What makes a battery "deep cycle" vs a starting battery?

Starting batteries (SLI) use thin lead plates packed closely together for maximum surface area — great for delivering high current briefly but terrible for sustained discharge. Deep cycle batteries use thicker, denser plates that withstand repeated deep discharges without warping. A starting battery can be permanently damaged by a single deep discharge to 20% SOC, while a proper deep cycle battery handles hundreds of such cycles.

How often should I equalize a flooded deep cycle battery?

The two makers this page names print different equalize voltages, so use your own battery's sheet. Trojan's T-105 datasheet prints an equalize charge of 16.20V for a 12V string, which is 8.10V on a single 6V battery, while U.S. Battery prints 2.55 plus or minus 0.05 volts per cell , which is 15.0 to 15.6V on a six-cell 12V battery. U.S. Battery also prints the interval: hold the equalize voltage for one to three hours after a normal charge, and repeat every 30 days. Equalizing reverses stratification, where the acid settles to the bottom of the cells and wears the plates unevenly. Skip it on AGM, gel and LiFePO4 batteries, because a controlled overcharge damages them.

Are 6V golf cart batteries better than 12V deep cycle batteries?

For large banks they are the usual choice, though neither maker publishes a cycle-life comparison against a 12V deep cycle battery, so treat the folklore about them lasting half again as long as folklore. What is printed is the capacity: Trojan rates the T-105 at 225Ah over 20 hours and U.S. Battery rates the US 2200 XC2 at 232Ah, both 6V, both in the GC2 case. Wire two in series and you have a 12V 225Ah bank, which is the state this calculator opens in; series raises the voltage and leaves the amp-hours alone. The trade-off is more wiring, more weight and monthly watering. Many full-time RVers run golf-cart banks, and our RV battery runtime calculator shows how they hold up over a weekend.

Deep Cycle Battery Runtime Calc — AGM & LiFePO4 DoD

6 min read

A deep cycle battery is built for sustained discharge, where a starting battery is built for short bursts. But "deep cycle" covers everything from a cheap marine/RV combo battery to a Trojan T-105 golf cart cell, and the four chemistries behave differently at the same load. This deep cycle battery runtime calculator asks for the chemistry first, then corrects the runtime for how fast you are pulling: a lead-acid bank gives back less than its label says when the load rises, and the correction is fitted to figures Trojan, Lifeline and Victron print. It opens on two T-105s in series, a 12V 225Ah bank. For marine-specific use with trolling motor thrust tables, try our marine battery runtime calculator.

Trojan T-105 amp-hour ratings at the 20-hour and 5-hour rates beside the maker-recommended depth of discharge.

Deep Cycle Battery Types Compared

Feature	Flooded (Golf Cart)	AGM Deep Cycle	Gel Deep Cycle	LiFePO4 Deep Cycle
Maker-printed DoD advice	20-50% (Trojan)	50% maximum average (Concorde)	50% or 80%, with the cycle count printed for each (Victron)	100% (Battle Born)
Printed cycle life	Trojan publishes no single figure for the T-105	1,500 cycles at 30% discharge, 600 at 50%, 400 at 80% (Victron AGM Deep Cycle)	1,800 cycles at 30% discharge, 750 at 50%, 500 at 80% (Victron Gel Deep Cycle)	3,000-5,000 deep discharge cycles (Battle Born)
Rate correction used here	n = 1.15	n = 1.15	n = 1.19	None below 1C
Maintenance	Add water monthly	None	None	None
Cost per Ah (early 2026)	$0.50-1.00	$1.50-3.00	$2.00-3.50	$3.00-6.00
Weight per Ah	~0.6 lbs	~0.6 lbs	~0.6 lbs	~0.25 lbs

One cell in that table is empty on purpose. Trojan publishes depth-of-discharge advice for the T-105 but no cycles-at-a-depth figure for that model, so none is shown; the AGM and gel rows come from Victron's own GEL and AGM datasheet, which prints a three-row cycle design life for each range, and the LiFePO4 row from Battle Born. Read the AGM and gel rows as what they are: one maker's design life for its own cells, not a figure for every battery of that type. 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. The weight-per-Ah column is our own estimate; no published source backs those figures. Check your results against your product data or a licensed professional before acting.

Trojan T-105 prints 250Ah at the 100-hour rate, 225Ah at 20 hours and 185Ah at 5 hours. — Every point is from Trojan's own T-105 datasheet: the same battery gives back 250Ah drawn slowly and 144Ah at its printed 75-amp test point.

Why a Fast Discharge Costs You Amp-Hours

A lead-acid battery gives back less total energy when you pull harder, and its maker measures exactly how much less. Trojan prints four ratings for the same T-105: 250Ah over 100 hours, 225Ah over 20 hours, 207Ah over 10 hours and 185Ah over 5 hours. Against the 20-hour rating most labels quote, the 5-hour figure is 17.8% lower.

This is the Peukert effect, named after the German scientist who quantified it in 1897. High current builds lead sulfate on the plates faster than it dissolves, and the blocked area stops taking part in the reaction.

The calculator above does not ignore this. It applies a rate correction of the form t = a x (I/C20)^-n, with a and n fitted by least squares to constant-current points those makers print: Trojan for flooded, Lifeline and Victron for AGM, Victron for gel. The fitted exponents are 1.15 for flooded, 1.15 for AGM and 1.19 for gel, and the fit reproduces the printed tables to within about 10% for flooded, 6% for AGM and 2% for gel. Lifeline separately prints a recommended Peukert constant of 1.12 for its own line. Outside the range of rates a maker prints, the result carries a live warning rather than a silent extrapolation.

LiFePO4 takes no rate correction here, because Victron measures its own nominal capacity at a 1C discharge already. It does get the nominal voltage right: Victron prints 12.8V and 1280Wh for a 100Ah block, so feeding a flat 12V would understate the pack by 6.25%.

Worked Examples

Runtime from Golf Cart Batteries in an Off-Grid System

Context

You wire four 6V 225Ah golf cart batteries in series for a 24V bank. Your evening loads average 300W and you limit discharge to 50% DoD.

Calculation

Bank: 4 x 6V = 24V, 225Ah (series does not increase Ah)

Current: 300W at 90% efficiency is 333W from the bank, which is 13.9A at 24V, or 0.062C

Runtime to empty at that rate: 14.8 hours; at 50% DoD, 7.4 hours

Interpretation

About 7 hours 24 minutes, enough for an evening with a recharge due by morning. The old identity formula gave 7.65 hours here; the rate correction takes roughly a quarter of an hour off, because 0.062C is a little faster than the 20-hour rate the 225Ah label is measured at.

Takeaway

Golf cart batteries are the cheapest deep-cycle option but require regular watering. For a maintenance-free comparison, check our AGM battery runtime calculator.

Comparing Deep Cycle Types for a Marine Setup

Context

You are choosing between a 105Ah flooded deep-cycle and a 100Ah AGM for powering fish finders, radios, and lights (60W total) on your boat. Both are 12V.

Calculation

Both draw 66.7W from the bank at 90% efficiency, which is 5.6A at 12V

Flooded 105Ah at 0.053C, 50% DoD: 8.8 hours

AGM 100Ah at 0.056C, 50% DoD: 8.7 hours

Interpretation

Seven minutes apart, which is noise. The AGM holds up a shade better because its own fitted exponent is flatter, but at this load neither chemistry is working hard. The AGM costs more and is sealed, vibration-resistant and maintenance-free, which are the real advantages on a boat.

Takeaway

Either battery handles a full day of fishing electronics. For trolling motor loads (much heavier), size a dedicated battery bank using our battery size for inverter calculator with your motor's wattage.

Frequently Asked Questions

Glossary

Peukert Effect

The drop in total delivered capacity as discharge current rises. Trojan prints the same T-105 at 225Ah over 20 hours, 207Ah over 10 hours and 185Ah over 5 hours. LiFePO4 shows almost none of it, which is why Victron measures its own nominal capacity at a 1C discharge.

Deep Cycle Battery

A battery designed to be regularly discharged to 50% or more of its capacity and recharged. Unlike starting batteries (which deliver brief high-current bursts), deep cycle batteries have thicker plates that withstand repeated deep discharges.

Flooded Lead-Acid

The oldest and cheapest deep-cycle battery type. Contains liquid electrolyte that must be periodically topped up with distilled water. Vents hydrogen gas during charging and must be installed upright in a ventilated space.

Sizing a deep cycle bank for solar storage? The solar battery bank size calculator matches your panels to the right battery capacity.

Deep cycle batteries vary wildly in quality and real-world performance. The cheapest marine/RV battery at the auto parts store is not in the same class as a Trojan T-105 or a quality LiFePO4 cell. Buy the best you can afford, respect the DoD limits, and your batteries will return the investment many times over. For a detailed look at standard flooded lead-acid chemistry, including the Peukert equation in depth, see the lead-acid runtime calculator. Our LiFePO4 vs lead-acid comparison quantifies the cost-per-cycle advantage of each chemistry.

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

Last updated: September 22, 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.