33+3 BATTERY DRAIN

[furma2n]

I own hull # 110 with a single CAT 3126 engine. I am having trouble with the house battery losing voltage while standing idle. During the winter I had new batteries and a new charger installed, as well as adding galvanic isolators to both shore power lines.

I just returned from a trip 2 days ago, and this morning the house battery reads 12.4 volts.........there has been no load on it since we docked.

Any ideas??

[N4QC]

Make sure that the water levels in the batteries are correct. Only use distilled water if they need to be topped off.

Make sure that the tops of the batteries are clean. There should be no crud around the battery posts.

[Mariner]

Read up a bit on my experiences, as I've posted every excruciating detail. First off, 12.4v is not dead. It's not even low. That's about all you can expect a battery of that type after sitting for a day or two. It is, after all, a 12 volt battery. It may read 13 or 14 volts durring or shortly after charging, but in a static state, that it going to float down to 12.5 or so. If it drops below 12 volts, then you probably have a parasitic drain somewhere.

The key thing is whether or not you're able to start the engine. So long as you can, I think you are just fine.

[Guest]

Thanks for the replies............the batteries are new and all connections are clean and tight; full with distilled water. The reason I became concerned is that Nigel Calder's book indicates that 12.4v is 75% of charge....it seems to me that a new battery should not lose 25% of its charge in 2 days with no load, but maybe I am worried for nothing.

[Mariner]

I've never seen a deep cycle battery that hung out at 13.5-14vdc for very long after the charger was turned off. That said, I'm not exactly the authority on electricity.

The question is if you leave it for another 48 hours, does it drop to 9vdc? If so, then you have a drain.

[DougSea]

Thanks for the replies............the batteries are new and all connections are clean and tight; full with distilled water. The reason I became concerned is that Nigel Calder's book indicates that 12.4v is 75% of charge....it seems to me that a new battery should not lose 25% of its charge in 2 days with no load, but maybe I am worried for nothing.

You might want to get a hydrometer and test the electrolyte of the battery. These are nice (assuming you have lead acid batteries) because you can check each cell. My big house battery is an 8D that's as old as the boat. I've found by using a cheap hydrometer that at least one cell is not staying up to full charge and this battery will be replaced over the winter.

Doug

[pbirman]

12.4 - 12.6 V is the normal state for a "12V" battery. Actually the word "battery" means it is a collection of cells, in this case six cells each rated about 2.1 volts. Under charge the voltage will rise to 13.8 - 14V owing to a transient effect called the "surface charge" which dissipates in an hour or so after the battery charger shuts off.

Cell voltage is not a useful way to estimate a battery's state of charge since it is virtually the same from full charge to exhaustion. As one poster here suggested, a hygrometer to measure the specific gravity of the "water" (actually sulphuric acid - be careful!) is the best way --- probably the only way. A second test is to measure the internal resistance of the cells. a measure of how much voltage is lost under heavy load. Unless you have a heavy load battery tester, the best way is to see (as has also been suggested) how well the battery starts the engine or supports some other heavy load.

If one cell of a 6-cell battery goes bad, you will notice a loss of two volts and that is about the only fault that a simple terminal voltage test will reveal.

Paul