A starting battery is designed to deliver a large current for a few seconds to crank an engine, and then be recharged right away. A deep cycle battery is designed to deliver a steady, smaller current for hours and be drained and recharged many times. Starting batteries use many thin plates for surface area; deep cycle batteries use thicker plates that survive deep discharge. Using the wrong type shortens battery life quickly.
Deep cycle vs starting battery
| Starting (cranking) | Deep cycle | |
|---|---|---|
| Current | Very high for seconds | Moderate for hours |
| Discharge depth | Shallow | Deep, repeatedly |
| Plates | Many thin plates | Fewer, thicker plates |
| Rated by | CCA or MCA | Amp-hours or reserve capacity |
| Typical use | Cranking engines | Trolling motors, electronics, house loads |
For a boat
Most boats with a trolling motor or many electronics run two systems: a starting battery for the outboard and one or more deep cycle batteries for the trolling motor and accessories. A battery switch, a battery isolator or a DC-to-DC charger keeps the house or trolling batteries from draining the starting battery. Small boats with one battery often use a dual-purpose battery.
For a trolling motor
Always use deep cycle batteries for a trolling motor. A starting battery run flat every trip will lose capacity within a season. On 24- and 36-volt motors, use matched batteries of the same type and age; see 24V and 36V trolling motor batteries and how to wire a 24 volt trolling motor.
For a winch or other high-current loads
Winches draw very high current in short bursts, which suits a starting or dual-purpose battery, ideally recharged by an engine running at the same time. A deep cycle battery works if it can supply the current the winch maker specifies.
Deep cycle and starting batteries in parallel
Do not wire a deep cycle and a starting battery directly in parallel. Different types and ages charge and discharge differently, so one drags the other down. Keep them as separate banks and connect them only through a switch, isolator or DC-to-DC charger.
Lithium deep cycle batteries
LiFePO4 batteries are deep cycle batteries that tolerate deeper discharge and last far more cycles than lead-acid. Not all lithium batteries are rated for engine starting, so check before using one for cranking. See lithium vs lead-acid and how long lithium batteries last, and our Ionic 12V 300Ah LiFePO4 deep cycle battery with Bluetooth and heater ($1,599.00).
Related
See marine battery vs car battery and how to charge trolling motor batteries.
More guides: boat battery switches · how many amp hours you need for a trolling motor · LiFePO4 vs lithium-ion.
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