|Battery Technology Overview
(Information was compiled from web sources and Minn Kota's FAQ section. The following is written with electric trolling motor usage as the exclusive application for the listed battery types, and comparison focus of flooded cell vs AGM battery types.)
There are basically four different deep cycle battery technologies:
1. Standard Lead Acid
2. Lead Calcium
3. Gel Cell Batteries
4. AGM (Optima is AGM)
Standard Lead Acid:
These are deep cycle batteries which use lead plates twice as thick as automotive batteries. They are the least expensive of all deep cycle battery options. Most deep cycle batteries will hold their charge about 6 months and have a limit of 300 to 500 charge/discharge cycles.
The most expensive deep cycle battery costing around $400/battery! Lead calcium plates dramatically increase the battery life. The design is sealed but gasses will still vent. They are less maintenance with low water consumption than lead acid, less thermal corrosion, and can accept higher charging voltages without damage. They have up to 12 month storage and a limit of 500 to 700 charge/discharge cycles.
Their major drawback is excessive cost and poor performance in a hot climate as water consumption changes to a high rate..
This design basically suspends the electrolyte into a silica gel compound which helps eliminate spilling. Advantages are no maintenance, totally spill proof, greater than 12 month charge storage, and 600 to 1000 charge/discharge cycles.
Major drawbacks to gel cell batteries are they are more easily damaged if overcharged, requires a special charge capable of delivering a lower regulated voltage to charge these batteries.
AGM (Absorbed Glass Mat) batteries have two variations:
1. Cylindrical Cells
2. Conventional Cells
AGM batteries use a fiberglass like material which keeps the acids/electrolyte in suspension between the lead plate. They are designed as the cylindrical variety like the Optima, or as a conventional looking battery. There is little difference in specifications between AGM batteries which are cylindrical or conventional construction.
Cylindrical designs offer improved vibration resistance. "Conventional" AGM batteries are constructed with parallel plates like a standard marine battery, but does use the Absorbed Glass Mat between the plates. From what I understand, conventional AGM designs do not provide the vibration resistance of their cylindrical counterparts. Vibration resistance is a major benefit for applications that encounter constant high speed travel over rough waters.
AGM batteries are similar to gel cells in terms of overall benefits. The primary benefits of AGM batteries are that they are maintenance free, provide highest resistance to shock and vibration, greater than 12 month charge storage life, and have 600-1000 charge/discharge cycles. Other advantages is that AGM batteries charge more efficiently, that is it takes less current to bring the batteries up to charge. AGM and Gel Cell batteries are more efficient in that they can dispense a charge at a higher rate than standard marine batteries (flooded cell batteries). Flooded cell batteries can not deliver more than 25% of their AmpHour capacity without affecting available capacity.
The primary disadvantage of AGM batteries is that they are prone to thermal runaway at high temperatures causing the battery to be destroyed. This occurs because an electro chemical reaction takes place when overheated when being charged. The cause may be too high of a charging voltage, high ambient temperature, or a combination of both. Exceeding 14 volts (or less under higher temperatures) will result in a catastrophic failure of the battery. AGM batteries are also very expensive, nearly 3 times the cost over their flooded cell counterparts.
From what I can tell, for trolling motor batteries, it would appear that the most important rating of a battery is it's AmpHour Rating. AmpHours are a unit of measure for a batteries total electrical storage capacity. Basically this rating looks at the battery's storage capacity and gets its number by multiplying the current draw by the time it takes to drain the battery at that rate. 5 amp draw x 20 hours = 100 AmpHour battery.
Marine Cranking Amps are short burst ratings and does not provide an indication of what the total storage capacity of the battery is. The only information here is how many amps the battery can deliver for 30 seconds in 32 degrees (maintaining at least 1.2 volts per cell or 7.2 volts for a 12 volt battery). The battery may be able to deliver high current for short durations, but will benefit from a recharge after delivering that capacity. The AmpHour Rating will more accurately indicate the battery capacity for prolonged current draw that occurs with trolling motor usage. This rating is most useful when the battery is used as a cranking battery to start the boat motor.
Are AGM Batteries Worth The Expense?
Looking at the largest capacity Optima, the Optima D31M with a 75 AmpHour rating and a price of $185.00. Compare this to the Minn Kota MK-18 Excide lead acid marine battery with an Amp Hour rating of 95 AmpHours and a price of $65.00. In terms of AmpHour ratings, the Minn Kota MK-18 battery will offer two more hours of life before discharge over the Optima D31M when drawing 10 continuous amps, based on single battery usage.
Other than vibration resistance and more charge/discharge cycles available on the Optima, I don't see how the $370.00 for two Optima D31M (at 59 lbs each) provide a significant performance advantage over the Minn Kota battery ( 55 lbs) costing $120-$125.00 for the pair.
Email me if I am wrong, but I see the benefit of the Optima is for high current draw trolling motors covering large distances where the Minn Kota or other flooded battery types will start to tap into their reserve capacity. Because of the Optima's sensitivity of thermal runaway (which might occur under heavy loads at high temperatures) any potential benefits of longer life is not necessarily guaranteed with the Optima (or any battery).
Standard lead acid marine batteries will deliver on an average three years of use. This seems to be the consensus amongst people I have talked with. At $370.00 for two Optima D31M batteries, that amount would buy 6 standard marine group size 27 batteries at about $60.00 for each battery . If you sum up the capacities of purchasing standard marine batteries every three years and a total cost of $360.00 over this time (not including inflation) the three sets of standard batteries would deliver a combined total of appx. 9 years of service, and a combined total of 900-1500 charge/discharge cycles. The Optima would offer 4 to 6 years of service and 600-1000 charge/discharge cycles. The standard lead acid batteries will also deliver up to three hours longer life per charge/discharge based on 10 amps over AGM batteries including the Optima.
It may be difficult to drop $370 for a 24V trolling motor, but the AGM may have some performance benefits under heavy loads. If money is no object I would say go for it but make sure your charging system is sound. When a battery does fail, realize that it will cost about $185 to replace each battery.
Other than the vibration resistance benefits of the cylindrical AGM type batteries, and ability to deliver a higher charge than standard marine batteries, in most users situations these benefits may not be needed and wouldn't outweigh the high cost for AGM batteries as most trolling motors are not run continuously or at a high rate. In addition, because of thermal runaway vulnerabilities of AGM batteries, potential risk to the alternator in cranking applications, plus sensitivity to improper charging damage as with other battery types, the long term payoff may not be realized.