"Only a rich man can afford to buy cheap batteries" the commonest disappointment is to turn the key and nothing happens, and we  often meet boat owners who are unaware they have unsuitable  batteries.

 
 

There are different types of battery;

1/Cranking or starting battery, this as the name would suggest its purpose is to supply the power to rotate the engine by electric starter motor fast enough for it to compress and ignite the fuel air mixture.

Once the engine is running the starter motor is not required, so the power the battery supplies to the starter motor is for a short time only, lots of power and in a short burst. Sometimes the engine may be reluctant to start for various reasons, so the battery has to be capable of containing sufficient reserve power to cover this event. The power used is measured as quantity delivered in a measured time.

Several variations of ratings may be applied to a battery including;

 MCA (marine cranking amps) is the commonly used rating for marine/RV starting power. It is the number of amperes (THE POWER) a lead acid battery at 32 degrees F (0 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). This lets you know how much power you have to start your boat's engine. MCA (marine cranking amps) or CA (Cranking amps), which are generally 20% higher than CCA (cold cranking amps).

CCA (cold cranking amps) is a measurement of the starting power of a battery at freezing point (zero degrees F.) under a load (ampere draw) for 30 seconds with the end voltage maintained at 1.20 volts per cell.

NOTE: Do not confuse Cold Cranking Amperage (CCA) with Marine Cranking Amperage (MCA) or Cranking Amperage (CA). MCA and CA is a higher battery rating measured at warmer temperatures.

NOTE; Starting batteries are usually rated at "CCA", or cold cranking amps, or "MCA", Marine cranking amps - the same as "CA". cranking amps.

IN SHORT; starting batteries deliver high power in short bursts.

It is achieved because a cranking/starting battery has a lot more and thinner plates to increase the plate surface area in contact with the acid. Starter batteries are not designed to be discharged completely and then recharged, their life is reduced if this should happen. On motor vehicles the alternator will cut in when the engine runs and supply plenty of power for the vehicles system and to top up the battery replacing the power lost to start the engine. Under normal conditions the battery would not be more the 20% discharged at any time.

 

2/ Service or deep cycle battery, it is good practice on well found boats to carry an additional battery or batteries that can be used to supply the electronics and domestic devices on board, this is sometimes known as the "house" battery. These do not need produce the huge burst of power the starter battery needs. They do need to be able to supply a steady flow of power for a long period of time and be discharged and recharged many times over.

However a deep cycle battery can provide a surge when needed, but nothing like the surge a starting battery can. A deep cycle battery is designed to be deeply discharged over and over again down to 80% (something that would ruin starting battery very quickly). To accomplish this, a deep cycle battery uses thicker plates. This means the battery for this use is more efficient if built specifically for this task.

Typically, a deep cycle battery will have two or three times the RC (reserve capacity) of a starting battery, but will deliver one-half or three-quarters the CCAs. In addition, a deep cycle battery can withstand several hundred total discharge/recharge cycles, while a starting battery is not designed to be totally discharged.

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 rating compared to a similar size starting battery. As a general rule, if you are going to use a true deep cycle battery also as a starting battery, it should be oversized by at least about 25% compared to the existing or recommended starting battery group size to get the same cranking amps. 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 boats are more heavily loaded with power sucking "appliances", such as electronics, pumps, televisions, fridges and lighting systems etc. that are more suited for deep cycle batteries.

NOTE; "Marine" batteries are usually actually a "hybrid", and fall between the starting and deep-cycle batteries, though a few are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. "Hybrid" types should not be discharged more than 50%. Any battery with the capacity shown in CA or MCA may not be a true deep-cycle battery. It is sometimes hard to tell, as the term deep cycle is sometimes over used. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open - not much of an option.  Unfortunately many boats are equipped with batteries brought from cut price suppliers; many are designed for automotive use, you take it back under warrantee to complain it has failed in a short time and you will find find the true cost of a cheap battery.

Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate  to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive type charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries.

Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death.  The newer AGM (absorbed glass mat) batteries have all the advantages (and then some) of gelled, with none of the disadvantages.