Run the numbers above with 50% DoD and 85% efficiency — those are the honest defaults for lead-acid. If the runtime falls short, you need a bigger bank or a different chemistry. For a head-to-head comparison with lithium, read our LiFePO4 vs lead-acid runtime comparison. The deep cycle runtime calculator covers flooded, gel, and golf-cart batteries with Peukert-effect guidance.

Last updated: March 17, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.