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Old 10-07-2010, 10:35   #1
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Location: Belgrade, Maine
Boat: J/40 Watermusic
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Electrical - Engine Starting Question

The battery box in my "new" J/40 is large enough to hold four batteries. For the past month, while I've been working on the boat (since it was launched from winter storage in Maine), I have had two group 24 wet-cell marine starting batteries in the box, wired to a single circuit (#1). These two batteries have without difficulty started the Volvo diesel three or four times. As there were no batteries connected to circuit #2, I left the boat battery switch on "Both" all the time. I have thrown a charger on these batteries from time to time and kept up their charge.

A few days ago, I installed two group 31 gel-cell batteries and wired them to the other circuit (#2). Yesterday, I checked that both sets of batteries were fully charged--they both showed about 12.6 volts--and then tried to start the engine on circuit #1 (the wet cells). No go... it turned over slowly, just as if it were connected to a mostly drained battery. I waited a couple of minutes and tried again with the same results. I checked the charge on the wet cells, which was down a bit at first but it rebounded to 12.6 volts after five minutes.

At that point, I put the battery switch over to "Both" and started the engine with no problem.

So, does my battery switch need to be on "Both" to start the engine, no matter the charge on the batteries? Very curious.
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Old 10-07-2010, 11:01   #2
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First of all gel-cell and AGM batteries have a higher specific gravity than most wet-cell batteries and, as a result, have a higher standing "full" voltage. They should be closer to 12.9 V.

Second, inspect the entire positive and negitive circuit of the gel-cell batteries as the starter "sees" them. I suspect that you have a long cable run in there not "seen" to the wet batteries.

Third, there is no doubt that your gel-cell bank should spin that starter faster than the flooded bank due to higher voltage and lower internal resistance (assuming that your parasitic cable and switch resistances are the same for both banks).

I suspect that your charger is not applying the correct float voltage (13.8 or 13.9V) to the gel-cell batteries.
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Old 10-07-2010, 11:33   #3
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I would check the connections on the first set of batteries. You might have a bad cable or bad connection. It is also possible you have a bad contact in the switch. The contact being used in the both position is probably different than the contact in the 1 position. If it is corroded or burned you may be getting a high resistance connection which can carry voltage under no load but cannot adequately carry the starting amperage. When you switch it to both there is at least one clean connection to one of the banks allowing you to start the engine.
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Old 10-07-2010, 13:41   #4
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Mixing flooded batteries (your Group 24 starting batteries) with gelled batteries (your new Group 31 batteries) is not a good idea due to the differences in their needed voltages for proper charging. (Note that mixing flooded batteries and AGM batteries is generally OK, since they have similar voltage needs for charging.)

Moreover, the gels are more delicate than the flooded batteries. It's pretty crucial to get their charging and float voltages right and, especially, to avoid overcharging. If you treat them right, they can last a very long time, but they're also easy to kill with bad charging practices.

In the case you described, I'd speculate that either your cabling/connections are dirty or inadequate or the wet cell batteries are weak.

What you should do depends on your electrical setup and your cruising plans. Since you've already got two new gelled batteries, you might consider buying one more group 31 and using it for starting, while tossing the old flooded batteries. BTW, two starting batteries is overkill. You don't need them.

That way, at least you'd have all new batteries of the same type, requiring the same type of charging. Then, you'd need to be sure your battery charger and any other onboard charging devices can be set for gel cell charging.


PS: are you SURE you have gelled batteries? Many folks mix up gelled batteries and AGM batteries....they're not the same, and require different treatment.

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