Battery test

[Allan]

I have just removed my battery from the boat for it's final winter charge and wish to test it. It is a 75 amp/hr "deep discharge" battery. I seem to remember being taught a 20 hour test was the best way to test this type of battery. I plan to run a 45w (3.75amp) load to see if this will run for 20 hours. Can anyone confirm if this is correct or not?
Also I am considering fitting a small solar panel to keep the battery topped up while the boat is on it's mooring. I have no charging available on the boat, but only use the battery for navlights.
I have seen a table of voltages against state of charge but I can't remember where! Does anyone have this and is it a reliable way of checking the state of charge?
Thanks in advance,
ALLAN

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[StephenW]

another way is a heavy load on the battery for 30 seconds - like cranking the engine while pushing the decompression switch (watch out for water-filling the exhaust if still in the water - best on dry land). Voltage shouldn't drop below 10.5V.

I wouldn't completeley discharge the batteries as anything below 50% will cause damage as I understand it.

Read Nigel Calders Electrical and Mechanical Boat Handbook

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[Allan]

Stephen, thanks for your reply. This is a domestic battery and unlike a start battery should be OK when taken below 50%.
ALLAN

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[VicS]

Based on information published a few years back in PBO I take 12.7 volts as fully charged and 11.7v as virtually discharged with a linear relationship between these and state of charge. The voltages are measured with digital meter (ie very high resistance) when the battery has rested for 12 hours after charging or discharging.

I have a 5watt solar panel which I rate as one of the most useful bits of kit I have bought for the boat. Other than this I only have a very small charging output from the outboard which on a good day is only run for a few minutes.

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[alan]

The test as you describe is detailed on the following web site:
www.attfield.dircon.co.uk

look under "related topics" and then "battery:safe testing".

If you put the data in an Excel spread sheet you can then make pretty graphs of your battery performance, and compare them with the same test next year or if you suspect that your batteries are not performing.

Alan.
Nettuno, Italy.

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[TonyBrooks]

I do not think that is the case.

It should have thicker plates than an engine battery so would be more susceptable to damage by excess current flow. Its the charging current that may cause damage on such a battery when deply discharged and then connected to a modern alternator. The higher teh current flow, the more the plates "dance" and flex.

I would never exceed a charge rate of about 20% of amp hour capacity if I could avoid it, and its worked OK for me - so far.

Tony Brooks

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