• Sizing Your House Battery Bank

    Attachment 1112
    With the demand to keep manufacturing costs low, many manufacturers of cruisers up to forty feet don't provide an adequate house battery system. For the average daytime boater, this normally does not present much of a problem, but when you begin to spend overnights on the hook the inadequacies quickly become apparent.

    Some boaters never experience this problem because they run their generators constantly. Of course, this adds to the expense of boating in both fuel burn and maintenance, but they are willing to accept that to keep the comforts of home. Logically, there are certain times of the year when you have no choice and will have to use the air-conditioning to cool off or the heat to stave off the chill. However, there are also those days when the temperatures are more comfortable and neither heat nor air is needed.

    Sitting back with a glass of wine in hand on a fall evening listening to a Loon's cries or crawling into the rack with the hatches open listening to a barn owl sing you to sleep without the puttering of a generator, now those are the true pleasures of boating.

    So the decision is made to add more batteries. How many batteries do you need?

    Computing Power Consumption

    To answer that question you have to determine how much energy you are using. More specifically you need to determine how many amps you are using before re-charging your batteries. If you have a clamp on ammeter handy, you may be able to get a rough idea of how many amps you are using by taking a reading from the positive feed off the house battery bank. Otherwise with a little bit of work you can get a pretty accurate idea of your power consumption.

    Your energy needs are going to fluctuate depending on what you are doing. For instance, if you are cruising you will likely have radios, radar, a stereo and other assorted items consuming energy. Then while anchored you will probably have a different list of items eating away at those precious amps. At first glance you may think you use more amps while cruising since you have more equipment operating, but don't forget that the engine's alternators are busy whirring away providing power to the system. However, for our purposes, we will ignore any inputs into the system and just compute our consumption.

    Step 1

    Begin by making a list of all the DC powered items on the boat along with their power consumption. Power consumption can be listed as watts or amps and can generally be found on the piece of equipment or in the owners manual. To keep things simple any values listed as watts will be converted to amps using this formula:

    watts / volts = amps

    For example, you have four 50 watt halogen light fixtures, compute the amps.

    4 x 50 watts = 200 watts
    200 watts / 12 volts = 16.7 amps per hour

    If, for some reason, you cannot find an item's power consumption, you can use a clamp on ammeter. With the item powered off place the ammeter around the house's power supply cable and note the amp reading. Then switch the item on allow the reading to stabilize and note the new reading. The difference between the two will yield the power consumption of the item.

    When you have all the amperage values for the items listed, make a copy of the list. You will compute the power consumption for the two most common scenarios, cruising and anchored. The highest power consumption value between the two is what you will use to compute your battery capacity.

    Step 2
    Usage times are likely to vary each time you use your boat but that is okay you are looking for an average. Approximate the usage of each of the items in hours and if possible tenths of an hour. Six minutes is the equivalent of one-tenth of an hour. In this example, lets assume you use those halogen lights an average of 1.3 hours while cruising.

    1.3 hrs x 16.7 amps = 21.7 amp/hrs

    So you will note 21.7 amp/hrs on your cruising list. When you have computed the amp/hrs for each item on the list add them up to determine the total amp/hours consumed as shown below.

    Attachment 1110
    Now that you have computed the total amp/hrs used while at anchor, perform the same calculations from cruising. The situation that yields the highest value is the one you should use to determine your battery needs. In this example, we will use 100 amp/hrs since it will make the computations easier to follow.

    Battery Concepts

    Battery technology has vastly improved over the last few years from the less-expensive Flooded Wet Lead Acid, Absorbed Glass Mat AGM and the more expensive Gel Batteries. Each type is unique and offers benefits as well as drawbacks. You will have to determine the amount you want to spend and which type is best for your situation. Batteries are one of those components that you will truly get what you paid for, so I recommend sticking with more reputable manufacturers.

    House battery banks are the workhorse for the cruising boater. They are regularly discharged to levels that would destroy an automotive battery and frequently experience longer times between full charge cycles, both of which are stressful to the battery. Even though deep cycle batteries are designed with thicker plates to handle these stresses, the battery's life span is still decreased..

    Keeping in mind that a battery cycle is a complete discharge then a full re-charge, look at the Cycle Life example below for the Trojan AGM battery.

    Attachment 1114
    If you only discharged the battery 10% the battery's lifespan will be approximately 4,300 cycles and decreases slightly down to approximately 4,000 cycles at 20% discharge. At 30% discharge the cycle life has dropped to 3,000 cycles, which is a 31% decrease in the batteries lifespan. Now only discharging your battery bank 30% before recharging is a bit unreasonable unless you have a large battery bank and a small energy consumption.

    More realistic is a 50% discharge, which yields around 1,200 cycles. This is a good target for regular use, and here is why.

    Battery acceptance rate is the amount of current the battery will accept during the charging cycle. This is an important concept to understand when re-charging batteries.

    A fully discharged battery can have a battery acceptance rate of up to 100% of its rated capacity. So, for our example, 100 amp/hr battery it would accept 100 amps and in theory be completely recharged in one hour. However, this is not the case. As the state of charge increases, the battery's acceptance rate decreases. At a 50% state of charge, the battery's acceptance rate has dropped to approximately 30% of its rated capacity. Now our example battery will only accept 30 amps of charging current. When a battery reaches a state of charge between 75-80%, the acceptance rate drops sharply, and from this point, it can take several hours top off the battery.

    What does all this mean to me? If you are out on the water and want to avoid running that generator or engine any longer than you have to, plan on discharging your batteries down to 50%. Then, since it can take several hours to charge the batteries above an 80% charge plan on stopping there. You can always top them off back at the slip.

    If you are going to spend a lot of money on batteries, it will serve you well to invest in a decent battery monitor. Xantrex LinkLite is an example of one that provides functions that will enable you to properly manage your battery bank. I am not specifically promoting that particular monitor but it will give you an idea of what to look for. There are cheaper models that cost much less but really don't provide you with useful information.

    If you don't have a dedicated battery monitor, you are probably wondering, How do I know when I reach the proper percentages? This procedure is not very accurate, but it will get you close. You can use the table below and the voltmeter on your DC electrical panel.

    Attachment 1115
    Note in the table above that a 50% state of charge is the equivalent of 12.1 volts while 80% is 12.5 volts. However, the values in the chart are open circuit voltages which is the battery voltage with no load. It should be obvious that it is impractical to disconnect your battery each time and check the voltage so here is what you can do.

    While discharging, when your voltmeter reaches approximately 12.0 volts, power off all DC items and remove the cables from your battery. Allow the battery to sit idle for 30 min to an hour to stabilize. Periodically check the voltage and when it stops climbing note the value. Now re-connect the cables on the battery and check the voltage indication on your DC panel meter. Accounting for any errors on the DC panel's voltmeter, you should be able to determine a voltage value to stop discharging that will give an open circuit voltage close to 12.1 volts. In the future all you will have to do is monitor the voltage and when it reaches your determined value, begin charging the batteries.

    Now you may be thinking all you have to do is charge until the voltmeter on the panel displays 12.5 volts and you are done. However there is one problem with that. As soon as you start charging the voltmeter will read the charging voltage, not the battery voltage.

    You will need to determine the charging profile of your specific battery charger which is normally available in the datasheet. Most newer chargers closely follow charging profiles specified by the battery manufacturers. Look at this charging profile recommended by Trojan for their Flooded/Wet batteries.

    Attachment 1116
    As you can see starting at 20% state of charge, they recommend a high constant current and then at 90% the current drops dramatically. The charging voltage starts out low and at 90% levels off at 2.35 volts per cell or 14.1 volts. From 20% to 90% is considered the bulk charging stage. The battery is charging as rapidly as it can. At 90% state of charge this is known as the absorption stage. The battery's acceptance rate rapidly declines.

    Now consult the charging profile for your specific battery charger to determine the absorption stage voltage. On my Xantrex charger, the absorption stage voltage is 14.4 volts, which is slightly above what Trojan recommends but well within limits. So how would you know to stop charging the battery? When you see the charging voltage begin to level off at 14.4 volts you can stop charging. Once again, the above procedure will get you in the ball park, but for the best results, you should add a good battery monitor to your system.

    Sizing the Battery Bank

    So to manage your batteries efficiently you will be charging to 80% and discharging to 50% or only using 30% of your battery's capacity. With our boats demand of 100 amp/hrs per day, this will require 333 amp/hrs of capacity (100 amp/hrs divided by 30% equals 333 amp/hrs). Now you can see how this works, for instance, if you wanted to last two days before charging you would need 666 amp/hrs of battery capacity.

    To summarize, you need to determine your energy usage and how often you want be recharging the batteries then use a good battery monitor to manage your system and size your bank appropriately.
    Hendo78, sealady and gary peterson like this.


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