Batteries don't die, their owners kill them.
For our purposes out on the road, there are three types of batteries available with which to work: cranking, deep cycle and dual purpose. And regardless of the type, most people are experts at killing them.
Cranking (or starting) – used to start the engine. They are designed with lots of thin lead plates which provide more surface area for the batteries to provide those cranking amps for a short burst to turn the starting motor. These are measured in cranking amps (CA), marine cranking amps (MCA) or cold-cranking amps (CCA).
Deep Cycle – Designed to discharge power at a slower rate for an extended period of time. They have fewer, yet thicker lead plates which allow them to be discharged more deeply without damaging the plates. These are measured in Amp hours (Ah) and reserve capacity (RC). The Ah is the number of amps per hour which can be drawn for a specified hourly rate (usually over 20 hours) before the battery dies, and the RC is the number of minutes the battery can carry a specific load (usually measured at a 25 amp draw) before the voltage drops below a certain useful level. A typical deep cycle battery will have three times the RC of a cranking battery.
Dual Purpose – a hybrid battery that is somewhere in between cranking and deep cycle, which has lead plates thicker than cranking batteries yet thinner than deep cycle batteries. These are usually marketed as “marine deep cycle” batteries, or as “truck” batteries. Internally they are the same, with the only difference being the terminals. Hybrid batteries aren’t the ideal solution for starting or for deep cycling, but they are ideal for certain situations, like the starting of a small motor on a boat and running the low amp draws of a trolling motor, running lights or small inverter loads, and will have enough capacity to hold up until they are recharged at the end of the day on the water. When used in battery banks they will also start a truck, and can be used for the same low amp draws of small inverter loads. A microwave oven is not a small inverter load.
The key to not killing your batteries is to use the proper battery for the situation, to use the battery for the purpose for which it was designed. Use a starting battery for starting the engine and for handling the electrical needs of the vehicle itself, and use a separate set of deep cycle batteries (a house battery bank) for everything other than the vehicle needs: inverter loads like computer, TV, lights, microwave oven, and for the 12-volt loads like a roof vent fans and the 12-volt appliances that people keep plugging into their cigarette lighter socket and running off their cranking battery. If your wattage and amp hour needs are modest, then a dual purpose hybrid battery, or a bank of them, will work well.
Your deep cycle batteries should never be discharged more than 50 percent. The shallower the discharge between full recharges, the longer the batteries will last. The more times you go down below 50 percent, the shorter the lifespan of the batteries. Whenever you start charging the batteries you should full recharge them before removing the charging current. One of the worse things you can do is run the batteries down to the point where lights dim, fans slow down, and the inverter starts screaming, because you’ve now discharged them too deeply, and then you compound things when you crank the engine to recharge them but only run the engine for 15 or 30 minutes, thereby undercharging them. If you do this over and over, it’s known as chronically undercharging the batteries.
If you draw 100 amp hours out of a battery bank, and then crank the engine (or a generator, or use shore power and a battery charger) to recharge them, the alternator may be putting amps back into the battery bank at a rate of 30 amps. Do that for an hour and it’ll put 30 amp hours into the bank. Do it for 15 minutes and you’ve put 7.5 amp hours into the battery bank. Clearly, 15 minutes ain’t gonna cut it to replace 100 amp hours. Neither will 4 or 5 hours, actually. As the battery begins to fill up, the amount of current (amps) the battery can take in drops, so the last ten or fifteen percent of charge can take several hours all by itself. If you fail to give the batteries that ten for fifteen percent, or undercharge them, the batteries will develop sulfation. Sulfation occurs when a lead acid battery is deprived of a full charge. Lead acid must periodically be charged 14–16 hours to attain full saturation charge.
During use, small lead sulfate crystals form on the lead plates of the batteries as a result of the chemical reactions to create the electricity. During prolonged charge deprivation, if you go too long between fully recharging the batteries, the lead sulfate converts to a stable crystalline that deposits on the negative plates, encrusting them. This leads to the development of large crystals, which reduce the battery’s active material that is responsible for high capacity and low resistance, as the electrical charge cannot pass through this crust of sulfate. Sulfation also lowers charge acceptance so charging will take even longer because of the increased internal resistance. When you notice that your batteries don’t seem to last as long as they used to and that you have to recharge them more often, it’s because they are sulfated. Sulfation is a normal consequence of the electrochemical process of storing and retrieving the energy of a battery, but there is no point in speeding that process up.
You can reverse some of the sulfation of a battery by what is called an “equilazation charge” where you overcharge the battery with a voltage that is higher than normal, but it must be done according the battery manufacturer’s procedures and by using a controlled voltage charger. This is done once a week, once a month, or one a year depending on the battery type and the way the battery is used. Four-stage “smart” chargers typically have the bulk, absorption, float and equalization stages. You cannot equalize your batteries with most chargers found at automotive stores.
The best way to know how many amps you’ve drawn out of your batteries, and how many you’ve put back in when recharging them, is to use a battery monitor. I prefer the Xantrex battery monitor for its simplicity and functionality, but Trimetric and others are available, as well. A battery monitor removes the guesswork by keeping track of all amps in, all amps out, how fast the amps move in and out, and will tell you at a glance the state of charge of the batteries, and the amp hour capacity. It’s a fuel gauge for your batteries. It is also equipped with an internal programmable alarm relay, to automatically fire up a generator when needed, or to turn off devices when the battery voltage drops below (or goes above) certain programmable boundaries (which can prevent you from discharging more than 50 percent). A good battery monitor isn’t cheap ($200 to $250 online), but you don’t want to skimp on a monitor. My motto is: never skimp or tires or batteries. And that includes the battery monitor.
Batteries that should last 4-6 years will need to be replaced in 2-3 years unless you monitor and care for them. So, if you discharge too deeply and don’t fully recharge when charging them, if you use the wrong battery type for your needs, you are promoting battery sulfation, and just killing your batteries.