The only electrical load on a designated starting battery is the engine starter motor. Sizing a starting battery is easy: from the engine manufacturer's literature, determine the cold cranking amperage needed to reliably start the engine. Secondary house battery banks or dedicated batteries are often used for windlasses and other designated heavy loads, and sizing these banks is the same as for the main bank.
The total capacity of a house battery bank relates to two things:
- Your total electrical load drawn from that house bank, including inverter loads (but NOT loads supplied by direct AC power sources such as shore power or a gen-set ).
- The reserve battery capacity you'd like to have, which is the amount of time you can live solely off battery power before needing to recharge.
Rules of thumb when sizing house battery banks:
|Your usable battery capacity is the amount of energy available when the batteries are between 50 and 90 percent of full charge. This means that your usable capacity is about 40 percent of the total capacity. Battery life is extended dramatically if you don't discharge below the 50 percent level on a regular basis. Topping up the last 10 percent of charge usually takes too long with an engine-driven charging source since charging current drops significantly during the latter stages of charging.|
|Not all of the charging power that reaches the batteries actually gets stored as electrical energy—some is lost in the process. It's a good idea to make provisions for about 15 percent battery losses. In addition to these rules of thumb, I usually recommend sizing total battery capacity as if no renewable chargers were present. That way the more power renewables produce, the less you'll need to be concerned with a regular engine-charging routine.|
Example: If we assume an electrical load of 110 amp-hours per day and a one-day reserve capacity, sizing total battery capacity would be as follows:
|110 amp-hours per day load x 1 day of reserve capacity = 110 amp-hours of usable battery capacity needed.|
|Since 110 amp-hours of usable capacity = 40 percent of total battery capacity, therefore: 110 amp-hours divided by 0.4 = 275 amp-hours of total battery capacity before losses.|
|275 amp-hours of total capacity x 1.15 (accounts for 15 percent battery losses) = roughly 320 amp-hours of battery capacity required.|
Increasing your electrical load or the length of your reserve capacity increases the total battery capacity required. For instance, if you wanted two days between regular engine-charging cycles, you'd need to double your total battery capacity.
Before running out to buy your batteries, check to see if the total battery capacity you've calculated is suitable for your charging sources. To make the most efficient use of your alternator, total battery capacity of wet batteries should be at least four times the amperage delivered during bulk charging (when your alternator is producing the most current). Gel battery capacity can be two-to-three times the maximum alternator current. With 320 amp-hours of total capacity, you could have a 100-amp high-output alternator producing about 80 amps when it's warm. Larger amperage alternators require more battery capacity to keep charging current levels from dropping prematurely.
Although weight and space are always concerns on a sailboat, having sufficient battery capacity ensures good system performance, a reliable supply of power when you need it most, and extended battery life.
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