Battery Types and Deep Cycle Batteries


    Courtesy of Melex Electrovehicles

    Starting Batteries

    Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead “sponge”, similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).

    Deep Cycle Batteries

    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 specialise 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 brands with thin plates up to the true deep cycle brands, such as Trojan, US etc. In general, you get what you pay for.

    Marine Batteries

    Marine batteries are usually actually a “hybrid”, and fall between the starting and deep-cycle batteries. 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%. Starting batteries are usually rated at “CCA”, or cold cranking amps, or “MCA”, Marine cranking amps – the same as “CA”. 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 overused. CA and MCA ratings are at 32 degrees F, while CCA is at zero degree F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open – not good for the warranty!

    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 Trojan) 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.

    Battery Construction Materials

    Nearly all large rechargeable batteries in common use are Lead-Acid type. (There are some NiCads in use, but for most purposes the very high initial expense, and the high expense of disposal, does not justify them). The acid is typically 30% Sulfuric acid and 70% water at full charge. NiFe (Nickel-Iron) batteries are also available – these have a very long life, but rather poor efficiency (60-70%) and the voltages are different, making it more difficult to match up with standard 12v/24/48v systems and inverters. The biggest problem with NiFe batteries is that you may have to put in 100 watts to get 70 watts of charge – they are much less efficient than Lead-Acid. What you save on batteries you will have to make up for by buying a larger solar panel system. NiCads are also inefficient – typically around 65% – and very expensive. However, NiCads can be frozen without damage, so are sometimes used in areas where the temperatures may fall below -50 degrees F. Most AGM batteries will also survive freezing with no problems, even though the output when frozen will be little or nothing.

    Industrial Deep Cycle Batteries

    Sometimes called “fork lift”, “traction” or “stationary” batteries, are used where power is needed over a longer period of time, and are designed to be “deep cycled”, or discharged down as low as 20% of full charge (80% DOD, or Depth of Discharge). These are often called traction batteries because of their widespread use in forklifts, golf carts, and floor sweepers (from which we get the “GC” and “FS” series of battery sizes). Deep cycle batteries have much thicker plates than automotive batteries.

    Plate Thickness

    Plate thickness (of the Positive plate) matters because of a factor called “positive grid corrosion“. This ranks among the top 3 reasons for battery failure. The positive (+) plate is what gets eaten away gradually over time, so eventually there is nothing left – it all falls to the bottom as sediment. Thicker plates are directly related to longer life, so other things being equal, the battery with the thickest plates will last the longest.
    Automotive batteries typically have plates about .040″ (40/1000″) thick, while forklift batteries may have plates more than 1/4″ (.265″ for example in the Rolls-Surrette) thick – almost 7 times as thick as auto batteries. The typical golf cart will have plates that are around .07 to .11″ thick. The Concorde AGM’s are .115″, The Rolls-Surrette L-16 type (CH460) is .150″, and the US Battery and Trojan L-16 types are .090″.
    Most industrial deep-cycle batteries use Lead-Antimony plates rather than the Lead-Calcium used in AGM or gelled deep-cycle batteries. The Antimony increases plate life and strength, but increases gassing and water loss. This is why most industrial and golf cart batteries have to be checked often for water level if you do not have Water Miser or Hydrocaps. The self discharge of batteries with Lead-Antimony plates can be high – as much as 1% per day on an older battery. A new AGM typically self-discharges at about 1-2% per month, while an old one may be as much as 2% per week.

    Sealed Batteries

    Sealed batteries are made with vents that (usually) cannot be removed. The so-called Maintenance Free batteries are also sealed, but are not usually leak proof. Sealed batteries are not totally sealed, as they must allow gas to vent during charging. If overcharged too many times, some of these batteries can lose enough water that they will die before their time. Most smaller deep cycle batteries (including AGM) use Lead-Calcium plates for increased life, while most industrial and forklift batteries use Lead-Antimony for greater plate strength.
    A few industrial batteries have special caps that convert the Hydrogen and Oxygen back into water, reducing water loss by up to 95%. The popular “HydroCaps” and Water Miser Caps for flooded batteries do the same job for conventional (”wet”), golf cart, and fork-lift batteries to varying degrees. Lead-Antimony batteries have a much higher self-discharge rate (2-10% per week) than Lead or Lead-Calcium (1-5% per month), but the Antimony improves the mechanical strength of the plates, which is an important factor in electric vehicles. They are generally used where they are under constant or very frequent charge/discharge cycles, such as fork lifts and floor sweepers. The Antimony increases plate life at the expense of higher self discharge. If left for long periods unused, these should be trickle charged to avoid damage from sulfation – but this applies to ANY battery.
    As in all things, there are trade offs. The Lead-Antimony types have a very long lifespan, but higher self discharge rates.

    Battery Size Codes

    Batteries come in all different sizes. Many have “group” sizes, which is based upon the physical size and terminal placement. It is NOT a measure of battery capacity. Typical BCI codes are group U1, 24, 27, and 31. Industrial batteries are usually designated by a part number such as “FS” for floor sweeper, or “GC” for golf cart. Many batteries follow no particular code, and are just manufacturers part numbers. Other standard size codes are 4D & 8D, large industrial batteries, commonly used in solar electric systems.

    Some common battery size codes used are: (ratings are approximate)


    34 to 40 Amp hours

    12 volts

    Group 24

    70-85 Amp hours

    12 volts

    Group 27

    85-105 Amp hours

    12 volts

    Group 31

    95-125 Amp hours

    12 volts


    180-215 Amp hours

    12 volts


    225-255 Amp hours

    12 volts

    Golf cart & T-105

    180 to 220 Amp hours

    6 volts


    340 to 415 Amp hours

    6 volts

    Gelled batteries, or “Gel Cells” contain acid that has been “gelled” by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. 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 (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive 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.Gelled Electrolyte

    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.

    AGM, or Absorbed Glass Mat Batteries

    A newer type of sealed battery uses “Absorbed Glass Mats”, or AGM between the plates. This is a very fine fibre Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse. Some makes available internationally are Concorde, Lifeline, Fullriver, Panasonic, Optima, and US AGM batteries. These are also called “starved electrolyte”, as the mat is about 95% saturated rather than fully soaked. That also means that they will not leak acid even if broken.
    AGM batteries have several advantages over both gelled and flooded, at about the same cost as gelled:
    Since all the electrolyte (acid) is contained in the glass mats, they cannot spill, even if broken. This also means that since they are non-hazardous, the shipping costs are lower. In addition, since there is no liquid to freeze and expand, they are practically immune from freezing damage.
    Nearly all AGM batteries are “recombinant” – what that means is that the Oxygen and Hydrogen recombine INSIDE the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombining is typically 99+% efficient, so almost no water is lost.
    The charging voltages are the same as for any standard battery – no need for any special adjustments or problems with incompatible chargers or charge controls. And, since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits.
    AGM’s have a very low self-discharge – from 1% to 3% per month is usual. This means that they can sit in storage for much longer periods without charging than standard batteries. The Concorde batteries can be almost fully recharged (95% or better) even after 30 days of being totally discharged.
    AGM’s do not have any liquid to spill, and even under severe overcharge conditions hydrogen emission is far below the 4% max specified for aircraft and enclosed spaces. The plates in AGM’s are tightly packed and rigidly mounted, and will withstand shock and vibration better than any standard battery.
    Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. AGM’s will cost 2 to 3 times as much as flooded batteries of the same capacity. In many installations, where the batteries are set in an area where you don’t have to worry about fumes or leakage, a standard or industrial deep cycle is a better economic choice. AGM batteries main advantages are no maintenance, completely sealed against fumes, Hydrogen, or leakage, non-spilling even if they are broken, and can survive most freezes. Not everyone needs these features.
    AGM batteries are also able to be shipped as non-hazardous cargo, unlike conventional “wet” cell batteries.

    See also….

    What is a Battery?