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Author Topic:   Battery Use
wood duck posted 08-29-2008 11:06 PM ET (US)   Profile for wood duck   Send Email to wood duck  
I have a 2008 205 Conquest with a 200 Verado with 55 hours. According to the BW manual, it says to start on both batteries, and then switch over to a single battery while underway and that running on both batteries may cause electrical damage. I have talked to several boaters, including Whaler owners, who disagree and recommend starting and running on both batteries. I have been following the manual, but now I am not certain. I am asking for some feedback.
jimh posted 08-30-2008 08:52 AM ET (US)     Profile for jimh  Send Email to jimh     
The reasoning behind the recommendation to parallel the batteries for engine start is related to the Verado motor. It needs plenty of electrical power to start, and its engine control unit (ECU) probably does not tolerate any voltage sags, especially during start up. A modern engine won't start if its microprocessor that controls everything is not properly running and executing its stored instructions.

The reasoning behind the recommendation to switch to a single battery for operating the boat is to prevent the batteries from being in parallel. With the primary battery distribution switch in the BOTH position, the batteries are connected in parallel by very heavy conductors. When batteries are connected in parallel they tend to behave like the weaker of the two batteries. Charging the batteries in parallel is not as effective as charging them separately because the charging current will not always distribute equally.

In some instances batteries are designed to operate in parallel, but typically when this is done there is some circuitry to help the batteries share both the load current and the charging current equally. However, when the batteries are just strapped in parallel by the battery distribution switch, it is only the internal resistance of the batteries which controls how they behave. If you are lucky, you have two batteries with internal resistances that are matched perfectly, and there is no problem. However, batteries are quite individual devices, and even among batteries of the same age, same manufacturing lot, and same use history, it is common for variations in the internal resistance to develop, particularly as the battery ages.

The worst situation occurs when one battery has a major problem such as a shorted cell. That battery will kill another battery in parallel with it. Less severe problems occur when one battery absorbs most of the charging current, leaving the other battery at less than full capacity.

jimh posted 08-30-2008 09:00 AM ET (US)     Profile for jimh  Send Email to jimh     
Another concern with parallel battery operation: the charging current. Two batteries could absorb very high charging current. This might be an excessive load for the charging source. Outboard motor battery charging circuits are often able to supply rather high charging current, but there is always in the back of my mind a concern about their endurance. I don't think in the main that the battery charging circuits on most motors were really designed to deliver the peak current all the time. Thus, if you hang two batteries in parallel, the may create a very high charging current load, possibly right up to the maximum available from the motor. That puts your motor charging circuit at its maximum rated power. OK, it should be able to do that, but maybe you don't want to find out if it really can pump out that kind of current all the time.
seabob4 posted 08-30-2008 11:38 AM ET (US)     Profile for seabob4  Send Email to seabob4     
Remember, Verados do require a minimum of 800 CCA, which is not a very common battery. We use Exide Nautilus 1000s, 1000 MCA/800 CCA. We do a lot of twin Verado installs, and we do NOT parallel the batteries when starting. Rather than consulting the Whaler owners manual, read the Verado manual you got with the boat owners paperwork package. Read the section about battery requirements.

As I have stated in past threads, Verados are extremely voltage dependent. Good strong batteries are a must. The easiest way to achieve this is to install an onboard battery charger and plug it in after each outing. Get in the habit of doing this and it will make your life much simpler. Good batteries will also eliminate false Smartcraft error messages as the entire Smartcraft/DTS system relies heavily on good voltage.

JMHO.

wood duck posted 08-30-2008 08:39 PM ET (US)     Profile for wood duck  Send Email to wood duck     
Thanks, Jim for the thorough explanation, it is quite clear to me now. Thanks, Seabob4, for the recommendation to install an onboard charger, it makes sense for me to do that.

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