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I have a 5 year old Interstate 12V cranking btry. Put it on the charger for a few hours then tested it after it rested about 8 hrs and again at 24 hours and both times got 12.57

Does that reflect near 100% charge? I'm thinking it does. Yes I know it's 5 years old but I want to hear from those that know if this btry is still holding a full charge. Thanks!
 

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nothing wrong with that. my dekkas are going back in for the 6th season 12.4 on both.
Open Circuit Voltage (12 V) Open Circuit Voltage (6 V) Approximate charge Relative acid density 12.65 V 6.32 V 100% 1.265 g/cm3 12.45 V 6.22 V 75% 1.225 g/cm3 12.24 V 6.12 V 50% 1.190 g/cm3 12.06 V 6.03 V 25% 1.155 g/cm3 11.89 V 6.00 V 0% 1.120 g/cm3 Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full charge.
The following is common for a six-cell automotive lead-acid battery at room temperature:

  • Quiescent (open-circuit) voltage at full charge: 12.6 V
  • Unloading-end: 11.8 V
  • Charge with 13.2–14.4 V
  • Gassing voltage: 14.4 V
  • Continuous-preservation charge with max. 13.2 V
  • After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
  • Wait at least 12 hours after charging to measure open circuit voltage, the resting time allows surface charge to dissipate and enables a more accurate reading.
  • All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes.
 

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You have to put a load on the battery (usualy like a light or something around 5 amps). Let it run for 5 mins. This will remove any surface charge.

One other thing you need to do is a load test. You can take it to pepboys or advanced auto, and they will test it for free.
 
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