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post #1 of 19 Old 03-23-2010 Thread Starter
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battery test

Hi everyone, we just bought a Catalina 25 and it came with two 12 volt Stowaway Marine deep cycle batteries, 105 amphours each. They appear to be fairly new and the PO said that he kept them on a charger (what kind I'm not sure) over the winter.

I brought them inside and charged them up and switched them to the float charger which they have been on for a few days. The other day I wanted to see what they would do if I took them off of the charger and let them sit for a little while. When I unplugged them they were at a full 12.8 volts. I let them sit for two days and when I checked them again they were down to about 11.5 volts. From what I know about batteries, this high of a self-discharge rate isn't good... Are these batteries sunk or is there something else I can do to them? I checked the electrolyte levels and they are full. Thanks!

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post #2 of 19 Old 03-23-2010
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jj,

That amount of discharge in so short a time is not encouraging. If they are not sealed batteries, before spending any serious money, go to an auto-parts store and buy a hydrometer (~$5-$7) and test the specific gravity. The hydrometer will be calibrated and likely have a "good, fair, and dead" range that is good enough for a quick diagnosis.

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post #3 of 19 Old 03-23-2010
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Do a load test on them
Charge them fully then connect all the load you have ( not starter motor or anything else that will burn out) for at least 20 minutes. You should see the volts drop off dramatically(11V) then come back up. How good your batteries are depends on how far they come back and for how long. Take voltage every couple of minutes and store these for future reference when you need to make a decision on when to replace.
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post #4 of 19 Old 03-23-2010
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Did you check the electrolyte levels in the cells. If they're low, the batteries will appear to self-discharge rather rapidly.

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post #5 of 19 Old 03-23-2010
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Low voltage in a battery is likely due to something preventing the electrolyte form interacting with the plates:
- A highly sulfated battery - lead sulfate crystals
- Low electrolyte levels
- Degraded plates
- Poor electrolyte

Suggestions to bring them back:
- Check the electrolyte level
- Use a hydrometer to test electrolyte (see post 2)
- Apply an equalization charge (High voltage [16V] and low amperage [<5A]) to the batteries. DO NOT TRY THIS UNLESS YOU KNOW WHAT YOU ARE DOING.

If it were me, I'd buy new batteries.


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post #6 of 19 Old 03-23-2010 Thread Starter
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Quote:
Originally Posted by sailingdog View Post
Did you check the electrolyte levels in the cells. If they're low, the batteries will appear to self-discharge rather rapidly.
I did check the electrolyte level and the batteries are full.

Doesn't a hydrometer just test the charge of the battery? What is the difference between testing the electrolyte fluid with a hydrometer and testing the voltage with a multimeter?

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post #7 of 19 Old 03-23-2010
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A Hydrometer tests the specific gravity of the electrolyte in the battery. Electrolyte is a solution of sulfuric acid and water. As the battery discharges, the sulfuric acid weakens, and the solution becomes less dense. If the battery seems to discharge after removing it from a charger, it MAY be because the electrolyte in the immediate vicinity of the plates (high density) is diffusing with the electrolyte throughout the battery.

While the multimeter will show you the voltage that the battery is holding at a given point of time, the hydrometer will show you the charge that the battery can store.

Here is a description of the reaction...

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post #8 of 19 Old 03-23-2010
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Quote:
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Doesn't a hydrometer just test the charge of the battery? What is the difference between testing the electrolyte fluid with a hydrometer and testing the voltage with a multimeter?
Here is a "cut & paste" (sorry Dave)

6. Battery Testing can be done in more than one way. The most accurate method is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer, to measure voltage use a digital D.C. Voltmeter. A quality load tester may be a good purchase if you need to test sealed batteries.

For any of these methods, you must first fully charge the battery and then remove the surface charge. If the battery has been sitting at least several hours (I prefer at least 12 hours) you may begin testing. To remove surface charge the battery must be discharged for several minutes. Using a headlight (high beam) will do the trick. After turning off the light you are ready to test the battery.

State of Charge Specific Gravity Voltage
12V 6V
100% 1.265 12.7 6.3
*75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0

*Sulfation of Batteries starts when specific gravity falls below 1.225 or voltage measures less than 12.4 for a 12v battery, or 6.2 for a 6 volt battery. Sulfation hardens on the battery plates reducing and eventually destroying the ability of the battery to generate Volts and Amps.

Load testing is yet another way of testing a battery. Load test removes amps from a battery much like starting an engine would. A load tester can be purchased at most auto parts stores. Some battery companies label their battery with the amp load for testing. This number is usually 1/2 of the CCA rating. For instance, a 500CCA battery would load test at 250 amps for 15 seconds. A load test can only be performed if the battery is near or at full charge.

The results of your testing should be as follows:

Hydrometer readings should not vary more than .05 differences between cells.

Digital Voltmeters should read as the voltage is shown in this document. The sealed AGM and Gel-Cell battery voltage (full charged) will be slightly higher in the 12.8 to 12.9 ranges. If you have voltage readings in the 10.5 volts range on a charged battery, that typically indicates a shorted cell.

If you have a maintenance free wet cell, the only ways to test are voltmeter and load test. Any of the maintenance free type batteries that have a built in hydrometer(black/green window) will tell you the condition of 1 cell of 6. You may get a good reading from 1 cell but have a problem with other cells in the battery.

When in doubt about battery testing, call the battery manufacturer. Many batteries sold today have a toll free number to call for help.

and here is its home source...Battery Basics: How to Provide Care and Maintenance for Your Battery | BatteryStuff.com

Wayne
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post #9 of 19 Old 03-23-2010
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Hydrometers allow you to test:

(1) the state of charge (SOC) of each cell in a flooded battery or battery bank; and
(2) the differences in specific gravity levels between the cells (or the need for equalization).

Voltmeters allow you to test the SOC of the whole battery, or each whole battery in a battery bank, providing the battery has not been under charge or load for a few hours. They can be used effectively on all types of batteries.

Neither method tells you much about the capacity of the battery, i.e., it's ability to store and deliver energy in the rated amount. That a battery is at 100% SOC says nothing about its capacity. As batteries age, sulfate, stratify, corrode, contaminate, and physically distort and deteriorate they will eventually reach a final stage wherein a 100% SOC battery may be incapable of even turning over your engine. We've all seen that in cars.

The best way to test capacity of deep-cycle batteries is with a constant load calculated to reach total discharge (10.5VDC) after a 20-hour period. Note that battery testers of the type used in garages do not do the same thing. They put a heavy load on the battery for a short period...not at all the same thing as a proper capacity test for house batteries.

The next best way to test capacity is probably with a sophisticated battery analyzer like the Midtronics tester series. These test the internal resistance of batteries and are the industry standard these days. These testers are not infallible, and they're expensive ($500-600 and up). Their advantage is that they are quick and easy to use, give pretty repeatable results if used correctly, and generally seem to represent battery capacity pretty well.

Bottom line: given what the OP observed, I'd consider these puppies toast and get some new ones and treat them well. Only then will you know what you've got.

Bill
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post #10 of 19 Old 03-23-2010
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Wayne's post made me think about rewording some of my post above.
A Hydrometer tests the specific gravity of the electrolyte in each cell of the battery. You need to test each cell to get a reading on the overall health of the battery.

Also; As the battery cell discharges; lead, lead peroxide, and sulphuric acid of the cell are gradually changed into lead sulphate and water.
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