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

Trojan prints the recommendation for its own flooded deep-cycle batteries: discharge 20 to 50% of rated capacity, "even though the battery is capable of being cycled to 80%". Concorde makes the same call for Lifeline AGM, recommending an average depth of discharge no greater than 50% of the 20-hour rating. What you give up in depth you get back in cycles: Victron prints 1,800 cycles at 30% discharge for its gel deep-cycle line, 750 at 50% and 500 at 80%, and 1,500, 600 and 400 at those depths for its AGM line. Past 80% you are into hard sulfation, where the crystals stop dissolving back and the capacity does not return with a charge.

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 anything that cycles daily (solar storage, an RV, a boat), LiFePO4 wins on cost per cycle even though it costs more on the shelf. Battle Born prints 100% depth of discharge and 3,000-5,000 deep discharge cycles for its 100Ah 12V battery, so a $400 pack delivering 100Ah for 3,000 cycles works out at $0.0013 per Ah cycled, or about $0.0001 per Wh at the 12.8V nominal Victron prints for LiFePO4. A $150 sealed lead-acid 100Ah battery held to 50% gives 50Ah a cycle, and Victron prints 600 cycles at 50% discharge for its AGM deep-cycle line, which is $0.005 per Ah cycled or $0.00042 per Wh. That is about 4 times more per unit of energy used. For standby duty such as a UPS or emergency lighting the battery barely cycles at all, cycle life stops mattering, and lead-acid is still the cheaper buy.

Lead-Acid Battery Runtime Calc — 50% DoD

7 min read

Lead-acid is still the cheapest way to buy a kilowatt-hour of storage, and still the chemistry most likely to fall short of its label. A 100Ah battery does not hand over 100Ah at every current: the harder you pull, the less you get back. The lead-acid battery runtime calculator below applies that loss instead of ignoring it, using a rate correction fitted to the discharge tables Trojan, Lifeline and Victron publish for their own batteries. Pick flooded, AGM or gel, enter your load, and the runtime you see already carries the derate. If you are sizing a bank for solar storage, the solar battery bank calculator works to the lower depth limits lead-acid needs.

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

The Peukert Effect in Lead-Acid Batteries

Amp-hour ratings are measured at the 20-hour rate. Trojan's own data sheet for the T-105 puts the whole ladder on one line: the same battery is rated 250Ah at the 100-hour rate, 225Ah at the 20-hour rate, 207Ah at the 10-hour rate and 185Ah at the 5-hour rate. One battery, four numbers. Pull harder and you get less, because at high current lead sulfate forms faster than it dissolves back into the electrolyte and blocks the plate surface before the plate is spent.

The calculator above does not assume the rated amp-hours arrive at any current. It works out the current your load actually draws, then reads the time off a curve fitted to printed discharge points: time to full discharge = a x (I / C20)^-n, where I is the current, C20 is the 20-hour rating, and (a, n) is the fitted pair for the type you picked. Your depth of discharge then takes its share of that time. The rate correction is fitted to the constant-current discharge tables Trojan, Lifeline and Victron print for their own batteries, and it reproduces those printed points within about 10% for flooded, 6% for AGM and 2% for gel.

Two checks against the sheets themselves. Trojan prints 447 minutes at 25A for the 225Ah T-105; the tool computes 7 hours 31 minutes to full discharge against that printed 7 hours 27 minutes. Lifeline prints 195 minutes at 25A for its 105Ah GPL-31T; the tool computes 3 hours 17 minutes against the printed 3 hours 15 minutes. The flat-capacity arithmetic this page used to show gave 9 hours and 4 hours 12 minutes for those same two batteries, and that gap is where the missing hours came from.

What the correction does not cover is temperature or age. Trojan and Victron both print capacity-versus-temperature curves and neither is in the engine, so a cold bank does worse than the figure above. Push the load past the fastest discharge the makers printed and the result panel says so, because past that point the tool is extrapolating rather than reading a curve somebody measured.

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

Flooded and sealed are the same lead-acid chemistry in different packaging, and the packaging is what you live with day to day. Every figure below is one the maker prints for its own product, with the maker named in the cell.

Feature	Flooded (FLA)	Sealed (SLA/VRLA)
Maintenance	Trojan says to check the water level once a month until you learn your own interval	Maintenance-free
Gassing	Releases hydrogen when charging, so it needs ventilation	Minimal gassing (recombination)
Mounting	Must sit upright, because the 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 the maker prints	Trojan prints none on the T-105 sheet; it recommends 20-50% depth of discharge	Victron AGM: 1,500 cycles at 30% discharge, 600 at 50%, 400 at 80%. Victron gel: 1,800, 750 and 500 at the same three depths
Self-discharge	5-15% per month (Trojan, T-105 sheet)	About 2% per month (Lifeline GPL-31T)
Charge acceptance	Moderate	Good (AGM takes a higher charge rate)

Flooded wins on cost per amp-hour and is the value play for an off-grid cabin where you can reach the caps once a month. Prices and rates on this page are our own estimates as of the date shown; they change often. Enter your own figures for a current result. Sealed AGM and gel cost more and give up nothing to spillage or hydrogen, which is why they win in an RV, a boat, or a cupboard. Look at where the printed cycle rows land, though: Victron rates its gel line 750 cycles at 50% discharge against 500 at 80%, so the depth you choose moves the replacement date more than the badge on the case does.

Worked Examples

Running a 500W Load From a 200Ah Flooded Bank

Context

A 200Ah 12V flooded bank runs a 500W inverter load. Flooded is held to 50% depth of discharge to protect cycle life, and the inverter and wiring together return about 85% of what leaves the plates.

Calculation

Current drawn from the plates: 500 W / 0.85 / 12 V = 49.0 A

Rate against the 20-hour rating: 49.0 / 200 = 0.245

Flooded fit, time to full discharge: 0.5964 x 0.245^-1.1529 = 3.02 hours

At 50% depth of discharge: 3.02 x 0.50 = 1.51 hours

Interpretation

The tool prints 1 hour 31 minutes. The flat-capacity arithmetic this page used to show gave 2 hours 2 minutes for the same bank, and the missing half-hour is the rate loss: at 49A the battery is working at nearly five times its 20-hour current, and it returns less than its rating for it.

Takeaway

A 500W draw on a single 200Ah bank is heavy duty for lead-acid. Either double the bank or cut the load. For how the depth you pick shapes cycle life, see our depth of discharge guide.

Checking the Tool Against Trojan's T-105 Sheet

Context

A Trojan T-105 is a 6V 225Ah flooded golf-cart battery. Trojan's data sheet prints how long it holds a constant 25A: 447 minutes, to 1.75 volts per cell. A 150W DC load at 6V draws exactly that 25A, and a DC load has no inverter in the way, so efficiency is 100%.

Calculation

Current: 150 W / 1.00 / 6 V = 25.0 A

Rate: 25.0 / 225 = 0.111

Time to full discharge: 0.5964 x 0.111^-1.1529 = 7.51 hours, or 7 hours 31 minutes

Trojan prints 447 minutes, which is 7 hours 27 minutes

At the 50% depth the tool defaults to: 7.51 x 0.50 = 3.76 hours

Interpretation

The tool lands within 1% of Trojan's own printed figure, which is the whole point of the fit: it is calibrated against tables like this one rather than against the nameplate. The 3 hours 45 minutes on screen is the half of that discharge you should actually plan to use.

Takeaway

Reach for the printed sheet for your own battery when you have it, and use the tool when you do not. To size the panels that put this bank back, use our solar panel and battery sizing calculator.

Frequently Asked Questions

Glossary

Peukert Effect

The capacity a lead-acid battery gives up falls as the discharge current rises. Trojan rates one T-105 four different ways on the same sheet: 250Ah at the 100-hour rate, 225Ah at the 20-hour rate, 207Ah at the 10-hour rate and 185Ah at the 5-hour rate. The calculator applies a correction fitted to printed points like those rather than assuming the rating arrives at any current.

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 discharge rate a battery's Ah rating is measured at. A 200Ah battery at the 20-hour rate delivers 10A for 20 hours, down to its end-of-discharge voltage. Trojan and Lifeline both state the same criterion for that endpoint: 1.75 volts per cell. Draw more than the 20-hour current and the amp-hours you actually get fall below the rating.

Planning solar charging for your lead-acid bank? The solar charge time calculator accounts for the slow absorption phase that lead-acid requires.

Lead-acid stays the cheapest bank to buy and the heaviest to live with, and half the nameplate is all you should plan to spend. The defaults above (flooded, 50% depth, 85% efficiency) are the honest starting point, and the runtime they return already has the rate loss taken out of it, so it is not the optimistic nameplate figure this page used to print. If it falls short, the answer is a bigger bank, a smaller load, or a different chemistry. For the head-to-head with lithium, read our LiFePO4 vs lead-acid runtime comparison. The deep cycle runtime calculator covers flooded, gel and golf-cart banks.

More Battery calculators

Browse all battery calculators — LiFePO4, lead-acid, AGM, and lithium-ion runtime, charge time, and capacity sizing.

Last updated: September 23, 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.

Methodology reviewed by Doc. dr. sc. Damir Topić, Assistant Professor, FERIT Osijek.