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Author Topic:   Starting Battery Discharged; Affect on Service Life
swist posted 07-07-2014 03:11 PM ET (US)   Profile for swist   Send Email to swist  
Don't get old. I left some stuff powered on in my Montauk and forgot to turn the battery switch OFF, and then it sat in the Arthur storm deluge for 3 days with the bilge pump running a lot. Battery dead as a doornail.

I put in a spare so I could use the boat and put the battery on a charger (an elcheapo Sears 6 amp). When I attached the leads from the charger, its ammeter read zero for a couple of seconds and slowly (over a period of about 15 seconds) went up to 6 amps. It's been on the charger for about 8 hours and the ammeter now reads 4.5 amps and seems not to be going down much any more. The voltage is 12.7 if I disconnect the charger, but of course that is with zero load.

I know starting batteries don't like complete discharges. Does this charging current pattern indicate anything useful?

Will the battery still give useful service? Or is it simply that its life has been shortened?

jimh posted 07-07-2014 03:21 PM ET (US)     Profile for jimh  Send Email to jimh     
If the battery was otherwise in good condition, one deep discharge shouldn't kill it.

Here's an anecdote: at age 85 my Mother decided she wanted a new car. The first time she drove it she left an interior map reading lamp on--she had hit the switch inadvertently when adjusting the mirror. A week later she calls me and says the car won't start. I drive over and find the battery completely flat. A slow recharge to full with a charger--like the one you described--got the battery back. I think it lasted five years, which is darn good for five Michigan winters.

I suppose you could assess the battery by taking it to a battery retailer and asking them to test it on one of their modern battery chargers which can estimate the cranking current capacity without actually drawing the full cranking current. But then those testers are probably designed to sell new batteries.

You really cannot deduce too much from the resting terminal voltage other than the state of charge. If you had a really fancy instrument you could measure the internal resistance, but since you don't have a measurement from before the discharge, you won't have much to compare with, other than perhaps a new, fresh battery of the same brand and model.

Perhaps the best indicator will be to monitor the battery's terminal voltage when you crank over the engine. If it seems to go really low, below 9-Volts, the battery may be in trouble. (I see my rather large cranking battery dip to the 9-volt region under engine cranking on my NMEA-2000 instrument; I haven't tried actually watching a voltmeter on the battery terminals.)

swist posted 07-08-2014 07:50 AM ET (US)     Profile for swist  Send Email to swist     
Thanks Jim,
I sometimes wonder if the susceptibility to deterioration of starting class batteries under deep discharge isn't overstated. In my 50 years of driving, I've had my share of dead batteries, sometimes close together on the same battery due to an alternator or other problem. I don't recall ever replacing a battery that seemed to die well before its time.

Obviously the marine environment demands more attention to this, which is why I asked the question. No one needs a battery to suddenly fail out in the North Atlantic because it has been severely discharged at one (or more) points in its history.

Jefecinco posted 07-08-2014 09:35 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
A dual battery arrangement with an isolator is not a bad idea for any boat but for a boat that ventures offshore I believe it is essential for safety. If a dual battery arrangement is inappropriate for some boats or budgets carrying a booster/jump battery pack is an alternative.

I've never had a battery failure on my boats. However I've had automobile batteries suddenly fail with no warning whatsoever. Battery failures with no warning may be a little more common in the climate of the deep south. I know automotive batteries for our area have different model designations than do those for other parts of the country.


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