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.
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 | $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.
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.
Frequently Asked Questions
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.
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Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
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.