Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. This gives less surface area, thus less "instant" power like starting batteries need.
Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf car battery is quite popular for small systems and RV's. The problem is that "golf car" refers to a size of battery (commonly called GC-2, or T-105), not the type or construction - so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up the true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
Using a deep cycle battery as a starting battery
There is generally no problem with this, providing that allowance is made for the lower cranking amps compared to a similar size starting battery. As a general rule, if you are going to use a true deep cycle battery (such as the Concorde SunXtender) also as a starting battery, it should be oversized about 20% compared to the existing or recommended starting battery group size to get the same cranking amps. That is about the same as replacing a group 24 with a group 31. With modern engines with fuel injection and electronic ignition, it generally takes much less battery power to crank and start them, so raw cranking amps is less important than it used to be. On the other hand, many cars, boats, and RV's are more heavily loaded with power sucking "appliances", such as megawatt stereo systems etc. that are more suited for deep cycle batteries. We have used the Concorde SunXtender AGM batteries in some of our vehicles with no problems.
It will not hurt a deep cycle battery to be used as a starting battery, but for the same size battery they cannot supply as much cranking amps as a regular starting battery.
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