Boat Electrics (2)

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.

Previously I wrote about your 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. I provided a sample chart with a list of all the electrical items in our boat. Estimate the time of use for each item over a 24 hour period. Typical items include any Bilge Pump, Fresh Water Pump, Shower Pump, Macerator, Navigation lights, Tunnel Lights, Galley lights, Saloon lights, Bathroom lights, Kitchen lights, Bedroom lights, Inverter, Television, Radio, Fans, Heating Pump and Fridge. Estimate the total amps consumption for each item over a period of 24 hours.

TIP: When choosing items such as lighting LED provides a low power options. On my boat if I turned on all the internal lights - the power consumed was over 20 amps. When the lights were replaced by direct LED replacements the load was reduced to just over 2 amps.

A battery is not fit for purpose at the end of its useful life. For instance when it will not supply the needed power budget for a given length of time. It is a false economy to delay replacing batteries that are not performing very well. You will be consuming diesel to make up the charge by running the engine when not on the move.

TIP: Never mix old and new batteries in a leisure bank. An old battery will help to reduce the performance down on a new battery. 

TIP: All electrical devices have a working voltage. Try to keep all your devices on your boat at 12 volts working. 

If the voltage of any device is not 12 volts. Then you will need to use an inverter. Turning on an inverter will consume some power for the inverter itself to work. It will also consume power converting 12 volts DC  to say 230 volts AC.  

Now lets suppose that the item we want to use is rated at 250 watts at 230 volts we can use a simple calculation called ohms law.  This is a typical value for a mains flat screen television.

Ohms law uses four things which are just notional values. They are Resistance, Voltage, Amperes and Watts. If you know the values of two of the items you can calculate the other unknown values.  For instance 1 volt multiplied by 1 amp = 1 watt.  1 volt at 1 amp = 1 ohm of resistance. 

Now lets suppose that the item we want to use is rated at 250 watts at 230 volts we can use a simple calculation called ohms law.  (a typical value for a mains flat screen television) So if we divide the 250 watts by the voltage which is 250 volts we can calculate that 1.08 amps are required to power the device. However, if we are converting 12 volts to generate the same power we can calculate the amps needed at 12 volts.  This is 20.8 amps. However, the inverter will also use some power to work and this would be 1 or 2 amps at 12 volts. rounding the figure out that's about 23 amps. 

The state of charge of a battery will extend or shorten the life of a battery. The more the battery is kept at full charge the longer it will last. If a brand new battery is allowed to remain discharged for a period of just a few weeks it will fail. Under charging or a flat battery will shorten its life as the plates will quickly sulphate. (You don't need to know any more than that)

Tip: when buying new batteries some outlets have a very high turnover and so batteries will have had a short storage time on the shelf.

Tip: some retailers will dry store a battery only filling with acid at the time of sale. Ask the retailer if they dry store their batteries. 

Over charging a battery will shorten its working life and also causing gassing which will release hydrogen which is an explosive mixture. So its a good idea when working with batteries to wear some eye protection as a precaution. High temperature within a battery will shorten its life in some cases by buckling the plates or by driving water out of the acid and  the concentrated acid will damage the plates. (You don't need to know any more than that)

The state of charge of a battery will extend or shorten the life of a battery. The more the battery is kept at full charge the greater the number of cycles it will last. If a brand new battery is allowed to remain discharged for a period of just a few weeks it will fail.

Tip: Batteries have a certain number of cycles of charge and discharge that they will complete. The deeper a battery is discharged the less cycles it will last.

To be continued.