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Author Topic:   Calculating Engine Starting Battery Size
jimh posted 01-02-2015 01:07 PM ET (US)   Profile for jimh   Send Email to jimh  
The best method to select a battery of proper size for outboard engine starting is to adhere to the recommendations of the manufacturer.

A related consideration is the ability of the outboard engine to maintain the starting battery at full charge over the typical operating cycle of the boat. To assess if a starting battery will be maintained at full-charge by an outboard engine, one has to compute the amount of charging current available over a period of time, that is, a value of Ampere-hours of charging current. Then one derates this by a factor of less than one, because no battery is perfectly efficient at absorbing charging current and converting the energy into stored electrical energy.

When computing the charging current available, any current available must be offset by other loads connected to the battery. For example, if a charging current of 10-Amperes were available, and if other loads consumed 7-Amperes, then only a net of 3-Amperes of current is charging the battery. If the engine runs for only 0.5-hours between starts, this means that the alternator of the engine can only supply 1.5-Ampere-hours of charging current. If the battery is only 0.66-percent efficient at absorbing charging current, this means the net charge of stored electrical energy is only 1-Ampere-hour of restored charge between engine starts.

One would then assess how much energy was consumed by each engine start. That is probably more than 1-Ampere-hour. It is hard to know with precision. We can estimate that perhaps a peak current of 1,000-Amperes will flow (based on the engine maker's recommendation of a battery that can supply that much current). We can estimate that the current flows for 2-seconds. That suggests consumption of only 0.5-Ampere-hour, but I think that estimate is low. Battery ratings for Ampere-hours are very dependent on the rate of the current discharge, and at a very high rate of current discharge the Ampere-hour rating of the battery will be much lower than the specified rating.

A rule of thumb for starting batteries might be: for each normal, quick-cranking start you have to run the engine at least 20-minutes to restore the charge lost in the start, if we assume the engine will be running above idle speeds and generating significant charging current.

To assess the ability of a particularly battery to keep up with the current demanded from it, the best way is to track the terminal voltage over time. If the battery terminal voltage is trending downward over several days of typical use, then the charging system is not keeping up with the load demand.

The most important aspect of battery capacity is that it be sufficient, even under adverse conditions, to get the engine started. Once the engine is started, we can expect the battery to be recharged. Not all engines are completely self-sufficient at idle speeds in satisfying their own electrical current demands. Some engines may still be drawing current from a battery at idle speeds. Some engines will be able to supply ample excess current for battery charging, even at idle speeds. Recall that a battery only receives charging current when there is more current available than needed by the other loads.

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