Deep Cycle Vs Starter Battery: What's The Difference?

There's more to automotive and marine batteries than just delivering electricity to what they're bolted up to. Well, actually, that does pretty much sum them up, but how they deliver that juice varies quite a bit, depending on their application.

You can't just grab any battery that fits and always expect it to work. Besides ensuring it's the appropriate size and provides the proper amount of cold cranking amps (which surprisingly isn't a euphemism), you must determine whether it's meant for starting a car — a starter battery — or designed to provide sustained power over longer periods, which is what a deep cycle battery does.

No, the "deep cycle" name doesn't come from riding a bicycle below sea level; it's a form of battery that's geared more towards marine, RV, and, heck, even overlanding use. But only as a method of providing power to accessories such as lights, a stereo, and other things that an alternator isn't powering. Let's take a closer look at both batteries, including their positives and negatives.

A starter battery is intended to provide a quick, hearty jolt of electricity to spin the starter motor, which then spins the engine to fire it up. From there, any accessories such as lights, the stereo, and the climate system get their juice from the alternator, which also slowly sends electricity back to the battery so that it may continue to jolt for a long while (assuming it's rated positively by Consumer reports).

Starter batteries provide high cold cranking amps (CCA), which is the number of amperes they can deliver at 0 degrees Fahrenheit for 30 seconds while holding at least 7.2 volts. The higher the CCA, the stronger the cranking power. Inside, starter batteries' thin plates allow for maximum current flow, hence the quick burst of energy instead of sustained output. This is why you may have killed your battery or come close to doing so after having your stereo, lights, or HVAC system on for a while without running the engine.

In comparison, deep cycle batteries are measured in amp-hours (Ah) rather than CCA. Their use is focused on providing a steady electric output for things like trolling motors on a boat, RV or boat entertainment amenities, an overland build's refrigerator and/or lights, and more. Unlike starter batteries, deep cycle batteries are designed to sustain these loads over extended periods before recharging. Typically, they're designed to last a long time using elements like higher-density paste, thicker plates, and different grid compositions, with compositions hinging on battery type. Still, it's important to read up on any model's specs before use.

Utilizing a starter battery when you should have a deep cycle battery will not be effective and will shorten its life. A deep cycle will provide insufficient CCA in a starting application and not play nicely with modern cars' battery management systems, activating fault codes. However, there is such a thing as a dual-purpose battery, which is primarily used in smaller marine and camper applications (the latter with appropriate wiring from under the hood). Dual-purpose batteries are for when someone wants a single battery and space is at a premium. But, as the saying goes, "jack of all trades, master of none." Dual-purpose batteries can't lay down the cranks like a starter battery, and they also can't provide as much extended juice as a full-on deep cycle unit.

You can run both a starter battery and a deep cycle battery; just ensure one isn't doing the job of the other with their respective wiring. If packaging is a concern in a trail-ready rig, lithium iron phosphate (LiFePO4) deep cycle batteries are always smaller and lighter than their lead-acid and absorbent glass mat (AGM) counterparts. Most LiFePO4 deep cycle batteries have other benefits over these alternatives, too, such as built-in battery management systems that protect them from short-circuiting, overheating, overcharging, or over-discharging. 

The difference between deep cycle and starter batteries ultimately lies in how they dispense power and, thus, their intended purposes. The former is for a longer-term draw, whereas the latter simply provides enough quick juice to fire up an engine.