Sunday, 8 January 2012

Assault and Battery (2)

Continued from Assault and Battery (1)
Batteries are designed to fulfil a specific purpose. There are two main types that we are interested in on our boat, Starter or Automotive type and the Leisure or Deep Cycle type.

The main difference between an automotive and leisure battery is the amount of instantaneous amperes that can be delivered and the number of charge and discharge cycles the battery will support. The number of cycles for an automotive battery is a few tens. (50-80)The number of cycles for a leisure battery is typically a few hundred cycles. (300-500) The deeper the discharge of the battery the fewer the cycles the battery will complete. The maximum discharge cycle on an automotive battery is up to 75% of capacity. At which point the battery will run out of cranking power. The maximum discharge cycle on a leisure battery is typically to 50% of capacity for the maximum number of cycles.

The automotive battery is the one we use to start the engine. Its internal construction is designed to provide a huge amount of electrical power that is needed to crank over an engine. These instant high energy requirements (hundreds of amperes) differ from the typical requirements needed from a leisure battery.

The leisure battery is constructed so that its power delivery can be provided over a much longer period in time but only at a few tens of amperes rather than the instantaneous hundred of amperes needed from an automotive battery.

We measure the capacity of a battery in amperes hours. If the battery is rated at 100 ampere hours. In theory the battery should be able to deliver 1 ampere for 100 hours or 100 amperes for one hour before it is considered as discharged. I said in theory, because of various technobabble reasons, it don't quite work that way!

The slower you take amps out of a battery the more amps in total you will get. The faster you take amps out of a battery the fewer amps in total you will get. This is another one of the technobabble reasons you don't need to understand. You just need to accept that it happens this way.

Often the battery capacity will be quoted at two time related discharge rates. Typically quoted at a 8 hour and 20 hour rating. The 20 hour rating is the capacity of the battery determined over 20 hours at 80° F (26.7°C). A battery rated at 100 A.H. for 20 hours means that if you divide 20 into 100 the battery can be discharged at 5 amperes continuously for 20 hours. Likewise, if the rating was at 8 hours then divide 8 into 100. This would mean that the battery could discharge 12.5 amperes for 8 hours.

The lower the discharge rate, the more in total ampere hours you will get. Now these figures are useful when selecting a capacity size (in ampere hours) for your battery bank. But you would also need to know what the average power consumption you have on your boat. Most boaters do not have any idea of the power budget for their boat. You can forget about the starter battery, it has only the one function. Your power budget is what is used over time by your electrical items on your boat.

Deep Cycle Battery bank sizing can be one of the more complex and important calculations in your electrical system design. If the battery bank is over sized, you risk not being able to keep it fully charged. If the battery bank is sized too small, you won't be able to run your intended electrical items for as long as you'd planned.

The first thing you’ll need to know is the amount of energy you’ll be consuming per day. It’s worth the time to do a careful evaluation of exactly what the amperes for each of the appliances you plan to use and for what lengths of time. Keep track of this information on an electrical load list. You’ll can then refer to this list later for sizing new or replacement components. A loads list is simply a tally of all electrical loads that will be used in the completed system. Everything from lights, to television sets, to hairdryers, to cell phone chargers should be included on your list.

Typical items include lighting, fridge, television, stereo and pumps.  All the electrical items on your boat will consume some power. Some consume power for long periods and some for short periods.  Make a list of all the items such as TV and refrigerator and do a quick estimate of the time in use and the amount of amps each item draws. Many appliances will give you their power consumption figure on a small plate attached to the item or in the instruction manual.

It is possible to monitor your power consumption using an ammeter to measure the power being drawn out of the battery bank. You can turn all the appliances off, turn each one back on in turn and and make a note the amperes being used. Now estimate the amount of use a day for each item or over a few days make a note of the actual usage and you will have a reasonable idea of the ampere hours for your boat. Remember that fridges do not run continuously but come on for a short period two or three times and hour.

There is a secondary use for the list and that is to provide a later comparison. For instance on Rosie we have started to replace the small halogen bulbs (which consumed about 2 ampere each) with LED lights. Because the lights are switched in banks of four and six bulbs we made significant savings. We found that six LED lights consumed slightly less than a single halogen bulb.

Continued in Assault and Battery (3)


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