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?

Equalization (controlled overcharge at 15.5-16.0V for 12V batteries) should be done every 30-90 days depending on use. It reverses stratification — where the electrolyte acid concentration settles to the bottom of the cells, causing uneven wear. Skip equalization for AGM, gel, and LiFePO4 batteries — it will damage them. Only flooded lead-acid benefits from this process.

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

For large battery banks, yes. 6V golf cart batteries (like Trojan T-105 or US Battery US 2200) have thicker plates and more electrolyte per cell than 12V deep cycle batteries. Two 6V 225Ah batteries wired in series give you 12V with 225Ah — and they typically outlast a single 12V 200Ah deep cycle battery by 30-50% in cycle life. The trade-off is more wiring and a heavier, larger bank. Many full-time RVers use golf-cart banks — see how they perform with our RV battery runtime calculator .

Deep Cycle Battery Runtime Calc

4 min read

Deep cycle batteries are designed for sustained discharge — unlike starting batteries that deliver short bursts. But "deep cycle" covers everything from cheap marine/RV combo batteries to premium Trojan T-105 golf cart cells. This calculator estimates runtime for any deep cycle type, but your results depend heavily on which type you are running. For marine-specific use with trolling motor thrust tables, try our marine battery runtime calculator.

Deep cycle battery capacity at C/20 vs C/0.5 discharge rates with recommended depth of discharge.

Deep Cycle Battery Types Compared

Feature	Flooded (Golf Cart)	AGM Deep Cycle	Gel Deep Cycle	LiFePO4 Deep Cycle
Best DoD	50%	50%	50-60%	80-100%
Cycle Life at 50% DoD	600-1,000	400-800	700-1,200	5,000+
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
Peukert Effect	Significant	Moderate	Moderate	Minimal

Flooded golf cart batteries (like the Trojan T-105 at 225Ah/6V) offer the best value per kWh but require regular water checks. AGM eliminates maintenance at a higher price. LiFePO4 costs the most upfront but wins on cost-per-cycle and weight.

Deep cycle capacity varies by discharge rate from 100Ah at C/20 down to 68Ah at C/0.5. — Faster discharge rates yield less total capacity due to the Peukert effect — plan for 80-85Ah usable at typical off-grid loads.

The Peukert Effect: Why High Loads Kill Your Runtime

Lead-acid batteries have a dirty secret: they deliver less total energy at higher discharge rates. A battery rated at 225Ah when discharged over 20 hours (11.25A) might only deliver 170Ah when discharged over 5 hours (34A). That is a 25% capacity loss just from pulling power faster.

This is the Peukert effect, named after the German scientist who quantified it in 1897. The effect is caused by internal chemical reactions that cannot keep up with high current demands. Thick lead sulfate forms on the plates faster than it can dissolve, blocking further reaction.

The Peukert exponent ranges from 1.0 (no effect — ideal battery) to about 1.4 for cheap flooded lead-acid. AGM batteries are slightly better at 1.05-1.15. LiFePO4 batteries have a Peukert exponent near 1.0, meaning they deliver close to their rated capacity regardless of discharge rate.

The practical takeaway: if you are running heavy loads from lead-acid batteries, reduce the rated Ah by 15-30% in your calculations. The simple formula this calculator uses is accurate for moderate loads but optimistic for high-draw applications on lead-acid banks.

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)

Usable: 225 x 24 x 0.50 x 0.85 = 2,295 Wh

Runtime: 2,295 / 300 = 7.65 hours

Interpretation

About 7 hours and 40 minutes — enough for an evening but you will need to recharge by morning. The Peukert effect may shorten this by 10-15% at higher discharge rates.

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

Flooded at 50% DoD, 85% eff: 105 x 12 x 0.50 x 0.85 = 535 Wh → 8.9 hrs at 60W

AGM at 50% DoD, 88% eff: 100 x 12 x 0.50 x 0.88 = 528 Wh → 8.8 hrs at 60W

Interpretation

Nearly identical runtime. The AGM costs more but is sealed, vibration-resistant, and maintenance-free — real advantages on a boat where acid spills and tipping are real risks.

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 phenomenon where lead-acid batteries deliver less total energy at higher discharge rates. A 100Ah battery might deliver only 85Ah at a 20A draw rate vs the full 100Ah at a 5A rate. LiFePO4 batteries are nearly immune to this effect.

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 <a href="/solar/solar-battery-bank-size-calculator">solar battery bank size calculator</a> 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.

Last updated: March 18, 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.