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Author Topic:   Battery Charging and Engine Start Cycles
jimh posted 02-02-2012 09:07 AM ET (US)   Profile for jimh   Send Email to jimh  
A lead-acid storage battery is typically used to provide the electrical energy to start most larger engines. As soon as most engines start, they generate enough electrical power to provide the electrical energy they need to run, and there is typically surplus electrical energy available to use for recharging the battery. It occurs to me that a useful measure of the battery condition might be found by comparing the total running hours with the total start cycles. Many modern engines provide this information as part of their diagnostic data, available from the engine's electronic controller.

As it happens, I have some data from my modern outboard engine, and it shows the accumulated running time as about 204-hours. The number of engine starts is shown as 252. From those data we can find the average time the engine runs for each start as 48-minutes.

Modern engines also provide data about their operating speed. Again, from my engine's history I see that 64-percent of the operating time is spent at engine speeds of 400 to 1,600-RPM, and 30-percent of the operating time is spent at engine speeds of 3,000 to 4,500-RPM. We have accounted for 94-percent of the engine operating time in those two ranges.

In the lower engine speed range we can consider that the available charging current will be limited to perhaps 10-amperes. In the higher speed range we can consider the engine can supply maximum charging current, so let us say 35-amperes. Now we can approximate the amount of charging current the engine will produce per operating hour as

Charging current = 0.64 x 10-Amperes + 0.30 x 35-Amperes
Charging current =6.4-Amperes + 10.5-Amperes
Charging current = 16.9-Amperes

Since the average running time is 48-minutes, we can estimate the number of Ampere-hours of charge that the engine can provide in its typical start-run-stop cycle to be

16.9-Amperes x 48-minutes x 1-hour/60-minutes = 13.52 Ampere-Hours

As long as the engine cranking process does not consume more than 13.52-Ampere-hours of battery charge, the outboard engine will maintain the battery at full charge.

jimh posted 02-02-2012 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
Now the inquiry must turn to the engine start process. I do not have any measured data--although I plan to acquire some with new Ammeter I just bought--for the amount of current drawn during engine starting. Let us say that the engine cranking current will be 200-Amperes. (This seems like a reasonable approximation based on the size of the conductors used to deliver the battery current to the engine.)

On a modern engine that is in good tune, we can anticipate very rapid starting. It is very unusual in my experience with my modern engine for the length of engine cranking time to exceed two seconds.

Now we have data for current and time, so we can deduce the electrical power consumed:

200-Amperes x 2-seconds x 1-hour/3600-seconds = 0.11-Ampere-hour

We can now compare the energy consumed in starting with the average energy available for re-charge in our typical start-run-stop cycle:

Energy used per start = 0.11-Ampere-hour
Energy available for charging = 13.5-Ampere-hours

On this basis it appears that the battery can be maintained at full charge condition for an infinite number of start-run-stop cycles, as the available electrical energy from the charging system exceeds the energy used in starting the engine by a factor of more than 100-times.

Based on the data from my engine, I do not see much concern that the engine charging system will have any problem maintaining the engine cranking battery at full charge.

jimh posted 02-02-2012 09:28 AM ET (US)     Profile for jimh  Send Email to jimh     
Let us suppose that there has been an error in one of the factors used in these calculations. We could say the data in worst case could be to be in error by a factor of ten and in the direction of causing the most harm. For example, where I estimate the cranking current to be 200-Amperes we could suspect it might be ten times more, 2,000-Amperes. We have seen that the capacity of the system exceeds the demand by a factor of more than 100. Even allowing for a tenfold error, the system would still have a reserve capacity of more than ten times what is needed.
20dauntless posted 02-02-2012 10:13 AM ET (US)     Profile for 20dauntless    
Hi Jim, based on my experience the engine start battery recharges very quickly. I have a Blue Seas VSR installed so that the engine automatically uses the engine starting batter to start, charges it to a pre specified voltage, and then switches over to charge the house battery. After an engine start, even when it's only ~30 degrees outside, it only takes maybe 30 seconds for the voltage to get high enough on the starting battery for the VSR to switch over to the house battery. And this is with the engine running at idle. The starting battery is a group 24 and the engine is a Honda BF90D.
18AGAIN posted 02-02-2012 11:11 AM ET (US)     Profile for 18AGAIN  Send Email to 18AGAIN     
Hi Jim. The one thing not being considered is the use of all electronic equipement ie. VHF, plotter, lights, electric downriggers so how will all this be incorporated into your calculations
jimh posted 02-02-2012 12:13 PM ET (US)     Profile for jimh  Send Email to jimh     
The typical auxiliary electronic load while underway on my boat consists of the VHF Marine Band radio, a multifunction display device with GPS receiver, SONAR, and color display, and some automatic pumps.

The VHF Marine Band radio is typically in receive mode with the audio muted, and in that state the current draw is about 1-Ampere. The multifunction display probably draws 3-Amperes. The pumps probably run less than one minute per hour, so their current drain can be ignored. The typical current drain is thus about 4-Amperes. My estimates for available charging current from my modern outboard engine were conservative. For example, the engine is actually rated to deliver 50-Amperes; I only used 35-Amperes in my calculation. I don't think the extra 4-Amperes of the electronic load will affect the charging condition. Recall that there is a surplus of more than 100-times the necessary current.

jimh posted 02-02-2012 08:51 PM ET (US)     Profile for jimh  Send Email to jimh     
A storage battery has the potential to contain a certain amount of stored electrical energy. When the storage battery has reached the maximum amount of electrical energy it can store, it is said to be "fully charged." When we talk about an outboard engine charging a storage battery, we have to consider if the charging system is able to push enough electrical energy into the storage battery to cause the storage battery to become fully charged. The ability of the charging system to fully charge a storage battery depends on the voltage and current it can supply to the battery. The notion of a full-charge also depends on other factors like temperature.

The best battery charging systems monitor the battery voltage and temperature, and adjust the charging system to vary the current being forced into the storage battery. The typical outboard engine charging system, even those charging systems on what we would like to consider as modern outboard engines, may not be sufficiently sophisticated to be able to always and reliable provide the proper voltage and current to an attached storage battery to consistently cause the storage battery to reach full-charge.

Outboard engine charging systems seem to be generally designed around a constant voltage output, and such a charging system will tend to have the charging current taper towards zero as the battery terminal voltage rises toward full-charge. I don't know of any outboard engine charging system that monitors the battery temperature and uses the temperature to vary the charging system voltage output or current. Sophisticated battery charging systems, such as a photo-voltaic charging system designed to charge and maintain a large ampere-hour battery, typically employ a temperature sensor for the battery. The charging current and voltage are adjusted to suit the battery temperature. The charging voltage and current are also changed according to an algorithm to best condition the chemistry of the battery and to push it to full-charge. Outboard engine charging systems tend to be less sophisticated. Sophisticated charging systems are also adjustable to match the characteristics of particular batteries. Not all batteries are alike, and each battery may have its own preference to charging terminal voltage and current profiles.

It is thus possible that a particular outboard engine charging system may not be able to perfectly charge and maintain a particular battery. We cannot be certain that a particular battery used with a particular engine will produce an electrical energy storage system where the battery is always returned to the absolute peak of its energy storage capacity.

jimh posted 02-02-2012 11:39 PM ET (US)     Profile for jimh  Send Email to jimh     
What I have shown here with the data from my modern outboard engine's history report made available by its electronic controller is that in my case, for my boating habits, there is very ample capacity for the engine to provide sufficient charging current to maintain the storage battery at full charge. I demonstrated that the ratio of available charging current to necessary charging current exceeded 100-to-one.

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