The Boat House Battery is crucial to the reliability and functioning of just about everything on board. This covers the important task of selecting suitable batteries for use in service (house battery power) roles. Most problems arise from improper battery selection. Battery bank capacities are either too small, with resultant power shortages, or so large that the charging system cannot properly recharge them, resulting in premature battery failure due to sulfation. Initially, it is essential to list all the equipment on your boat along with power consumption ratings. It takes time to do and I am surprised at how many people have never done this audit exercise. It is like not knowing what is in your gas tanks before heading off on a long road trip.
Equipment power consumption ratings can usually be found on equipment nameplates on each item of gear or within equipment manuals. It is recommended that the ratings, that are usually expressed in watts, be converted to current in amps. To do this, divide the power by your system voltage. Calculate the current consumption for a 12-hour period while in port or anchored. The calculation makes a base assumption that the engine will not be operated, and no generator with battery charging will be operational, or any renewables used. This is the baseline calculation. While motoring, all power is being effectively supplied from your engine alternators, and when the boat house battery is charged the alternator effectively supplies all power.
Create a simple table on your laptop or whatever works for you. I have added a page for you to use with the typical power consumption, so print off the page and insert your own values and then calculate your vessel loadings.
Load Calculation Table. To calculate the total system loading for a boat house battery, multiply the total current values by the number of hours to get the amp-hour rating. If equipment uses 1 amp over 24 hours, then it consumes 24 amp-hours.
Capacity Calculation. Select the column that matches the frequency of your charging periods. The most typical scenario is one of the boat at anchor, or on a mooring and operating the engine every 12 hours to pull down refrigerator temperatures with an engine driven eutectic refrigeration compressor.
Eg. Total consumption is 120 Ah over 12 hours = 10 amps/hour
Boat House Battery Capacity De-rating. As we wish to keep our discharge capacity to 50% of nominal battery capacity, we can assume that a battery capacity of 240 amp-hours is the basic minimum level. In a perfect system, this would be a minimum requirement, but certain realities must now be introduced into the equation. The figures below typify a common system, with alternator charging and standard regulator. Maximum charge deficiency is based on the premise that boat batteries are rarely above 70% charge and cannot be fully recharged with normal regulators. There is reduced capacity due to sulfation, which is typically a minimum of 10% of capacity. The key to maintaining optimum power levels and avoiding this common and surprising set of numbers is the charging system.
Nominal Capacity 240 Ah
Maximum cycling level (50%) Deduct 120 Ah
Maximum charge deficiency (30%)Deduct 72 Ah
Lost capacity (10%) Deduct 24 Ah
Available Battery Capacity 24 Ah
Boat House Battery Amp-hour Capacity. It is important to discuss a few more relevant points regarding amp-hour capacity, as it has significant ramifications on the selection of capacity and discharge characteristics.
Fast Discharge (Peukerts Equation). The faster a battery is discharged over the nominal rating (either 10 or 20 hour rate), the less the real amp-hour capacity the battery has. This effect is defined by Peukerts Equation, which has a logarithmic characteristic. This equation is based on the high and low discharge rates and discharge times to derive the Peukert coefficient 'n'. Average values are around 1.10 to 1.20. If we discharge a 250 amp-hour battery bank, which has nominal battery discharge rates for each identical battery of 12 amps per hour at a rate of 16 amps, we will actually have approximately 10–15% less capacity. Battery discharge meters such as the E-Meter incorporate this coefficient into the monitoring and calculation process.
Slow Discharge. The slower the discharge over the nominal rate, the greater the real capacity. If we discharge our 240 amp-hour battery bank at 6 amps per hour we will actually have approximately 10–15% more capacity. The disadvantage here is that slowly discharged batteries are harder to charge if deep cycled below 50%.
Battery Load Matching. The principal aim is to match the discharge characteristics of the battery bank to that of our calculated load of 10 amps per hour over 12 hours. Assume that we have a modified charging system so that we can recharge batteries to virtually 100% of nominal capacity. The factors affecting matching are s follows.
Discharge Requirement. The nominal required battery capacity of 240 Ah has been calculated as that required to supply 10 amps per hour over 12 hours to 50% of battery capacity. In most cases, the discharge requirements are worst for the night period, and this is the 12-hour period that should be used in calculations. What is required is a battery bank with similar discharge rates as the current consumption rate. This will maximize the capacity of the battery bank with respect to the effect defined in Peukerts coefficient.
Battery Requirements. As the consumption rate is based on a 12-hour period, a battery bank that is similarly rated at the 10-hour rate is required. In practice you will not match the precise required capacity, therefore you should go to the next battery size up. This is important also as the battery will be discharged longer and faster over 12 hours, so a safety margin is required. If you choose a battery that has 240 amp-hours at the 20 hour rate, in effect, you will be installing a battery that in the calculated service has 10-15% less capacity than that stated on the label, or approximately 215 Ah, so you are below capacity. This is not the fault of the supplier, but simply a failure to correctly calculate and specify the right battery to meet system requirements.
Load Calculations. It is essential to list all equipment on board your boat along with the various power consumption ratings. Ratings can usually be found on equipment nameplates or within equipment manuals. Insert your own values into the Actual column. Calculate power used for 12 hours. To convert power (in watts) to current (in amps), simply divide the power value by your system voltage. Add up all the current figures relevant to your vessel and multiply by hours to get an average amp-hour consumption rate. Space is reserved to add in specific values. Most of these items will be on when anchored or moored, but many will not be relevant if at a marina connected to a battery charger.
Additional Load Calculations. Other basic load characteristics have to be factored in to load calculations. Add up all the current figures relevant to your vessel and multiply by expected run times to get an average amp-hour consumption rate.
Intermittent Loads. It is often hard to quantify actual real current demands with intermittent loads. My suggestion is simply to use a baseline of 6 minutes per hour, which is .1 of an hour.
Motoring Loads. Certain loads are only applicable when motoring. Loads must be subtracted from charge current values, and actually may impact on charging system efficiency at low speeds. Loads include navigation lights, refrigeration clutch, watermaker clutch and ventilation fans. The boat house battery is critical so choose it well and also your boat battery.