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Old 11-17-2015, 07:50 PM   #11
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Originally Posted by ornery View Post
Pardon me but a fully charged battery should be 13+ volts. ALSO put a DIGITAL volt meter on the battery and just watch for voltage leak. That is see if the voltage drops noticeably. If your battery isn't holding a charge you will see the volt meter dropping. Set the meter to at least 2 decimal places and see if the voltage drops.
Sorry but WRONG on both counts.
1. a 100% fully charged battery is 12.7 volts. The 13+ volts you see immediatey after charging is called a surface charge and is entirely even a badly damaged battery will read over 13. This is WHY I said to set the battery aside for 24 hours to allow the surface charge to disappate BEFORE taking a reading.
2. The same applies to your "leaky" voltage. Every single battery on earth will show leakyvoltage after charging as it slowly drops form 13.2+ V to 12.7 or 12.4 or whatever its' true state of charge is. Furthermore...this has no useful purpose in tracking down minor parasitic loads because they have nothing to do with the battery not holding a charge.
Don't believe me?? Here:

Source: Trojan BatteriesBattery Testing

Battery testing can be done in several ways. The most popular includes measurement of specific gravity, and battery voltage. Specific gravity applies to wet cells with removable caps, giving access to the electrolyte. To measure specific gravity, buy a temperature compensating hydrometer at an auto parts store or tool supply. To measure voltage, use a digital voltmeter in the DC voltage setting. The surface charge must be removed from a freshly charged battery before testing. A 12 hour lapse after charging qualifies, or you may remove the surface charge with a load (20 amps for 3 plus minutes).
State of Charge / Voltage/ Specific Gravity/ 12V 6V
100% 12.7 6.3 1.265
75% 12.4 6.2 1.225
50% 12.2 6.1 1.190
25% 12.0 6.0 1.155 Discharged 11.9 6.0 1.120 Source: Battery Charging Tutorial |

Finally...I'd note that I use 24 hours as a wait time instead of 12 because surface charge removal is temperature dependent... the colder...the slower. In northern climates you can often come back even two or more days later and still find the battery over 12.7 (or whatever its' true SOC is).
...a cheap $20 load tester from Harbor Freight can cut the wait down to about 3 minutes.
This shows an experiment done by a friend of mine on static state of charge and the need to let a battery rest...complete with pictures. Case closed..
Measuring A Lead Acid Battery State of Charge Photo Gallery by Compass Marine How To at

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Old 11-18-2015, 07:42 PM   #12
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Join Date: May 2012
Posts: 357
Testing a deep cycle battery isn't as simple as it might seem. Voltage and specific gravity testers can show the current charge state but do little to show battery capacity. Automotive battery testers place a large (100-200A or more) load on the battery for a very short time to simulate the starter motor load which is not what you're interested in for your house battery.

Both house batteries in my 2011 Georgetown failed in less than a year because they had two extended periods where they were left fully discharged. (One was before I purchased the new unit, the other was caused by a wiring short in the hydraulic jack system.) When tested with an automotive tester, they tested ok. I pulled one and took it to a local outlet store for a major battery manufacturer. They told me that their staffing that day prohibited them from using the special deep cycle tester they had.

I built my own deep cycle tester and ran a test on the battery myself. The battery initially tested good but, at the end of the test, I determined the battery capacity was only 50% of its rated value. I turned my documentation over to the dealer and FR replaced both house batteries. The replacements are now over three years old and are still in good condition.


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