Monday, 7 September 2015

Boat Electrics. (1)

Over the last few weeks I have been having a conversation with various people [on-line] about different battery issues. There is an expectation that the uninitiated people have and there is the reality that electrically aware people have. So I thought I could provide a few hints and tips towards understanding about wear and tear as well as the care of batteries used in a leisure bank on a boat.

A typical 12 volt battery consists of six cells. The cells can each hold a charge of up to 2.11 volts. By linking the cells together in series inside the battery, we can obtain a larger output voltage. Often when a battery fails it is just a single cell that has failed. A multi stage or intelligent battery charger should be used from time to time. What this does is equalise the voltage in each cell.  The three stages are Bulk, Absorption and Float. It brings up any cell that is low to match its cell mates inside the battery. Often its called an equalising charge. (You don't need to know any more than that)

A fully charged (wet cell) battery will give a voltage across the battery terminals measured with a volt meter of 12.7 volts. However, when charging a battery we will often use a charging voltage of about 14.4 volts. When the battery is removed from the charger the battery will display an elevated charging voltage for several hours. After about 12 hours the voltage will settle back to its quiescent state of 12.7 volts. (You don't need to know any more than that)

Batteries come in different constructions and types.  Some batteries are constructed to give a lot of amps for a short period in time. Typical of this type is a starter battery. It delivers a huge amount of power to turn over the engine. Think in 100's of amps. The other type is a leisure or traction battery which is constructed to supply less amps but for much longer periods and to survive a larger number of  deep discharge cycles than a starter battery. (You don't need to know any more than that)

Leisure battery construction come in several different groupings. Wet cell and zero  maintenance. Wet cell batteries are the cheapest option and require periodic topping up of the cell fluids. Zero Maintenance require little or no attention. My own personal preference is for wet cell batteries as I can check each cell for liquid levels and the first sign of a suspect battery is low fluid within a cell. (You don't need to know any more than that)

When we charge a battery we are putting power into the battery. The battery converts the electrical power into a chemical change inside the battery. When we take power out of a battery the chemical change is then reversed to supply the electrical  power.  (You don't need to know any more than that)

Batteries will not last forever, no matter how well you look after them. From the moment the battery acid is first poured into a battery - the deterioration in performance begins. (You don't need to know any more than that)

The correct size of battery bank needs to be calculated. The battery bank size depending on your requirements. I wanted our battery bank to provide enough power for two typical days operation without needing to run the boat engine or to plug into a shore line.  

You need to calculate the size of your boats power budget. Once you have the power budget figure. Only then can you then decide how big your leisure battery bank will need to be.  
TIP: All electrical devices have a working voltage. Try to keep all your devices on your boat at 12 volts working. 
In this instance an estimated 107 ah are required each day.  Over 48 hours an estimated 214 ampere hours would be required.  In this case I settled on 4 110ah batteries in the leisure bank.  I had a second power source available which would put some guestimated charge into the battery bank (Solar Panels) which was not used in calculating the power budget. 

In my case I decided that a cut off point for battery voltage at 12.40 volts. This is when about 25% of the available battery capacity would have been used leaving 75% available. This has proved to be quite workable. As on sunny days the solar panels often add considerably more that the guestimated 20ah a day.

To be continued.

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