There is a lot out there to know about batteries and how to keep them healty and happy for a long time. Here are some random pointers: Slower discharging rates are more efficiant. That means you would be better off having two batteries hooked parallel for your electrical subsystem rather than having two separate batteries. Its easier on your batteries and gives you more power. True deep cycle batteries have solid lead plates in them. Most marine batteries are a hybrid starting/deep cycle type battery using a course lead sponge for plates. Batteries can have several types of electrolite configurations. There is the standard flooded, gelled, and absorbed glass mat or "AGM". In my opinion, gelled batteries should be avoided for boat use unless you are equipped to charge them properly, and since AGM batteries cost 2-3 times more than flooded, you can replace them once and still come out ahead. And, the only real diffrence between battery brands is the way the plates are constructed, better batteries will have better plates that are less subject to damage from physical shock and vibration. True industrial deep cycle batteries should not be discharged below 20% "depth of discharge" and most "commonly found in supercenter stores everywhere" marine batteries should not be discharged below 50% capacity for maximum life. A common marine type deep cycle battery that is subjected to depth of discharge cycles of 50% will last twice as long as a battery that is discharged to 20%. Occasional drops below 50% are generally not harmful to your batteries, but repeated deep discharges are. Remaining battery capacity can be measured by its voltage or specific gravity. Since its impossible to check the specific gravity of some cells due to sealed vents, usually the only option is to do a voltage check. Flooded cell batteries voltage should be checked several hours after charging and then after a few minutes under load from something like a small light bulb. The reason for this is it takes time for the electrolyte to equalize. A normal fully charged battery will have an approximate charge of about 12.7 volts which steadily drops as it is used: 90%=12.50 80%=12.42 70%=12.32 60%=12.20 50%=12.06 40%=11.90 30%=11.75 20%=11.58 10%=11.31 00%=10.50 Deep cycle batteries are rated by Amp-hour Capacity and how much battery capacity you carry with you should be adjusted accordingly. An amp-hour is just that, one amp draw per hour. Batteries are usually rated on the 20 amp-hour standard, meaning the battery it completly discharged over a period of 20 hours down to 10.5 volts while the total amp hours are measured. The average trolling motor draw on max setting is about 1 amp per 1 pound of thrust. So if you use a trolling motor with a 40 pound thrust on max, you will use approx 40 amp-hours of battery capacity per hour of use. Since batteries are rated at max depth of discharge, and you only want to do 50% discharge, you can cut their rated capacities in half right off the bat when estimating your needs. Batteries should probably be subjected to a load test periodically to determine their remaining amp-hour capacity, say at the beginning of each season. This will give you an idea of how much life is left in them and how long you can operate on the water. The #1 cause of premature battery failure is due to failing to recharge a discharge cell within 12-24 hours. The sooner you can hook a discharged battery to a charger the better. Letting a cell self discharge and failing to recharge for several months is even more deadly. Self discharge losses can be anywhere from 1-15% per month depending on type of battery. Contrary to belief, storing a battery on a concrete floor is OK. Used to be in old times the cases were made of wood and asphalt (which is not exactly the ideal container for acid) the electrolyte would slowly leak onto the floor causing a short. Overcharging is another big killer of batteries. Gel type batteries MUST be charged a lower amperage than regular flooded batteries. Otherwise voids may form in the gel which won't heal causing a loss in battery capacity. Using a regular car battery charger can and will damage them. AGM's can take much more abuse than either a flooded or gelled cell and have none of the disadvantages of gel cells. The proper amp charging rate is designated by the manufacturer and should not be exceeded regardless of battery type. Battery charging is divided into three stages, bulk, absorbtion, and float. During the bulk stage, a charge is given to the battery at the maximum safe rate that can be accepted until the voltage gets to around 80-90% capacity. Voltage on the charger can be anywhere up to 15 volts, but the amps needs to be a set limit. Because of Ohm's law, voltage and amperage are directly related and need to be taken into consideration for the most rapid safe charge. Once the absorbtion stage commences, the voltage of the charger remains the same, but the amount of amps decreases due to internal resistance in the cells. This is where you need more volts to fully charge the battery in the least amount of time, again to Ohm's law. A good way to think of it is that 20 amp charger will only be pushing 5 amps when the battery is nearly fully charged. The last stage is where you "float" the battery. The charging voltage is usually around 12.8 volts. Float chargers use what is called "pulse width modulation" which senses tiny drops in voltage and sends short charging cycles to the battery which range from several microseconds to seconds. Keep in mind that most low end chargers are bulk charge only and have little or no voltage regulation which are not very good for your batteries as they can overcharge them. If your battery has lost a lot of its original capacity, you can equalize them. Equalization is a controlled overchaging of the battery to remove sulphation. If your charger has this feature, you put it into that mode once the battery has been fully charged and check the specific gravity of the cells once per hour. When the specific gravity stops changing, the battery has equalized and you get some of the amp-hour capacity back.