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 boat house 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 boat house battery 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 load.

**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 boat house 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 boat house 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 boat house 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 boat house battery bank with similar discharge rates as the current consumption rate.
This will maximize the capacity of the boat house 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.