The Lead acid battery is made up of a stack of lead plates, a 35% sulfuric acid and 65% water solution. This solution is called electrolyte which causes a chemical reaction that produce electrons. When you test a battery with a hydrometer you are measuring the amount of sulfuric acid in the electrolyte. If you're reading is low, that means the chemistry that makes electrons is lacking. So where did the sulfur go? It is stuck to the battery plates and when you recharge the battery the sulfur returns to the electrolyte.
The first thing you MUST keep in mind is that you are working on a device that contains a very corrosive acid, can generate highly explosive gases and 100's amps of electrical current. We must think safety when we are working around and on the batteries. Remove all jewelry, wear safety goggles and I wear my oldest clothes just incase I get some acid on them. On many boats there are two types of batteries, starting (cranking) and deep cycle (marine-golf cart). The starting battery is designed to deliver quick bursts of energy (such as starting engines) and have a greater plate count. The deep cycle battery as less instant energy but greater long-term energy delivery. Starting batteries should not be used for deep cycle applications because their construction is different it will cause them to fail sooner. The so-called Dual Purpose Battery is only a compromise between the 2 types of batteries. Battery Maintenance is an important issue. When laying up a battery it is best to select a cool well ventilated area to store in . Place the battery in a plastic tray or keep it in its battery box with the lid off . You should be able to get at the battery to regularly check its condition The battery should be clean. Using a baking soda and water mix. Be careful not to get any of the solution inside the battery or it will neutralize the acid. Serviceable battery needs to have the fluid level checked. Use only mineral free water, distilled water is best ..don't use tap water and Don't overfill battery.
The most popular way too test a battery is to measurement the specific gravity and battery voltage. To measure specific gravity, buy a temperature compensating hydrometer at an auto parts store. To measure voltage, use a digital D.C. Voltmeter. Testing the battery: You must first have the battery fully charged. The surface charge must be removed before testing. If the battery has been sitting at least several hours with nothing connected to it you may begin testing.
100% 12.7v 6.3v
75% 12.4v 6.2v
50% 12.2v 6.1v
25% 11.9v 6.0v
To charge a battery too 100% a smart charger is best this type of charger charges a battery in 3 steps The first step is bulk charging where up to 80% of the battery energy is replaced by the charger at the maximum power rating of the charger. When the battery voltage reaches 14.4 volts the absorption-charging step begins. This is where the voltage is held at a constant 14.4 volts and the current (amps) decline until the battery is 98% charged. Next comes the Float Step, this is a regulated voltage of not more than 13.4 volts and usually less than 1 amp of current. This in time will bring the battery to 100% charged or close to it. The float charge will not boil or heat batteries but will maintain the batteries at 100% readiness and prevent cycling during long term inactivity.
How Batteries DIE
Sulfating of Batteries starts when specific gravity falls below 1.2 or voltage measures less than 12.4 (12v Battery) or 6.2 (6 volt battery). Sulfating hardens the battery plates reducing and eventually destroying the ability of the battery to generate Volts and Amps. Hydrometer readings should not vary more than .05 differences between cells. You may get a good reading from all cells but 1. That bad cell will severely jeopardize the performance of the battery and place a potentially destructive load on the charging system capacity. If they have been in the boat all winter take them out and look at them carefully for signs of damage. If they froze they might have cracked. Remove the filler caps and check the fluid level. If you topped them up in the fall there should be little change. If one or more cells were low possibly exposing the plates fill the cell and let the battery sit over night on a paper towel. In the morning check the level inside the battery and examine if the paper towel is wet.
Before installing the battery(ies) Inspect the connections for discoloration caused by heat, corrosion, dirt, or worn parts. If the batteries are already connected remove the positive and negative connections from the battery before disassemble the connections and clean. Scrub the metal with a wire brush until the metal shines replace the worn parts and coat the connection with Vaseline it will protect the connection. Many battery problems are caused by dirty and loose connections.