What is the difference between a deep cycle and starting battery?

What is a Deep Cycle Battery?

A deep cycle battery is designed to provide reliable power down to the 80% depth of discharge (DOD) level. The battery will be able to be discharged down to this level time after time with no or minimal loss of capacity. Deep cycle batteries are constructed with thicker solid lead plates which reduces the instant starting power of the battery, but enables it to handle the constant cycling.

Due to the plate design of the battery, true deep cycle batteries will normally have a much lower CCA rating than their starting counterparts. A few vendors with true deep cycle battery offerings are Vision, Crown, CSB, and NorthStar.

The overall life span of a deep cycle battery has an inverse correlation with the depth of discharge - see below: 

As you can see by the above chart, this battery will have an increased cycle life at a lower DOD. Most manufacturers recommend running your deep cycle battery setup down to the 50% DOD mark for the best bang for your buck.

What is a Starting Battery?

A starting or cranking battery is designed to deliver a large burst of power for a short period of time. The plate structure of the everyday starting battery also differs greatly from that of the deep cycle. The starting battery plates will be much thinner and a sponge like texture instead of a thick solid plate. The reason behind this is that the sponge like material is porous and gives a very large surface area for the active material to generate power.

A starting battery will only be discharged between 2 and 4% on average when the vehicle is started and have well over a 1000 cycles before the battery experiences capacity loss. The downside to the thin sponge like plate of the starting batteries is that they are not designed to withstand cycling below that 2 to 4% mark. If a starting battery is deep cycled it will begin to rapidly shed the sponge like material on its plates and loose capacity, in reality you have about 100 to 125 cycles of 50% DOD in a starting battery before it begins to deteriorate.

In Conclusion

The main difference between a starting battery and a deepcycle battery comes down the the construction of the plates. The deep cycle battery will have fewer but thicker solid lead plates that will product low instant starting power but consistant long term power. The starting battery has many more plates that are thinner and porous enabling it to produce a strong instant power for a brief period of time which rapidly diminishes the longer the battery is cycled.