How is a deep cycle battery different to a car (starter) battery?

There are basically two types of lead acid batteries Starter (engine cranking) and Deep Cycle (Caravans, dual batteries). The starter battery is designed to deliver quick bursts of energy so generally has more plates in order to have a larger surface area that provides a high electric current for a short period of time. The plates are thinner and have different material composition. The deep cycle battery has less instant energy but greater long term energy delivery. Deep cycle batteries have thicker plates and can survive a number of discharge cycles.
We do not recommend using starter batteries for deep cycle applications because the plates are more prone to warping and shedding active material, this will reduce the life of the battery or possibly have a catastrophic failure i.e. a sudden death due to a shorted cell.
You can also get “hybrid” batteries which fall between starting and deep cycle batteries.