Good Battery Practices
This is the "Do list" which describes how to avoid all the problems listed in the first section. All boat batteries are lead-acid, which describes the chemistry involved in the battery. Some are described as wet cells, some as "Maintenance free" others AGM or Gel cells. Some are starting batteries, others deep cycle and some dual purpose, which means not optimal for anything, but they have their place. They are all lead-acid batteries and differ in slightly, the newer ones differ in ways to make them non-spillable or easier to maintain. If you are the exception and have some other batter chemistry you already know it, so what are you doing reading here?
1) Use an appropriate battery charger. Having the wrong charger is probably the best all-around, least obvious way to damage batteries. I'm not going to repeat the whole section about battery chargers that comes later in this general article. But don't ignore the charger. For best performance it has to match the batteries in the system, both as to battery type and as to battery size. "Charger" includes your engine alternators. It's all a system and it has to match. I don't believe in installing the least expensive battery charger in the boating store in anything but the smallest boats that have one or two small batteries. Once you have decided that replacing batteries every year or two is not the way to go, getting long life out of a set of batteries involves a lot more considerations.
2) Fully recharge your batteries at the earliest opportunity. That is usually as soon as you get back to the dock unless you are cruising. If you are cruising it means at least once every few days to once every few weeks, your experience being the determining factor. You can get by with partially recharging the batteries day after day, but sooner or later you need to fully recharge them. There is no shortcut to fully recharging batteries. It takes time. Even deeply discharged, batteries only accept so much charging current. Charging them faster than they will comfortably accept drives the battery temperature up, drives off water and probably hastens corrosion of the positive plate structure. The "rule of thumb" for battery charger sizing is to provide one amp of charger for every ten ampere hours of battery capacity. Some batteries are better at fast charging than others, but that is another story.
Batteries accept charge current more slowly the closer they get to being fully charged. Most of the charge goes into the battery in the first few hours. This is why cruisers get by with partial charges. A full charge may take 8 to 12 hours. In fact, "full charge" is defined in an interesting way. "Full charge' is not when the charging current goes to zero, but when it falls below a certain point and does not change "appreciably" for a certain length of time. The values of amps and minutes change with the type and size of the battery or battery bank. Neglecting to fully recharging the battery bank leads to sulfation, which reduces the effective size of the batteries. See the section on battery charging for more information.
3) Check wet cells periodically and add water as needed. Wet cells have been the most common (least expensive) batteries. Although all lead-acid batteries have electrolyte consisting of sulfuric acid and water, in "wet" cells its in the natural liquid state, sloshing around inside the battery. As the battery is charged, some of the water is broken down and escapes as gas. Consequently, you must monitor wet cells to make sure they still have enough water and replace the water as it leaves the battery.
Here's how to check: Unscrew the cover to each battery cell, which is the round plastic thing of which there are six in a row down the length of the top of the battery. If there is nothing to unscrew, you have some version of a maintenance free battery. Occasionally on large batteries the cell covers will be something that snaps into place and will require you to pry them off with a pliers. Some cell covers on maintenance free batteries meet this elementary physical description so you may have to read what is printed on the top of the battery. If it says "maintenance free" on any way, "Gel," "Lead Calcium," or "AGM" do not try to open the cell covers. If you really have to work at getting the cell covers off the battery you probably should not have removed them. Maintenance free batteries can lose electrolyte too, but not as easily and it just cannot be replaced,
You may need a flashlight and a mirror to see into the hole in the top of the cell. If the liquid in the battery is low, when you look at the liquid it will reflect a flat surface or you will see the battery's guts poking through the surface of the liquid or you will see no liquid at all. Ideally, the top of the liquid is about 3/4 inch below the top of the cell opening. As it approaches this level the liquid will contact a circular structure suspended from the top of the battery and the liquid will immediately change from looking mirror flat to being distinctly curved. This is as full as the battery should be. If the battery liquid is low add nothing but distilled or de-ionized water. The terms mean the same thing and refer only to the process used to purify the water. If you add whatever water you have handy you will probably "poison" the chemical reaction in the battery to a greater or lesser degree and have to purchase a new battery sooner. If you add sea water you will kill the battery immediately. Using a small cup helps to get water into tight spots without overfilling the battery. A rubber squeeze bulb with a long rubber tail is an ideal instrument to replace water in a battery. A turkey baster will work, just not last as long. A battery contains a lot more electrolyte that you think. Buy purified water by the multiple gallons at the grocery store.
4) Exercise hygiene around acid. There is an element of hygiene in filling batteries. As you insert and remove your turkey baster, you may drip electrolyte on the top of the battery or splash some on your skin. On your skin the electrolyte will feel somewhere between an itch and a burn. It will not dissolve your flesh instantaneously, but the feeling will let you know you want to take care of it fast. Wash it off with water as soon as you can. Don't panic, but move right along. I just suck on or lick at an itching spot on my arm (if there is no visible acid) until it goes away in a few seconds. Acid tastes sour, so you know when you have it. For an otherwise invisible irritation this is probably ok. But this is probably something you should not try yourself. Keep a little baking soda around and apply to neutralize acid bites. Flushing with a lot of water works too. Battery acid is corrosive. It will eat holes in your clothes if you carry a battery that has been sitting in acid that overflowed from overcharging without neutralizing it. The holes show up the next time you wash the garment. In bad cases I wear a wrap-around acid-proof apron and nitrile or vinyl gloves. Latex gloves will fall apart as soon as they hit acid. I also wear safety glasses to guard against splashes to my eyes. Acid in the eyes calls for continuous flushing with fresh water (as from a hose) and a call to 911. Really.
A diluted sulfuric acid burn is not anywhere nearly as bad as a nitric acid burn or a dousing with a strong base like sodium hydroxide. Sodium hydroxide (lye) starts to convert the fat in your skin to soap and suddenly everything is really slippery and your skin starts to decompose and turn white. Hydrochloric acid, to finish off the three common strong acids, at least evaporates if you leave a drop somewhere. Sulfuric acid does not evaporate and will pull water out of the air, so tiny spots appear to grow.
Once you finish with the battery, clean off the top with a wet rag and some baking soda. The baking soda neutralizes the acid, which will bite you later if you leave it in place. Again, sulfuric acid does not evaporate and will pull water out of the air. So you want to neutralize all the little drops. Acid will also migrate up around the positive battery post, so that's a good place to concentrate. I'm generally not happy unless I have a light coating of bicarbonate on the top of a wet cell to take care of anything that bubbles out or comes out as mist. You wipe it off before adding water again because you do not want to get it in the battery. Rinse out your turkey baster or hydrometer in clean water so the electrolyte left on the tip will not burn a hole where you store it.
Speaking of battery hygiene, do not just slide a new battery into the accumulated liquid in the bottom of a battery box. That is lazy and disgusting. The liquid is dilute acid from battery overcharging. Take the time to neutralize it and clean out the box. I also dump a respectable amount of baking soda in a battery box before I put in a new battery. People sometimes thing this looks strange. When it comes time to change the battery again there may be liquid in the battery box again. It will be a mess but not an acid mess. How much baking soda should you keep on hand? At least several pounds. I have never tried to neutralize all the acid in one group 27 battery, but I'd bet it takes at least 10 pounds of bicarbonate.