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Deep Cycle Battery Guide

How deep cycle batteries differ from starting batteries, depth of discharge and cycle life, capacity at C5, C10, C20 and C100 rates, and 6 V, 8 V and 12 V packs for golf carts, solar and marine use.

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Deep cycle batteries power equipment rather than starting engines: golf carts and utility vehicles, solar and off-grid systems, floor machines and onboard loads in boats and recreational vehicles. They are specified differently from starter batteries, and the numbers on their data sheets only make sense once the discharge rate and depth of discharge are understood. This guide covers the concepts a buyer needs to compare deep cycle batteries and to specify them correctly. Forklift traction batteries, which are a related but separate category, are covered in the forklift traction battery guide.

Deep cycle, starting and dual-purpose batteries

Trojan Battery describes the difference as follows:

Type Design intent
Deep cycle Can be deeply discharged and recharged many times during its service life; designed to power electrical equipment for long periods
Starting (automotive) Designed for brief bursts of high current; cannot withstand more than a few deep discharges before failure
Dual-purpose A compromise between the two

A starter battery used for repeated deep discharge fails early. Where one battery must both start an engine and supply loads, as on some boats and leisure vehicles, a dual-purpose battery is the compromise, and its cranking and cycling ratings should both be checked.

Depth of discharge and cycle life

Depth of discharge (DoD) is the share of the battery's rated capacity that is used before recharging. Cycle life depends heavily on it, and the relationship is not linear.

US Battery's user manual for its flooded deep cycle batteries gives an example: discharging to 80% of the 20-hour rated capacity yields about 675 cycles, while discharging to 40% yields about 1,475 cycles, more than double. US Battery recommends limiting depth of discharge to 50% of the 20-hour capacity to balance cycle life against runtime, and avoiding discharge beyond 80%, which it says causes irreversible damage.

Trojan's guidance is similar: it generally recommends discharging 20% to 50% of rated capacity for optimum life, even though its batteries can be cycled to 80%.

Manufacturers publish cycle life at a stated depth of discharge, and the figures vary widely between product lines. Trojan's 2026 product specification guide, for example, lists flooded solar lines rated at 600, 1,200 and 1,900 cycles at 50% DoD. When comparing batteries:

  • Compare cycle life at the same depth of discharge. A figure at 20% DoD is not comparable with one at 80%.
  • Check the end-of-life criterion used by the manufacturer.
  • Treat laboratory cycle life as a comparison tool, not a service life guarantee. Temperature, charging quality and maintenance all shorten or extend real life.

Capacity rates: C5, C10, C20 and C100

A battery delivers more ampere-hours when discharged slowly than when discharged quickly. Capacity is therefore always quoted with a discharge time, written as C5, C10, C20 or C100 for 5, 10, 20 or 100 hours. Trojan's published figures for its T-105, a 6 V golf cart battery, show the effect:

Discharge rate T-105 capacity (Ah)
5-hour (C5) 185
10-hour (C10) 207
20-hour (C20) 225
100-hour (C100) 250

Trojan measures these at 27 °C (80 °F) to 1.75 V per cell. The same battery can therefore be described as 185 Ah or 250 Ah depending on the rate, which is why a quotation must state the rate, and why ratings from different manufacturers must be converted to the same rate before comparison.

Typical uses of each rate:

  • C20 is the common reference for many deep cycle and starter batteries.
  • C100 figures appear in solar battery specifications, reflecting slow discharge over days; Trojan lists 100-hour capacities for its solar lines.
  • C5 is the reference for traction batteries under IEC 60254-1, and appears on golf cart and industrial data sheets.
  • Runtime in minutes at a constant current is also used. BCI rates golf cart batteries in minutes: group GC2 at 75 A and groups GC8 and GC12 at 56 A, to 1.75 V per cell at 27 °C. Trojan lists the T-105 at 447 minutes at 25 A and 115 minutes at 75 A.

Temperature also matters: capacity published at 27 °C is not directly comparable with capacity published at 25 °C, and available capacity falls in the cold.

6 V, 8 V and 12 V batteries and packs

Deep cycle batteries are built as 6 V (three-cell), 8 V (four-cell) and 12 V (six-cell) blocks and combined in series to reach the system voltage. BCI defines common golf cart case sizes:

BCI group Voltage Maximum dimensions L × W × H (mm)
GC2 6 V 264 × 183 × 277
GC2H 6 V 264 × 183 × 295
GC8 8 V 264 × 183 × 277
GC8H 8 V 264 × 183 × 295
GC12 12 V 327 × 183 × 277

Series arithmetic sets the number of blocks: six 6 V batteries make 36 V, eight 6 V or six 8 V batteries make 48 V, and four 12 V batteries also make 48 V. US Battery gives examples from the Club Car range: models such as the DS and Precedent use six GC8 8 V batteries, while the Precedent Champion uses a 48 V system with four GC12 12 V batteries.

For a replacement pack:

  • Match the configuration the vehicle or system was designed for. The block voltage, count and case size must suit the battery compartment and cabling.
  • Keep the pack uniform. Specify the same model, technology and rating for every block in the series string.
  • Record terminal type; golf cart and deep cycle batteries use a variety of post, stud and bolt-through terminals.

Applications

Application What to specify
Golf carts and utility vehicles Pack voltage, block voltage and count, BCI group, terminal type, capacity at C20 or minutes at 75 A / 56 A. See golf cart batteries
Solar and off-grid storage System voltage, capacity at the rate the system designer uses (often C20 or C100), cycle life at the design DoD, flooded or valve-regulated. See solar batteries
Marine and leisure Starting, dual-purpose or deep cycle duty; for starting, cranking ratings such as CCA or MCA; vibration and installation location. See marine batteries
Floor machines and industrial equipment Voltage, capacity rate, tray or compartment dimensions. See industrial batteries

Charging and maintenance of flooded deep cycle batteries

Deep cycle batteries only reach their rated cycle life with correct charging. Trojan and US Battery give consistent instructions for flooded batteries:

  • Water after charging. Trojan instructs users to fully charge batteries before adding water, and to add water to a discharged or partly charged battery only if the plates are exposed.
  • Equalise periodically. Trojan recommends equalising flooded batteries for 2 to 4 hours when specific gravity after charging is below 1.235 or varies between cells by more than 0.030; US Battery recommends equalising every 30 days or 30 cycles. Trojan warns that AGM and gel batteries must never be equalised.
  • Do not store discharged batteries. Trojan instructs never to store batteries unless fully charged, and explains that operating at a partial state of charge or storing discharged causes sulfation and can lead to premature failure.
  • Keep flooded batteries upright. Trojan specifies that flooded batteries must be placed upright at all times.

Valve-regulated deep cycle batteries (AGM and gel) need no watering but require charging profiles matched to their technology. The comparison of AGM with EFB starter batteries is covered in EFB vs AGM batteries.

Specifying deep cycle batteries

On a request, state the application, block voltage and quantity per pack, BCI group or dimensions, technology (flooded, AGM or gel), capacity with its rate (for example 225 Ah at C20), cycle life requirement at a stated DoD, and terminal type. Browse deep cycle batteries and send the specification through the battery requirement form.

References

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