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  Is This Deeply Discharged Battery Going To Be Okay?

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Author Topic:   Is This Deeply Discharged Battery Going To Be Okay?
swist posted 09-22-2010 02:35 PM ET (US)   Profile for swist   Send Email to swist  
You know why you install a battery switch? So you can be a MORON and forget to shut it off (and of course then leave the electronics powered up, and leave the boat alone for 2 weeks).

The battery is one of those combo deep-cycle/starting jobs, lead-acid group 24.

After this idiocy, the battery voltage read as zero. I replaced the battery and the old one was sitting on my workbench on its way to be returned to get the $10 core deposit back.

Just for laughs, I stuck a voltmeter on it. 6 volts. Hmm, probably a little bounce back now that there is no load.

Just for more laughs, hook up an (el-cheapo Sears 6A) charger. Takes a while (many hours) but charge current drops to below 2 amps and voltage reads 12.7 volts. It's now 3 weeks later. Still 12.7 and trying to charge it shoes no appreciable charging current (it's holding).

I assumed that a battery completely discharged for 2 weeks would be damaged. But maybe not? I'm tempted to keep this battery as a backup but since I can't really reproduce a serious load on my workbench, is it likely to die if it gets put back on the boat?

Hoosier posted 09-22-2010 02:50 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
Take it to an auto parts store like AutoZone and have them do a load test on it. That should tell you if it's shot. I have my boat battery load tested at the start of each season.
jimh posted 09-22-2010 06:14 PM ET (US)     Profile for jimh  Send Email to jimh     
A reliable test for predicting the time in the future when a lead acid storage battery will fail is not easily made. The best indicator is probably to track the history of the battery's internal resistance. Generally measurement of the internal resistance of a battery is not easily performed without very specialized test equipment. The general trend with a lead-acid storage battery is for the internal resistance to increase over time. The basis for prediction of failure is the amount of change in internal resistance over time from the initial value when the battery was new. If no historical data is available for a particular battery, you may be able to make a reasonable inference about the present value of internal resistance by comparison to the OEM specifications for the battery.

A load test in which a substantial current drain is imposed is one method of testing the state of charge and capacity of a battery, however, the process of the test itself causes the charge to be decreased. At the conclusion of the test the battery is no longer in the same condition it was before the test. The simple brute-force load test does provide data for evaluation of the battery. The current delivered and the battery terminal voltage under load can be measured. Exactly how one would make an interpretation of this data to derive a measure of battery health is unknown to me. If the terminal voltage drops very low, below 10-volts for example, the battery might be considered to be NO-GOOD. But exactly where one would set the threshold for GOOD versus NO-GOOD is not clear to me.


Some more modern battery test devices make a test of battery capacity by using a smaller load current and careful measurement of the terminal voltage. From those measurements they impute a corollary load current that may be delivered. The advantage of these tests is there is no large current discharge from the battery, and at the conclusion of the test the battery is in the same condition as before the test.

A simpler metric for evaluating a battery is simply its age. If the battery is five years old, it is probably time to replace it. This is particularly true if the battery is a sealed battery such as an AGM battery. AGM batteries are known for many things, but long life is not one of them.

jimh posted 09-22-2010 10:11 PM ET (US)     Profile for jimh  Send Email to jimh     
In this specific case under discussion here, a battery of unknown age was accidently discharged to a very low state of charge and allowed to remain flat for two weeks. We are asked to predict the future utility of the battery. About all I can say is, "it's variable."

My general impression--and this is gained from first hand experience with batteries, discharging them, and 40-years of driving a car in a winter climate, and not from scientific or chemical analysis--is that the farther along in its normal life span of providing useful service that a battery is when a deep discharge occurs, the more likely the deep discharge will cause a significant decrease in the battery's capacity to store a charge and supply current. A new or almost new battery seems to be able to tolerate a prolonged period of deep discharge better than a five-year-old battery. I suspect that there are some underlying reasons based in Chemistry that account for this. I cannot explain them.

Also, a battery that has been deeply discharged seems to recover better if the re-charge is done gradually and with a low charging current. In many cases, a deeply discharged battery will not accept much charging current, and it is not unusual that only a small charging current will flow at the beginning of recharging if the battery is really very flat and in a deeply discharged state. After a day or two, the charging current will increase, and the battery will accept more current. Finally, the charging current will again taper as the battery nears its full-charge point.

Most retailers of batteries now have a low-current battery tester that can give you a metric of the battery's state of charge and stored power, usually given as a figure of merit in cranking amperes. Let your battery float on the charger for a few days, re-fill the electrolyte as needed (if the battery is not a sealed battery), and then have it tested.

swist posted 09-24-2010 06:37 AM ET (US)     Profile for swist  Send Email to swist     
Just to set the record straight, the battery was 1-1/2 years old. If it were much older, I would agree that it isn't worth the trouble of trying to resurrect it (and risk using it).

And I would say the charging behavior descibed by Jim H

"Also, a battery that has been deeply discharged seems to recover better if the re-charge is done gradually and with a low charging current. In many cases, a deeply discharged battery will not accept much charging current, and it is not unusual that only a small charging current will flow at the beginning of recharging if the battery is really very flat and in a deeply discharged state. After a day or two, the charging current will increase, and the battery will accept more current. Finally, the charging current will again taper as the battery nears its full-charge point."

pretty accurately describes what I observed.

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