Optima AGM battery
Let your batteries go flat and unable to start your engine and you'll be dead in the water. Batteries tend to be forgotten about until we need them and by that time it may be too late and you could find yourself calling for a tow.
Batteries are expensive so it pays to look after them, in many cases batteries prematurely fail and require replacement more often than they should because they are not maintained correctly. It is helpful to think about how a boat's batteries are used. On a car the battery basically fulfills one function and that is to start the engine. With the car engine running the alternator takes over and powers all the electrical circuits until the engine stops. On a boat batteries are asked to work in a different way, sure they still start the engines but then when we get to that picnic spot or we drop the anchor in some quiet cove they have to supply all the electrical needs until we start the engine again. This puts quite a strain on the battery bank and one small battery as fitted to your car would soon go flat if this was all that was available on the average boat. Most boats will have at least two batteries, often one which is reserved for starting the engine and another for house loads; lights, stereo, fridge etc.
The situation is even worse with a sailboat as often times the boat is sailing without the motor running so the chances to recharge the batteries are even less. Batteries that are routinely undercharged will fail prematurely and conversely it is possible to overcharge batteries and this will also lead to their swift demise. One of the best ways to keep batteries in top condition is to connect up a battery charger when the boat is left unattended. If the vessel is kept in a slip or at a dock where power is available then this is easy. Do not be tempted to use a automotive battery charger however as these may be OK for short term use but will overcharge batteries if left connected for long periods. Far better is a proper marine battery charger which are designed to be left permanently connected to the batteries and will maintain them in tip top condition. Many marine battery chargers allow some degree of user interface because the different types of batteries require somewhat differing charging regimes but this is something that once set requires no further input unless one type of battery is substituted for another. On a boat that is trailed it might be worth considering fitting a marine battery charger that can be plugged in when the boat is stored for longer periods, this will ensure that the battery remains in tip top condition and will perform well when you next take the boat out.
If plugging in the boat is not an option then other methods are worth considering such as installing a wind generator or solar panel. Another possibility is removing the battery from time to time, taking it home and giving it a proper charge before taking it back next time you visit the boat.
It is worth mentioning that there are three types of battery that you can buy. This has nothing to do with size but is how the battery itself is manufactured. The first of these is the lead acid wet cell and these are the cheapest and also the most common. The next are gel batteries which uses a jelly like substance in place of a liquid electrolyte and the final common type is an AGM (absorbed glass mat) which gets its name from the acid which is held in a fiberglass mat between the plates of the cell. Both the gel and AGM batteries are pretty much maintenance free and do not require topping up of the cells as is the case with the traditional lead acid. However this convenience comes at a price with a good AGM costing in many cases three time the price of more traditional types. On boats that have two or more batteries in a bank it is sensible to change these as a set even if one appears still to be in good condition. As batteries age their ability to accept a charge changes and they loose some of their potential to supply their full rated output, if a new battery is added into the mix the older batteries will tend to drag down the newer battery and you could find yourself changing it sooner than you had planned.
Batteries should also be kept clean and dry, batteries that sit in a damp bilge will all suffer and in some cases this can lead to high rates of self discharging for no apparent reason. Also it is USCG requirement that batteries be properly restrained and can't come loose when the weather cuts up rough. Electrical connections to the terminal posts should be well made, secure with no corrosion. Sometimes you will see a white deposit on battery posts but this can easily be cleaned off with a baking soda solution.
Wet cell batteries need to have the electrolyte regularly checked which should be just above the level of the top of the plates when you look into the battery. Top off with distilled water if the levels are low and always exercise caution when working around batteries. Remove jewelry such as watches and metal bracelets that could cause an electrical short and wear goggles or other eye protection to avoid splashing acid into your eyes. Batteries give off hydrogen as they charge so make sure all naked flames are extinguished and never smoke near a battery as an explosion could result.
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